Viral hemorrhagic fever
Last updated Jun 21, 2012
A wide range of viruses can cause viral hemorrhagic fever (VHF) and hence are designated as hemorrhagic fever viruses.
- All possess single-stranded RNA (which requires reverse transcriptase for multiplication or amplification by polymerase chain reaction [PCR])
- All possess a lipid envelope
Hemorrhagic fever viruses belong to four taxonomic families, only one of which, Filoviridae, has been assigned to an order (Mononegavirales):
Specific hemorrhagic fever viruses in each of the four families and key characteristics are included in the table below. The table includes the predominant hemorrhagic fever viruses but is not a comprehensive list. For example, a new arenavirus (Lujo virus) was identified in 2008 as a cause of illness in humans in Africa (Peters 2010: Lymphocytic choriomeningitis, Lassa virus, and South American hemorrhagic fevers).
Characteristics of Hemorrhagic Fever Viruses
—Origin of family and genus names from Latin "filo" for "thread"
—Old World arenaviruses:
~Chapare virus (also found in Bolivia)
—Origin of family and genus names from Latin "arenosos" for "sandy"
—Phlebovirus (includes Rift Valley fever virus)
—Spherical to slightly pleomorphic virions, 80-120 nm in diameter
—Yellow fever virus
—Origin of family name from Latin "flavus" for "yellow" (yellow fever virus)
The pathogenesis of hemorrhagic fever viruses is not completely understood; however, key points include the following (Peters 2002):
- Hemorrhagic fever viruses enter the bloodstream through various mechanisms (eg, the bite of a mosquito or tick, inhalation, mucous membrane exposure, parenteral exposure), and all (except hantaviruses) cause disease during the period of viremia.
- The infectious dose for hemorrhagic fever viruses appears to be low (1 to 10 organisms) (Franz 1997).
- Endothelial infection occurs with most hemorrhagic fever viruses and may be limited or widespread, depending on the virus.
- Hemorrhagic manifestations occur as a result of thrombocytopenia or severe platelet dysfunction along with endothelial dysfunction.
- Increased vascular permeability is common and may result in periorbital edema and hemoconcentration. Vascular dysregulation also often occurs, manifested by flushing of the face and chest.
- Hemorrhagic fever viruses can cause necrosis and hemorrhage in most organs; however, hepatic involvement often is particularly prominent.
- Hantaviruses, New World arenaviruses, and Ebola, Marburg, and Lassa viruses cause cytokine activation (Gomez 2011; Feldmann 2011; Peters 2010: Lymphocytic choriomeningitis virus, Lassa virus, and the South American hemorrhagic fevers; Peters 2010: Marburg and Ebola virus hemorrhagic fevers).
- The relative lack of histologic lesions in fatal cases and the lack of immunopathology suggest that cytokines are the primary mediators of hemorrhagic fever in arenavirus infections and that they play a major role in the clinical features of filovirus infections as well
- Studies in primates demonstrate that Ebola and Marburg viruses lead to a dysregulation of the immune response, with elevated circulating levels of cytokines and chemokines (Geisbert 2003: Pathogenesis of Ebola hemorrhagic fever in primate models; Hensley 2011).
- Ebola, Marburg, yellow fever, and Rift Valley fever viruses have a marked cytopathic effect (ie, are highly destructive to the cells they infect).
- Ebola and Lassa viruses also appear to infect monocyte-derived dendritic cells; dendritic cells exposed to these viruses do not up-regulate, fail to secrete pro-inflammatory or immunoregulatory cytokines, and do not effectively stimulate T cells (Bosio 2003, Mahanty 2003). These changes delay an effective early host response.
- Ebola, Marburg, Rift Valley fever, and Crimean-Congo hemorrhagic fever viruses can cause disseminated intravascular coagulation (DIC); the other hemorrhagic fever viruses generally do not.
- Coagulation abnormalities noted with Ebola virus infection appear to be triggered by immune-mediated mechanisms rather than occurring as the result of direct cytolysis of endothelial cells (Geisbert 2003: Pathogenesis of Ebola hemorrhagic fever in primate models). Findings from a study in rhesus macaques suggest that a mixed anti-inflammatory response syndrome (MARS) occurs following Ebola virus infection, similar to what is seen in severe sepsis and septic shock (Ebihara 2011).
- Suppression of the host antiviral response appears to play a critical role in the pathogenesis of Ebola virus infection (Peters 2010: Marburg and Ebola virus hemorrhagic fevers; Ebihara 2006). Failure to mount an effective immune response may be related to an immunosuppressive amino acid sequence in the filovirus glycoprotein or the secretion of a soluble glycoprotein by infected cells (Peters 2010: Marburg and Ebola virus hemorrhagic fevers). In addition, extensive lymphoid necrosis (in the spleen, thymus, and lymph nodes) and lymphocyte depletion through apoptosis also may play a role in suppression of the immune response (Feldmann 2011); however, one report suggests that lymphocyte apoptosis may be not be essential for disease progression (Bradfute 2010).
A model of the pathogenesis of Ebola virus based on observations of infection in cynomolgus monkeys is as follows (Geisbert 2003: Pathogenesis of Ebola hemorrhagic fever in cynomolgus macaques):
- Ebola virus spreads from the initial site of infection via monocytes and dendritic cells to lymph nodes (likely via the lymphatics) and to the liver and spleen (via blood). At these sites, the virus infects tissue macrophages, dendritic cells, and fibroblastic reticular cells.
- A series of events then occur that lead to virus-induced immunosuppression and apoptosis of T lymphocytes. As the disease progresses, apoptosis of natural killer cells also occurs, which limits the innate immune response.
- Unchecked viral replication then leads to increased levels of additional proinflammatory cytokines, chemokines, and possibly other mediators, which cause vascular impairment and trigger the coagulation cascade, ultimately leading to DIC.
- DIC then results in hemorrhagic shock, failure of multiple organs, and death.
Characteristic pathologic features of selected hemorrhagic fever viruses are shown in the table below.
Pathologic Features of Selected Hemorrhagic Fever Virusesa
Major Pathologic Features
—Extensive hepatocellular necrosis with intracytoplasmic viral inclusions
—Apoptosis of lymphocytes and lymphoid depletion
—Extensive hepatocellular necrosis with intracytoplasmic viral inclusions
—Apoptosis of lymphocytes and lymphoid depletion
—Extensive reticuloendothelial involvement
New World arenaviruses (including Junin, Machupo, Guanarito, Sabia)e
—Multifocal hepatocellular necrosis with Councilman body formation, nuclear pyknosis, cytoplasmic eosinophilia, cytolysis, and mild inflammatory cell infiltrates composed of neutrophils and mononuclear cells
Rift Valley fever virusf
—Widespread hepatocellular necrosis and hemorrhage with focal cytoplasmic degradation and formation of eosinophilic or dark bodies
Yellow fever virusg
—Midzonal hepatocellular necrosis
Kyasanur Forest disease virush
—Focal hepatocellular degeneration, fatty infiltration, and necrosis
Omsk hemorrhagic fever virusi
—Scattered focal hemorrhages (particularly in the brain)
Most hemorrhagic fever viruses that cause disease in humans occur in relatively localized areas of the world (primarily sub-Saharan Africa and focal areas of South America). The major geographic location and general pattern of occurrence for each virus are included in the table below.
General Epidemiologic Features of Hemorrhagic Fever Viruses
Major Geographic Location for Animal or Human Disease
General Pattern of Disease Occurrence
—Sub-Saharan Africa (all except Ebola-Reston)
—First identified in 1976
—First identified in 1967
—First identified in 1969
New World arenaviruses
South America (except Whitewater Arroyo virus, which has been associated with several illnesses in the western United States)
—At least six different New World arenaviruses known to cause human disease
Rift Valley fever virus
—First identified in animals in 1930 and in humans in 1975
Yellow fever virus
—Has been recognized for centuries
Kyasanur Forest disease virus
Karnataka State, India (west-central area of country)
—First identified in 1957
Omsk hemorrhagic fever virus
Central Asia (Western Siberia)
—First identified in 1940s
- Ebola hemorrhagic fever is an important emerging infection in central Africa and has received much attention in recent years owing to the documented high case-fatality rates (50% to 90%) associated with outbreaks. Ebola virus was first recognized in 1976 when two outbreaks of VHF occurred in Africa during that year (one in southern Sudan and one in northwest Zaire [now the Democratic Republic of the Congo]) (Peters 1999;Pourrut 2005; WHO 1978: Ebola hemorrhagic fever in Zaire).
- Since 1976, nearly 20 outbreaks have been recognized in Equatorial Africa (CDC 2011: Known cases and outbreaks of Ebola hemorrhagic fever, in chronological order). Furthermore, a geographic pattern has emerged in which the Ebola-Zaire strain affects predominantly central Africa and the Ebola-Sudan strain affects Southern Sudan and East Africa. The only strain identified in West Africa is the Ivory Coast strain, although as of mid-2012, this virus had been detected only on one occasion (Pourrut 2005). The Ebola-Bundibugyo strain has been found in East Africa (in Uganda) (McNeil 2010, Wamala 2010).
- The incidence of filovirus infections (with Ebola virus or Marburg virus) continues to be relatively low. As of 2008, these viruses combined have caused approximately 2,317 clinical cases and 1,671 confirmed deaths (Leroy 2011).
- In some situations, human Ebola outbreaks have occurred in conjunction with deaths in animals species (including gorillas, chimpanzees, mandrills, and bush pigs) (Lahm 2007; Vogel 2007). Characterization of Ebola-Zaire isolates from wild ape carcasses implicated recombinant viruses from the apes as the cause of outbreaks among humans from 2003 through 2005 (Wittmann 2007). During outbreaks in rural settings, multiple introductions of the virus into the human population through wildlife likely occur; whereas, during outbreaks in urban settings, a single introduction of the virus in the community appears to be more common (Allaranga 2010).
A recent large serological survey of rural populations in Gabon, using a sensitive and specific enzyme-linked immunosorbent assay (ELISA), found an overall Immunoglobulin G (IgG) seroprevalence of 15.3% to the Ebola-Zaire strain (Becquart 2010, Nkoghe 2011). The survey lasted 3 years and included 4,349 adults and 362 children from 220 randomly selected villages, representing 10.7% of all villages in Gabon. These results suggest that a large portion of the population may have been exposed to low levels of the virus through a common source, such as bat saliva on contaminated fruits. The findings also indicate that mild illness or asymptomatic infection may occur with greater frequency than previously suggested and support the perspective that the Ebola-Zaire strain is endemic in Gabon.
In addition to disease occurrence in Africa, several outbreaks of Ebola virus infection have been recognized in cynomolgus macaques in the Philippines or in monkeys imported from the Philippines into the United States or Italy; these situations have all involved the Ebola-Reston strain (CDC 1990:Filovirus infections among persons with occupational exposure to nonhuman primates or their tissues; CDC 1990: Filovirus infection associated with contact with nonhuman primates or their tissues; CDC 1996: Ebola-Reston virus infection among quarantined nonhuman primates; Miranda 1999).
The first outbreak occurred in 1989 at a primate facility in Reston, Virginia, which is why the strain associated with these outbreaks is referred to as the Ebola-Reston strain. At that time, animal cases also occurred in primate facilities in Pennsylvania and Texas. All three locations had received monkeys from the same export facility in the Philippines.
- Subsequently, outbreaks also occurred at the primate facility in the Philippines that exported the infected animals.
- Several persons working with infected primates had serologic evidence of recent filovirus infection; however, as of mid-2012, no clinical illnesses associated with this strain have been reported in humans.
In 2008, the Ebola-Reston strain was isolated from pigs in the Philippines. At the time, an increase in mortality was occurring on swine farms and samples from pigs that had died demonstrated evidence of infection with porcine reproductive and respiratory syndrome (PRRS) virus. In additions, some pigs had evidence of co-infection with the Reston-Ebola strain (Miranda 2011).
- Several people in contact with infected pigs had serologic evidence of infection, and all were asymptomatic; these results suggest that pigs can transmit the Ebola-Reston strain to humans. The most likely routes of transmission from pigs to humans include direct contact with the blood, secretions, organs, or other bodily fluids of infected animals and possibly respiratory transmission (WHO 2009: WHO expert consultation on Ebola Reston pathogenicity in humans).
- Ongoing circulation of the Ebola-Reston strain in the pig population provides an unusual opportunity for continued genetic evolution to occur as a result of passage, adaptation, and possible natural selection of this virus (WHO 2009: WHO expert consultation on Ebola Reston pathogenicity in humans). Monitoring the virus in pig populations, therefore, is appropriate.
Experimental challenge studies have demonstrated that the Ebola-Reston strain can cause asymptomatic infection in pigs and that viral shedding can occur from the nasopharynx following infection. These findings suggest that infected pigs can pose a transmission risk to farm, veterinary, and abattoir workers (Marsh 2011). In response to the isolation of the Ebola-Reston strain from pigs, another group of investigators challenged domesticated pigs with the Ebola-Zaire strain. They found that pigs also can be infected with the Ebola-Zaire strain and that infected pigs develop severe respiratory disease (Kobinger 2011). Furthermore, infected pigs shed virus from the nasopharynx for approximately 14 days after infection and can transmit the virus to uninfected pigs.
Ebola hemorrhagic fever cases and outbreaks identified between 1976 and mid-2012 are outlined in the table below, which can be found on the Centers for Disease Control and Prevention (CDC) Web site (CDC 2011: Known cases and outbreaks of Ebola hemorrhagic fever, in chronological order).
Recognized Cases and Outbreaks of Ebola Hemorrhagic Fever, In Chronological Ordera
Cases, Deaths, CFR (%)
Zaire (now DRC)
318 cases, 280 deaths, CFR 88%
284, 151, 53%
1 case (survived); laboratory infection
1 case (died)
34, 22, 65%
Cases in macaques only; no human cases
Cases in macaques only; 4 asymptomatic human cases identified by serology
Cases in macaques only; 3 asymptomatic human cases identified by serology
Cases in macaques only; no human cases
52, 31, 60%
Epidemic in wild chimpanzees; 1 human case (survived)
DRC (formerly Zaire)
315, 250, 81%
37, 21, 57%
60, 45, 74%
2, 1, 50%
Cases in macaques only; no human cases
Cases in macaques only; 1 asymptomatic human case identified by serology
425, 224, 53%
65, 53, 82%
Republic of Congo
57, 43, 75%
Republic of Congo
143, 129, 89%
Republic of Congo
35, 29, 83%
17, 7, 41%
264, 187, 71%
131, 42, 32%
Pigs shown to be infected; 6 asymptomatic human cases identified by serology
32, 15, 47%
Marburg hemorrhagic fever, like Ebola hemorrhagic fever, is an emerging disease in sub-Saharan Africa, although Marburg virus infection appears to be less common and case-fatality rates may be somewhat lower than those for Ebola virus infection.
Marburg virus was first recognized in 1967 when outbreaks occurred simultaneously in laboratory workers in Marburg, Germany; Frankfort, Germany; and Belgrade, Yugoslavia (now Serbia). These outbreaks were related to contact with African green monkeys that were imported from Uganda.
The incidence of filovirus infections (with Ebola virus or Marburg virus) continues to be relatively low. As of 2008, these viruses combined have caused approximately 2,317 clinical cases and 1,671 confirmed deaths (Leroy 2011).
Since the initial outbreaks, several additional cases and outbreaks have been reported; all have occurred among persons living or traveling in rural southern and eastern Africa; recognized cases and outbreaks are shown in the table below (CDC 2009: Known cases and outbreaks of Marburg hemorrhagic fever, in chronological order).
Recognized Outbreaks of Marburg Hemorrhagic Fever, in Chronological Ordera
Frankfort, Germany, and Belgrade, Yugoslavia
—Cases occurred in laboratory workers who had been exposed to African green monkeys or their tissues; the monkeys originally were from Uganda.
1 index case and 2 secondary cases occurred; the index patient had been traveling extensively in Zimbabwe before illness onset; 1 death occurred (CFR 33%).
1 index case and a secondary infection in a healthcare worker; 1 death occurred (CFR 50%).
A single case occurred in a man who had been traveling extensively in Kenya (he died).
—Most cases occurred in young male miners who worked in an underground mine.
Angola (predominantly in the northern Uige Province)
227 of the cases died (CFR 90%).
All cases were young males who worked in a mine; 2 workers died (CFR 50%).
United States (exposure in Uganda)
The case occurred in a traveler returning from Uganda. He had visited a cave in Maramagambo forest at the southern edge of Queen Elizabeth National Park. The likely source of exposure was bats in the cave. The case survived.
Netherlands (exposure in Uganda)
This case also had visited the same cave in Maramagambo forest during a trip to Uganda; she died.
Lassa fever is a disease that has become endemic in West Africa over the past 30 years. Rodents are the primary reservoir for Lassa virus, and the disease has a seasonal pattern. Case-fatality rates are somewhat lower for Lassa fever than for Ebola and Marburg virus infections, although they can be as high as 15% to 25% among hospitalized patients. Ribavirin therapy has been shown to be efficacious in treating some cases.
- Lassa fever was first recognized in 1969 in northern Nigeria, when a small outbreak occurred among several nurses working in a rural missionary hospital.
- Subsequent outbreaks have been recognized in Nigeria, Sierra Leone, and Liberia, and the disease is endemic in areas of West Africa.
- Although infections can occur year-round, the incidence of disease is highest during the dry season (LeDuc 1989). This finding may be related to aggregation of rodents inside houses during the dry season because of limited food supplies outdoors (Fichet-Calvet 2007).
- An estimated 100,000 to 300,000 Lassa fever virus infections occur annually in West Africa (McCormick 1987: A prospective study of the epidemiology and ecology of Lassa fever).
- In a cross-sectional seroprevalence study conducted in the tropical forest area of Guinea, West Africa, the prevalence of Lassa virus antibodies was estimated at 12.9% and 10.0% in rural and urban areas, respectively (Kerneis 2009).
- Occasionally, cases are imported from Africa into other countries (Isaacson 2001, Macher 2006). One report documented 24 cases of imported Lassa fever through 2004, with cases occurring in Europe, the United States, Canada, Israel, and Japan. A Lassa fever case occurred in 2010 in a US traveler who had visited rural Liberia and had become ill while in that country (Amorosa 2010). He sought medical care in Pennsylvania upon returning to the United States, where his illness was confirmed as Lassa fever. The patient recovered with supportive therapy, and no secondary cases were identified.
- An investigation into a case of imported Lassa fever in Germany demonstrated that the risk of transmission following exposure to the index case was low. Thirty persons were identified who had high-risk contact with the index case and serologic evidence of infection developed in only one of them. This secondary case was a physician who cared for the patient on day 9 of her illness; the physician received ribavirin and remained asymptomatic (Haas 2003). Similarly, investigation of a case of imported Lassa fever in New Jersey did not identify transmission of the virus to five high-risk contacts (all family members) (CDC 2004: Imported Lassa fever—New Jersey).
New World hemorrhagic fever is caused by several different arenaviruses. Most cases have occurred in regional areas of South America, although Whitewater Arroyo virus was identified in recent years as a cause of VHF in California. These viruses appear to be transmitted via contact with rodents or rodent excreta. New World hemorrhagic fever is relatively uncommon and for some viruses (eg, Sabia and Whitewater Arroyo virus) only a handful of cases have been recognized to date.
New World arenaviruses that cause disease in humans include the following (Charrel 2003):
- Junin virus (Argentine hemorrhagic fever)
- This virus was first recognized in 1955.
- The disease was initially localized to rural populations in the northwestern region of Buenos Aires province.
- In the 1980s, infection became endemic in several provinces of Argentina (eg, Buenos Aires, Santa Fe, Cordoba, La Pampa) (WHO 1985: Viral haemorrhagic fevers: report of a WHO expert committee). These provinces are all located in the northeastern part of the country and are contiguous to each other.
- Traditionally, between 200 and 2,000 cases were reported annually; however, in 1993, 24,000 cases were reported (Lacy 1996). Vaccination of the local population has dramatically decreased the incidence of disease (Peters 2010: Lymphocytic choriomeningitis, Lassa virus, and South American hemorrhagic fevers).
- Disease occurrence is seasonal and peaks during the months March through June (corresponding to the corn harvest season).
- Machupo virus (Bolivian hemorrhagic fever)
- The disease was first described in 1959, and the etiologic agent was identified in 1965.
- Bolivian hemorrhagic fever is restricted to the El Beni district in northeastern Bolivia.
- Between 1959 and 1962, 470 cases resulting in 142 deaths were reported (CDC 1994: Bolivian hemorrhagic fever—El Beni department, Bolivia).
- No outbreaks were recognized between 1971 and 1994. In the summer of 1994 an outbreak involving 10 people occurred in northeastern Bolivia (CDC 1994: Bolivian hemorrhagic fever—El Beni department, Bolivia).
- More than 200 cases occurred in 2008 (Aguilar 2009).
- Small outbreaks tend to occur, with several years of quiescence between outbreaks.
- Guanarito virus (Venezuelan hemorrhagic fever)
- This virus was first recognized in 1989 when an outbreak involving more than 100 cases occurred in the Portuguesa state of central Venezuela (Salas 1991).
- Sabia virus (Brazilian hemorrhagic fever)
- This virus was first recognized in Brazil in 1990 when a single fatal case occurred (Lisieux 1994).
- Two additional infections have been identified; both were laboratory-acquired (one in Brazil and one in the United States) (Armstrong 1999, Barry 1995, Lisieux 1994).
- Chapare virus
- A new arenavirus was discovered in 2004. The virus, referred to as Chapare virus, is responsible for some cases of VHF in Bolivia that have occurred outside the epidemic area for Machupo virus.
- Genomic analysis of viruses from clinically ill patients revealed that Chapare virus is most closely related to Sabia virus (Delgado 2008).
- Whitewater Arroyo virus
- Three cases were reported from California from June 1999 to May 2000 (CDC 2000: Fatal illnesses associated with a New World arenavirus); two patients lived in southern California and one lived in the San Francisco Bay area.
- All three cases were fatal.
- These case reports suggest that Whitewater Arroyo virus can cause VHF in humans; however, documentation of additional cases would support these initial findings.
- Serum samples from patients with acute central nervous system disease or undifferentiated febrile illnesses that were submitted to the Arbovirus Diseases Branch, Division of Vector-Borne Infectious Diseases, CDC, from 1989 to 2000 demonstrated that two of 1,185 specimens had a fourfold rise in IgG antibody titers to Whitewater Arroyo virus, suggesting that this virus was the cause of illness (Milazzo 2011).
Rift Valley fever is a mosquitoborne disease primarily found in sub-Saharan and North Africa. The disease, which affects livestock (eg, cattle, sheep) and humans, was first recognized in animals in 1930 in Kenya (Daubney 1931). Epizootics in animals characteristically involve high rates of prenatal mortality and spontaneous abortions. Human illnesses generally are mild, although severe forms of disease (eg, VHF, meningoencephalitis) occur in about 1% of cases, and retinitis can occur in up to 10% of cases.
- Mosquito eggs infected with Rift Valley fever virus can remain dormant but viable in soil for years while awaiting heavy rains for hatching.
- Infections in ruminants can amplify the viral burden in an area, and seasonal movement of ruminants may enhance spread of Rift Valley Fever virus to previously uninfected areas (Chevalier 2005).
- Analysis of an outbreak in 2003 revealed that East/Central African strains were present in West Africa and may have flourished because of increased rainfall (Faye 2007).
- A major epidemic involving thousands of livestock cases and 18,000 human cases with approximately 600 deaths occurred in Egypt in 1977 (Meegan 1979). Additional outbreaks in Egypt have been reported (probably representing repeated introductions of the virus).
- Another large outbreak involving thousands of cases occurred in Somalia and Kenya in 1997 and 1998 (Woods 2002).
- More recently, a series of outbreaks occurred in 2006 and 2007 in several countries in East Africa (Kenya, Somalia, and Tanzania) (CDC 2007: Rift Valley fever outbreak—Kenya, November 2006–January 2007; Hassan 2011; Jost 2010; Mohamed 2010; Nderitu 2011). Analysis of specimens from animals in Kenya indicated that multiple viral lineages were responsible for the outbreak in that country (Bird 2008).
- Recent outbreaks also have occurred in Madagascar (2008 and 2009), Mauritania (2010), and South Africa (2010) (Andriamandimby 2010; El Mamy 2011; WHO 2010: Rift Valley fever, South Africa).
- Until 2000, the disease had been identified only in sub-Saharan and North Africa. However, in the fall of 2000, outbreaks occurred simultaneously in Yemen and Saudi Arabia, thought to have been introduced from Africa through the sheep trade (CDC 2000: Outbreak of Rift Valley fever—Yemen; CDC 2000: Outbreak of Rift Valley fever—Saudi Arabia; Shoemaker 2002). More than 1,000 cases occurred in Yemen, and 886 cases were identified in Saudi Arabia (Madani 2003).
- In 2007 and 2009, experts met to address possible introduction of the virus into the United States, as was seen with West Nile virus in 1999 (Britch 2007; Hartley 2011). Pathway analyses have identified ways in which Rift Valley fever might be introduced into the United States. Such information can help in developing an effective targeted surveillance plan for rapid detection and response. Possible mechanisms of introduction include importation of infected animals, entry of Rift Valley fever–infected people, mechanical transport of insect vectors, and smuggling of live virus (Kasari 2008).
Outbreaks of yellow fever were first recognized in the 1600s, and the disease is now endemic in sub-Saharan Africa (between 15°N and 10°S latitude) and in tropical regions of South America. The vectors for yellow fever virus include several mosquito species. Illness can range from mild to severe, with an overall case-fatality rate of 5% to 7%. Vaccines against yellow fever are available.
The WHO estimates that 200,000 cases of yellow fever, causing 30,000 deaths, occur worldwide each year (WHO 2011: Yellow fever). According to the WHO, the incidence of yellow fever has increased over the past two decades owing to declining population immunity to infection, deforestation, urbanization, population movements, and climate change.
- Three cycles have been recognized (WHO 2011: Yellow fever):
- A sylvatic or jungle cycle that primarily involves transmission between mosquitoes and nonhuman primates, with humans as incidental hosts.
- An urban cycle that involves transmission between mosquitoes and humans in urban areas. The most important mosquito vector is Aedes aegypti. Urban outbreaks can involve hundreds (or even thousands) of people and pose a substantial public health threat.
- An intermediate cycle that is found in villages in humid and semi-humid savannahs of Africa, where small epidemics occur. This form involves semi-domestic mosquitoes that can infect both humans and nonhuman primates.
- Outbreaks of urban yellow fever have occurred only rarely in the Americas since the mid-1900s owing to public health programs aimed at eliminating the mosquito vector; however, reinfestation of urban areas with A aegypti mosquitoes has raised concern about the re-emergence of urban yellow fever. This concern was heightened following the occurrence of an outbreak of urban yellow fever in Paraguay in 2008 (Vaughn 2010).
- In South America, most sylvatic yellow fever cases involve workers who spend time in the forested areas of Bolivia, Brazil, Ecuador, Colombia, and Peru.
- In Africa, sylvatic and occasional urban outbreaks occur (WHO: Yellow fever: situation updates). Outbreaks primarily occur in West, Central, and East Africa, with the largest number of outbreaks reported in West Africa. A relatively large outbreak occurred in southern Sudan in 2003 (Onyango 2004).
- Yellow fever is occasionally exported to Europe or North America (most often because of failure to vaccinate travelers to endemic areas).
The geographical risk assessment for yellow fever was updated in 2010 by the Informal WHO Working Group on Geographic Risk for Yellow Fever (Jentes 2011; WHO 2011: Revised recommendations for yellow fever vaccination for international travellers, 2011). This revised assessment takes into account recent trends in yellow fever occurrence, such as re-emergence of the disease in 2007 and 2008 in Paraguay and Argentina after more than 30 years and increased incidence of yellow fever in a number of countries in Central Africa.
Kyasanur forest disease, which is a tick-borne infection, is relatively rare and found only in one region in the southwestern part of India (Gritsun 2003, Pattnaik 2006). The major clinical manifestations are VHF or meningoencephalitis, and the case-fatality rate is 3% to 5%. Key features of the virus and the disease are: (CDC: Kyasanur forest disease: fact sheet)
Kyasanur forest disease virus was first recognized in 1957 when it was isolated from a sick monkey from the Kyasanur forest in Karnataka State, India.
- Rodents, monkeys, bats, and other small mammals are the natural reservoirs. Larger animals (such as goats, cows, and sheep) may become infected, but they don't have a role in transmission of the disease.
- Natural infections have been identified only in several districts of Karnataka State, India (located in the southwestern region of the country). Outbreaks occur periodically and are usually signaled by epizootics in the local monkey population.
- A seasonal pattern has been noted, with most of the cases occurring in the spring months.
- A variant of Kyasanur forest disease virus (referred to as the Alkhumra virus) was identified in Saudi Arabia in 1995 (Madani 2005). Since 1995, additional outbreaks and cases have been recognized in Saudi Arabia (Madini 2011).
- Another variant of Kyasanur forest disease virus (referred to as the Nanjianyin virus) was isolated in 1989 from the serum of a patient in Yunnan province, China. Results of a serosurvey conducted in Yunnan between 1987 and 1990 demonstrated that 169 (19.5%) of 867 healthy residents of western Yunnan province and 6 (3.7%) of 161 healthy residents of northwestern Yunnan province carried antibodies against this virus (Wang 2009).
Omsk hemorrhagic fever also is a rare form of tick-borne VHF and is limited to regions of Central Asia (CDC: Omsk hemorrhagic fever: fact sheet; Gritsun 2003; Ruzek 2010). Key epidemiologic features of the virus and the disease are:
- The disease was first recognized in the early 1940s in Omsk, Russia.
- Two relatively large outbreaks occurred in the 1940s: one in 1945 (involving at least 200 cases) and one in 1946 (involving about 600 cases) (WHO 1985: Viral haemorrhagic fevers: report of a WHO expert committee).
- From 1946 to 1958, 972 cases of Omsk hemorrhagic fever were officially recorded. After 1960, the incidence decreased substantially, and since that time Omsk hemorrhagic fever has been recognized infrequently. From 1988 to 1997, 165 cases were reported; 10 were associated with tick bites and 155 were in muskrat hunters and poachers. Small outbreaks were recognized in 1990 (29 cases) and 1991 (38 cases).Omsk hemorrhagic fever occurs in the western Siberia regions of Omsk, Novosibirsk, Kurgan, and Tyumen.
- Muskrats and water voles appear to be the main animal reservoirs.
- Although illness is acquired predominantly through the bite of an infected tick, exposure to muskrats (eg, through skinning or contact with blood, feces, or urine of an infected animal) also has been shown to be a mode of transmission.
- Omsk hemorrhagic fever virus can be transmitted through the milk of infected goats or sheep and has been isolated from aquatic animals and water, suggesting that the virus is relatively stable in the environment.
The modes of transmission and reservoirs vary somewhat by agent; these features are outlined in the following table.
For those viruses that are transmitted person to person, the period of communicability apparently begins after onset of symptoms. Transmission during the incubation period has not been demonstrated. In one instance, Ebola virus transmission may have occurred from an infected patient several hours before actual onset of symptoms (WHO 1978: Ebola haemorrhagic fever in Zaire), but later systematic studies of the same disease demonstrated that the greatest risk for transmission is late in the clinical course (Dowell 1999).
Risk of transmission from fomites in an isolation ward and from convalescent patients is low when healthcare workers follow recommended infection control guidelines for VHF (Bausch 2007: Assessment of risk).
For both filoviral and arenaviral infections, virus may persist in urine and seminal fluids for several months; therefore, patients infected with these agents should refrain from sexual activity for 3 months after clinical recovery (Borio 2002).
Reservoirs and Modes of Transmission for Selected Hemorrhagic Fever Virusesa
Modes of Transmission
Fruit bats (various species); possibly other reservoirs
—Person to person (most likely through contact with blood or body fluids)
—Direct contact with infected pigs (Ebola-Reston strain only)
(fruit bat species); possibly other reservoirs
—Contact with blood, tissues, or tissue cultures from infected monkeys
—Contact with infected bats or bat secretions (noted in confined spaces such as caves or mines in Africa)
Mastomys species (multimammate mice)
—Predominantly airborne through virus-containing aerosols of rodent excreta
New World arenaviruses:
Calomys musculinus (drylands vesper mouse)
Predominantly airborne through virus-containing aerosols of rodent excreta
Calomys callosus (large vesper mouse)
—Predominantly airborne through virus-containing aerosols of rodent excreta
Zygodontomys brevicauda (cane mouse)
Unknown, but presumably through aerosolized rodent excreta
Not known; presumably a rodent
Unknown, although laboratory-acquired cases appear to have been contracted through aerosols
Not known, presumably a rodent
Neotoma species (woodrats)
Unknown, but presumably airborne through aerosolized rodent excreta
Rift Valley fever virusb,j
Ruminants (eg, cattle, sheep, and possibly wild ruminants), rats in some areas (eg, Egypt)
—Bite of infected mosquito
—Possible consumption of raw milk from infected animals
Yellow fever virusb,k
Predominantly Aedes and Haemagogus mosquito species; A aegypti is most important vector for urban yellow fever
—Bite of infected mosquito
—Vertical transmission from mother to infant
Kyasanur forest disease virusb,k
Rodents, bats, and other small mammals (eg, striped forest squirrel, house shrew); monkeys (eg, black-faced langur, South Indian bonnet macaque) appear to be amplifying hosts
Ticks (Haemaphysalis spinigera)
—Bite of infected tick
Omsk hemorrhagic fever virusb,k
Rodents (including muskrats and voles)
Ticks (Dermacentor pictus, D reticulatus)
—Bite of infected tick
Animal studies using nonhuman primates have demonstrated that clinical infection can be caused by aerosolized preparations of some hemorrhagic fever viruses, including Ebola, Marburg, Lassa, New World arenaviruses, and yellow fever viruses (Alves 2010, Johnson 1995, Kenyon 1992, Reed 2011, Stephenson 1984). Additional viruses (Rift Valley fever virus and flaviviruses) have been shown to cause aerosol infections in the laboratory setting (Banerjee 1979, Smithburn 1949). These viruses are considered potentially suitable as biological weapons (Borio 2002, Bray 2003) because:
- They can be disseminated through aerosols.
- They have a low infectious dose.
- They cause high morbidity and mortality.
- They cause fear and panic in the general public.
- Effective vaccines are not available or supplies are limited.
- These pathogens are available and most can be readily produced in large quantities.
- Research on weaponizing various hemorrhagic fever viruses has been conducted in the past despite the lack of treatment options or protective vaccines.
Examples of countries that have either weaponized hemorrhagic fever viruses or conducted biological weapons research on these viruses include the following (MIIS):
- The Soviet Union produced weaponized Marburg virus and conducted research on Ebola, Lassa, Rift Valley fever, and yellow fever viruses and New World arenaviruses.
- The United States conducted biological weapons research on Lassa, Rift Valley fever, and yellow fever viruses and New World arenaviruses.
- North Korea may have weaponized yellow fever virus.
In 2000, the CDC published a list of Category A agents (ie, those that are most likely to cause mass casualties if deliberately disseminated, can be released as small aerosols, and require broad-based public health preparedness). The list included New World arenaviruses and Ebola, Marburg, and Lassa viruses (CDC 2000:Biological and chemical terrorism).
According to the Working Group on Civilian Biodefense, hemorrhagic fever viruses that pose serious threats as potential biological weapons include the following (Borio 2002):
- Ebola virus
- Marburg virus
- Lassa virus
- New World arenaviruses
- Machupo (Bolivian hemorrhagic fever)
- Junin (Argentine hemorrhagic fever)
- Guanarito (Venezuelan hemorrhagic fever)
- Sabia (Brazilian hemorrhagic fever)
- Rift Valley fever virus
- Yellow fever virus
- Kyasanur forest disease virus
- Omsk hemorrhagic fever virus
Several other hemorrhagic fever viruses have been identified as human pathogens; examples include Chapare virus (a New World arenavirus found in Bolivia), Whitewater Arroyo virus (a New World arenavirus found in the western United States), Alkhumra virus (a variant of Kyasanur forest disease virus found in Saudi Arabia), and Nanjianyin virus (a variant of Kyasanur forest disease virus found in Yunnan province, China). The role of these viruses as potential bioterrorism agents is unknown.
The working group determined that several important hemorrhagic fever viruses are less likely than those mentioned above to be used as biological weapons. These agents are not discussed further in this overview; they include:
- Dengue virus (is not transmissible by small-particle aerosol, requires mosquito-vector transmission, and primary dengue infection only rarely causes hemorrhagic fever)
- Crimean-Congo hemorrhagic fever virus (does not replicate to high concentrations in currently available systems [a barrier to mass production])
- Hantaviruses (do not replicate to high concentrations in currently available systems)
Although clinical features vary somewhat for the various hemorrhagic fever viruses, the clinical presentations overlap substantially. All of the agents cause a febrile prodrome associated with varying degrees of prostration; other notable features include the following.
- Bleeding manifestations occur in variable proportions of patients (eg, in about 30% of patients with Ebola or Marburg hemorrhagic fever and in only about 1% of patients with Rift Valley fever).
- A maculopapular rash may be noted early in the clinical course in some forms of VHF (notably in Ebola and Marburg hemorrhagic fevers).
- Severe exudative pharyngitis is a characteristic early feature of Lassa fever.
- Several agents cause meningoencephalitis in addition to VHF (eg, Rift Valley fever, Kyasanur forest disease, Omsk hemorrhagic fever viruses).
- Jaundice may be a prominent feature in some infections (eg, Ebola and Marburg hemorrhagic fevers, Lassa fever, Rift Valley fever, yellow fever).
Major clinical features for each disease are included in the tables below.
Clinical Features of Ebola Hemorrhagic Fever
The most reliable information suggests 3-13 days, although some reports indicate a range of 2-21 days.
—Abrupt onset of fever, severe prostration, headache, myalgias is typical.
—Maculopapular rash (predominantly on trunk) about 5 days after illness onset occurs in about 50% of patients.
—Leukopenia occurs early in clinical course; leukocytosis may occur later.
Complications (generally occur at least 2 wk after illness onset)
—Varies by virus subtype:
Clinical Features of Marburg Hemorrhagic Fever
2-21 days (average 5-9 days)
—Abrupt onset of fever, severe prostration, headache, myalgias is typical, but the patient may present with an influenza-like illness.
—Maculopapular rash occurs on the 5th to 7th day (trunk, face, neck, proximal regions of extremities) and is nonpruritic.
—Leukopenia occurs early in clinical course (1,000-2,000/mm3); leukocytosis may occur later.
Complications (generally occur at least 2 wk after illness onset)
Varies by outbreak (23%-93%)
Clinical Features of Lassa Fever
—Illness begins gradually with fever, weakness, generalized malaise.
—Most Lassa virus infections in Africa are mild or subclinical; severe multisystem disease occurs in only 5%-10% of infections.
—Leukocyte counts occasionally are decreased but most often are normal or moderately increased.
Complications (generally occur in 2nd and 3rd wk of illness)
—8th cranial nerve damage with hearing loss (may improve or may result in permanent hearing loss)
—Overall mortality (including nonhospitalized patients), 1%-2%
Clinical Features of New World Hemorrhagic Fever
7-12 days (range 5-19 days) following natural exposure
Gradual onset of fever, sore throat, myalgias, low back pain, abdominal pain
—Common early findings include:
—Transient alopecia and nail furrows may occur.
—Junin (Argentine hemorrhagic fever), 15%-30%
Clinical Features of Rift Valley Fever
Fever, headache, photophobia, retro-orbital pain
—Subclinical infection is common.
—Initial leukocytosis may occur, followed by leukopenia.
—Blindness following retinitis
—In 2000 outbreak in Saudi Arabia, 17% among symptomatic patients and 33.3% among hospitalized patients admitted to RVF unit at local referral hospital
Clinical Features of Yellow Fever
3-6 days (median, 4.3 days)
—Fever, headache, myalgias, facial flushing, conjunctival injection, relative bradycardia (Faget's sign)
—Subclinical infection is common (5%-50%).
—Leukopenia occurs early in clinical course; leukocytosis may occur later.
Clinical Features of Kyasanur Forest Disease
2-9 days (usually 3-8 days)
Sudden onset of fever, myalgias, headache
—Diarrhea and vomiting occur by 3rd or 4th day.
—Leukopenia (average: 2,000/mm3)
Iridokeratitis has been reported in survivors.
Clinical Features of Omsk Hemorrhagic Fever
2-9 days (usually 3-8 days)
Fever, headache, vomiting, enanthem on palate, hyperemia of skin on upper body and mucous membranes
—Initial febrile illness lasting 5-12 days occurs, followed by second phase several days later in 30%-50% of patients that is often more severe.
Transient alopecia may occur
In general, the clinical manifestations of VHF are similar in children and adults. One review of 33 pediatric cases of Lassa fever identified the following clinical presentations on the basis of age (Monson 1987):
- Fetal infection (18 pregnancies):
- Spontaneous abortion following onset of infection occurred in 16 cases (one mother later died).
- Fetal death occurred in 2 cases; spontaneous abortion appeared likely in both.
- All pregnancies terminated between the 3rd and 9th months.
- Congenital infection (1 case):
- One child died of Lassa fever 4 days after birth; the mother denied any recent febrile illness at the time of the child's death, but she was lost to follow-up and could not be reached when the positive culture was reported.
- Nursing infants (7 cases):
- Presenting symptoms included fever, vomiting, seizures, pneumonia, edema, anorexia, cough, diarrhea, irritability, stomatitis, obtundation, bleeding, lethargy, and abdominal tenderness and distention.
- Leukocyte counts ranged from 3,600/mm3 to 20,000/mm3.
- The case-fatality rate was 29%.
- Children over 2 years of age (7 cases):
- Presenting symptoms included fever, cough, edema, bleeding, abdominal pain, vomiting, seizures, sore throat, conjunctivitis, and obtundation.
- Leukocyte counts ranged from 3,000/mm3 to 42,000/mm3.
- The case-fatality rate was 14%.
- "Swollen baby syndrome" (4 cases):
- Four children (aged 4 days, 6 weeks, 7 months, and 9 years) presented with the triad of edema, abdominal distention, and bleeding, which investigators referred to as "swollen baby syndrome."
- Three of the four children died.
A review of 15 pregnant women with Ebola hemorrhagic fever demonstrated high rates of fetal loss (similar to those noted above for Lassa fever) and also demonstrated a high case-fatality rate for pregnant women (Mupapa 1999):
- Spontaneous abortion occurred in 10 women
- All the mothers presented with severe bleeding; all but 1 subsequently died.
- One woman delivered a stillborn infant and subsequently died.
- One woman delivered a full-term baby; the infant died 3 days after birth and the mother died from severe postpartum hemorrhage.
- Three additional women died in their third trimester of pregnancy.
A wide range of conditions (bacterial, viral, and parasitic infections as well as noninfectious causes) should be considered in the differential diagnosis of VHF. However, most of these conditions do not cause bleeding manifestations as a primary feature and most are not likely to occur epidemiologically as a point-source epidemic with simultaneous presentation of many cases. Primary agents to consider in the differential diagnosis are outlined in the table below.
Differential Diagnosis for Viral Hemorrhagic Fevers That Pose a Bioterrorist Threata,b,c
Bacterial and Rickettsial Infections
Septicemia caused by Gram-negative bacteria
Underlying illness is usually present.
Staphylococcal or streptococcal toxic shock syndrome
—Streptococcal TSS may be associated with necrotizing fasciitis.
Rapid progression to shock and often death may occur.
—Maculopapular rash that begins on the trunk (palms and soles often involved) is characteristic.
Often occurs secondary to bubonic plague (characteristic bubo present in groin, axilla, or cervical region).
—Symptoms of enterocolitis and abdominal pain may be more prominent with typhoid fever than with VHF.
Rocky Mountain spotted fever
—A history of tick exposure may be obtained.
—A history of tick exposure may be obtained.
—This is most often self-limited but may be severe in about 10% of patients.
—Respiratory symptoms are prominent.
—In late-onset form, a pustular rash usually is present before hemorrhagic manifestations.
—Presenting features usually include cough, coryza, and conjunctivitis.
—Occurs in persons without history of rubella vaccination (such as migrant workers).
—Usually occurs in immunocompromised children.
Usually hepatitis A, B, C viruses (hepatitis E and G viruses and other viruses less commonly)
—Hepatic findings predominate.
—Fever is cyclic (every 48 hr for P vivax or P ovale; every 72 hr for P malariae) or continuous with intermittent spikes (most common pattern for P falciparum).
Trypanosoma brucei complex
—Painful chancre may occur at site of tsetse fly bite.
Acute Conditions That May Be Associated With a Bleeding Diathesis
Hemolytic uremic syndrome
Usually occurs as complication of infection with Escherichia coli O157:H7 or other Shiga toxin–producing E coli
—Disease involves a triad of renal involvement, thrombocytopenia, and hemolytic anemia.
Thrombotic thrombocytopenic purpura
May occur as complication of infection with E coli O157:H7 or other Shiga toxin–producing E coli, although may be noninfectious
—Disease includes renal involvement, thrombocytopenia, hemolytic anemia, neurologic involvement.
Idiopathic thrombocytopenic purpura
—Low platelet count is predominant feature.
Peripheral blood smear shows characteristic features of leukemia.
Collagen vascular disease
Acute onset of febrile illness is not likely.
Most clinicians in the United States have little or no clinical experience with the syndromes that characterize VHF; therefore, a high index of suspicion is needed to make an accurate diagnosis.
The diagnosis of VHF should be considered for any patient who presents with:
- Acute onset of fever (less than 3 weeks' duration)
- Severe prostrating or life-threatening illness
- Bleeding manifestations (ie, at least two of the following: hemorrhagic or purpuric rash, petechiae [particularly in nondependent areas], epistaxis, hematemesis, hemoptysis, blood in stool, or other evidence of bleeding)
- No predisposing factors for a bleeding diathesis
In naturally occurring cases, an appropriate travel or exposure history will usually be present. In the setting of a bioterrorist event, such a history will not be present and multiple patients will likely present simultaneously.
The American Society for Microbiology (ASM), in collaboration with the CDC and the Association of Public Health Laboratories (APHL), has developed sentinel laboratory guidelines to provide information regarding the diagnosis of microorganisms that could be agents of bioterrorism (ASM: Sentinel level clinical microbiology laboratory guidelines).
According to the sentinel laboratory guidelines for unknown viruses, clinical laboratories should consult public health authorities before collection of specimens for diagnostic testing (ASM 2003).
The CDC recommends that patients with suspected VHF have paired acute blood specimens (ideally collected during days 0 to 4 and days 4 to 9 of the acute illness) tested at a World Reference Laboratory (eg, the CDC) with biosafety level 4 capability using multiple methods as appropriate for the timing of the sample, including virus isolation, reverse transcription PCR (RT-PCR), and IgM and IgG ELISA (CDC 2009: Imported case of Marburg hemorrhagic fever).
Guidelines have been published for packing and shipping of infectious substances, diagnostic specimens, and biological agents from suspected acts of bioterrorism (ASM 2011).
- Laboratory personnel are at high risk for infection with hemorrhagic fever viruses, and laboratory-acquired infections have been documented following exposure to a number of different agents (eg, some New World arenaviruses, and Ebola, Marburg, Lassa, Kyasanur Forest disease, and Rift Valley fever viruses) (Armstrong 1999; Banerjee 1979; Borio 2002; CDC: Biosafety in microbiological and biomedical laboratories; CDC 1994: Bolivian hemorrhagic fever—El Beni department, Bolivia; Smithburn 1949). This is of particular concern for arenaviruses, since all of them are infectious as aerosols (Peters 2010: Lymphocytic choriomeningitis virus, Lassa virus, and the South American hemorrhagic fevers).
- Hazards in the laboratory setting include (CDC: Biosafety in microbiological and biomedical laboratories):
- Respiratory exposure to infectious aerosols (such as those generated through centrifugation)
- Mucous membrane exposure to infectious droplets
- Accidental parenteral inoculation
- A report from the US Army Medical Research Institute of Infectious Diseases (USAMRIID) described a situation whereby a research scientist was potentially exposed to Ebola virus via a needlestick injury in 2004 (Kortepeter 2008). The report discusses approaches for confining a potentially exposed patient in a medical containment suite. USAMRIID has suggested a stepwise approach for handling such incidents that includes contacting appropriate experts, determining sufficient infection control measures, and addressing isolation logistics.
- To minimize risk to laboratory personnel, all laboratory staff should be alerted, if possible, to the potential diagnosis of VHF and designated laboratory workers should receive training in handling specimens from such patients in advance.
- Specimens from patients with suspected VHF should be referred to an LRN national laboratory (eg, the CDC) for diagnostic testing under appropriate BSL conditions.
- Point-of-care analyzers should be used, if available, for bedside analysis of blood gases and critical values such as electrolytes to minimize risks to laboratory workers (Borio 2002, Chance 2000, Lindemans 1999).
- If point-of-care analyzers are not available, then efforts should be made to limit potential exposures of laboratory personnel in the hospital clinical laboratory (Armstrong 1999, Borio 2002):
- Laboratory testing should be limited to critical diagnostic tests only.
- Laboratory specimens should be clearly identified, double-bagged, and hand-carried to the laboratory at prescheduled times.
- Specimens should not be transported in pneumatic tube systems.
- The number of laboratory technicians handling specimens from patients with VHF should be limited to dedicated trained personnel.
- Serum samples should be pretreated with Triton X-100 (10 mcL of 10% Triton X-100 per 1 mL of serum for 1 hour) to reduce the risk of accidental exposure, although the efficacy of this procedure has not been demonstrated.
- Heating of samples at 60°C for 1 hour will virtually inactivate infectivity but will still allow measurement of electrolytes, creatinine, and other heat-stable markers (Jahrling 1999). Routine procedures with automated analyzers may be conducted, but analyzers should be disinfected with a 1:100 bleach solution or as recommended by the manufacturer (CDC 1995: Update: Management of patients with suspected viral hemorrhagic fever—United States).
- Blood smears for malaria are considered noninfectious for hemorrhagic fever viruses after solvent fixation (CDC 1995: Update: Management of patients with suspected viral hemorrhagic fever).
- Laboratory workers in hospital clinical laboratories should wear appropriate personal protective equipment (PPE) (see Infection Control: Isolation Precautions) when handling specimens from patients suspected of having VHF. All specimens should be handled at a minimum in a class 2 biological safety cabinet following BSL-3 practices (Borio 2002).
- The hemorrhagic fever viruses included in this report (except for yellow fever virus) are classified as select agents and therefore are regulated under 42 CFR part 73 (Possession, Use, and Transfer of Select Agents and Toxins), which was published in final form in the Federal Register in March 2005 (HHS 2005). As specified in the Public Health Security and Bioterrorism Preparedness and Response Act of 2002, 42 CFR part 73 provides requirements for laboratories that handle select agents (including registration, security risk assessments, safety plans, security plans, emergency response plans, training, transfers, record keeping, inspections, and notifications). Select agents are biological agents designated by the US government to be major threats to public health and safety. A current list of select agents is published on the CDC Web site under information about the Select Agent Program (CDC/APHIS 2011). In addition, the CDC has published additional guidelines for enhancing laboratory security for laboratories working with select agents (CDC 2002: Laboratory security and emergency response guidance for laboratories working with select agents).
- Most hemorrhagic fever viruses are classified as WHO risk group 4. Specimens and isolates that are reasonably suspected to contain a hemorrhagic fever virus must be transported as "infectious substances." The US Department of Transportation (DOT) regulations and International Air Transport Association (IATA) rules require training of all individuals involved in the transport of dangerous goods, including infectious substances (DOT and IATA 2012).
The Laboratory Response Network (LRN) is a national network of approximately 150 laboratories. It includes the following types of labs: federal, state and local public health, military, food testing, environmental, veterinary, and international (located in Canada, the United Kingdom, Australia, Mexico, and South Korea) (CDC: Facts about the Laboratory Response Network).
The LRN structure for bioterrorism designates laboratories as sentinel, reference, or national. Designation depends on the types of tests a laboratory can perform and how it handles infectious agents to protect workers and the public.
- Sentinel laboratories, formally called "level A laboratories,"represent an estimated 25,000 hospital-based laboratories that have direct contact with patients. In an unannounced or covert terrorist attack, sentinel laboratories could be the first facilities to encounter suspicious specimens. These laboratories generally have at least BSL-2 containment capabilities. A sentinel laboratory's responsibility is to rule out conditions of interest or refer suspicious samples to the nearest LRN reference laboratory. Sentinel laboratories use the ASM Sentinel Level Clinical Microbiology Laboratory Guidelines to rule out microorganisms that might be suspected as agents of bioterrorism (ASM).
- Reference laboratories, sometimes referred to as "confirmatory reference," can perform tests to detect and confirm the presence of a threat agent. These laboratories ensure a timely local response in the event of a terrorist incident. Rather than having to rely on confirmation from laboratories at the CDC, reference laboratories are capable of producing conclusive results; this allows local authorities to respond quickly to emergencies. These are mostly state or local public health laboratories but also include military, international, veterinary, agriculture, and food- and water-testing laboratories. Reference laboratories operate with BSL-3 containment facilities that have been given access to nonpublic testing protocols and reagents. One of the roles of the LRN reference laboratories is to provide guidance, training, outreach, and communications to the sentinel laboratories in their jurisdictions.
- National laboratories have unique resources to handle highly infectious agents and the ability to identify specific agent strains through molecular characterization methods. These laboratories also are responsible for methods development, bioforensics, and select-agent activity.
The following factors are important in terms of LRN testing for VHF:
- Most of the hemorrhagic fever viruses currently require testing under BSL-4 conditions (CDC: Biosafety in microbiological and biomedical laboratories).
- BSL-4 agents are considered dangerous/exotic agents which pose high risk of life-threatening disease, aerosol-transmitted lab infections, or related agents with unknown risk of transmission.
- Yellow fever and Rift Valley fever viruses can be tested under BSL-3 conditions, which are available at many of the reference laboratories in the LRN. Laboratory personnel should be vaccinated as appropriate and additional lab engineering controls, such as high-efficiency particulate air (HEPA)–filtered exhaust air, should be in place. In general, specialized tests for these viruses generally are only available at BSL-4 (LRN national) laboratories.
- Although testing for hemorrhagic fever viruses is limited to LRN national laboratories, the LRN has an established communication network that allows for rapid response and transportation of clinical specimens in the event of a bioemergency.
Tests for Detection of Hemorrhagic Fever Virus Infection (available only at specialized laboratories)
Laboratory Testing for Most Viral Hemorrhagic Fever Viruses
Ebola and Marburg Virusesa
—RT-PCR, including quantitative real-time RT-PCR
IgG serologic testing has not been reliable; use of the IgG ELISA may address this issue.
Lassa Fever Virus and New World Arenavirusesb
—RT-PCR (can be used to diagnose all arenavirus infections)
—Machupo virus can be difficult to isolate.
Rift Valley Fever Virusd
The virus can be easily aerosolized, so attempts to isolate it should be limited to facilities with maximal containment.
Yellow Fever Viruse
—For virus isolation, tissue samples should be divided into aliquots that are frozen at −70°C.
- The common clinical material used for VHF PCR is serum or plasma. Collection of blood in ethylenediaminetriacetic acid tubes ensures the highest PCR efficiency compared with serum, heparin, or citrate tubes (Drosten 2003).
- Serology (either testing an acute-phase specimen for IgM antibody or testing paired sera) can be used to diagnose most VHF infections.
- As with other rare diseases, the positive predictive value of PCR in the absence of other corroborating medical or epidemiologic evidence is exceedingly low.
- Immunohistochemistry methods are rapid, work directly on patient samples, are relatively sensitive and specific, and can be used on formalin-fixed samples. These methods have been described for Ebola virus (Zaki 1999), Marburg virus (Geisbert 1998), and Rift Valley fever virus (demonstrated in lambs; see Van der Lugt 1996) and would presumably be equally effective with the other hemorrhagic fever viruses.
- Although cell culture is the "gold standard" of virus detection and identification, performance of cell culture with these viruses is time consuming and extremely dangerous, and should be performed on suspect cases only at BSL-4 laboratories.
- Generally, hemorrhagic fever viruses can be recovered from serum or virtually any infected tissue.
- Most hemorrhagic fever viruses will grow in Vero and other mammalian cell lines. Passage in laboratory animals may increase cell culture sensitivity (Drosten 2003).
Supportive care is essential for patients with all types of VHF and includes the following:
- Maintenance of fluid and electrolyte balance, with hemodynamic monitoring as needed
- Mechanical ventilation, as indicated
- Dialysis, as indicated
- Steroids, which have not been shown to be of value, may be considered in certain situations because adrenal involvement may occur with VHF (Abraham 2002, Annane 2002)
- Anticoagulant therapies, aspirin, nonsteroidal anti-inflammatory medications, and intramuscular injections are contraindicated
- Appropriate therapy for secondary infections
Convalescent human plasma has been shown to be effective in the treatment of Argentine hemorrhagic fever and has been suggested for treatment of other New World arenavirus infections (Enria 2008; Peters 2010: Lymphocytic choriomeningitis, Lassa virus, and South American hemorrhagic fevers).
Ribavirin has some in vivo and in vitro activity against:
- Arenaviruses (Lassa fever and New World hemorrhagic fevers) (Enria 1994; Enria 2008; Huggins 1989; Kilgore 1997; McCormick 1986)
- Bunyaviruses (Rift Valley fever and others) (Borio 2002); ribavirin appears to be effective in animals (Peters 1986) and could potentially be used cautiously in humans
Antiviral agents have not been shown to be effective, and are not recommended, for infections caused by:
- Filoviruses (Ebola and Marburg hemorrhagic fever)
- Flaviviruses (yellow fever, Kyasanur Forest disease, and Omsk hemorrhagic fever)
- Recombinant nematode anticoagulant protein c2 (a known inhibitor of tissue factor [TF]–initiated blood coagulation)
- Recombinant human-activated protein C (currently licensed treatment of sepsis)
- Therapies that target the mRNA of different viral genes directly through steric hindrance or degradation of the target sequence
- Small molecules that directly target Ebola virus glycoprotein
The recommended regimens for use of ribavirin are shown in the table below.
Ribavirin Therapy Recommendations for Patients With Viral Hemorrhagic Fever of Unknown Cause or Known to Be Caused by an Arenavirus or Bunyavirusa
Adults (including pregnant women)c
—Loading dose of 30 mg/kg (maximum dose, 2 gm) IV once
Loading dose of 2,000 mg PO once, then:
—Loading dose of 30 mg/kg (maximum dose, 2 gm) IV once
—Loading dose of 30 mg/kg PO once
The Working Group on Civilian Biodefense does not recommend prophylactic antiviral therapy for persons exposed to any hemorrhagic fever viruses (including Lassa virus) in the absence of clinical illness (Borio 2002) and others have echoed this approach (Peters 2010: Lymphocytic choriomeningitis, Lassa virus, and South American hemorrhagic fevers). Instead, the working group recommends that exposed persons be placed under medical surveillance. Specific recommendations include:
- Exposed persons are defined as those with exposure to the initial bioterrorist release and those who are close contacts of, or have high-risk exposure to, a patient with VHF.
- High-risk exposures and close contacts are defined in the section on Infection Control: Isolation Precautions below.
- Exposed patients should monitor their temperatures daily and report any temperature of 101°F (38.3°C) or higher.
- Exposed persons should also report any other symptoms suggestive of VHF (see the section on Clinical Characteristics).
- If symptoms suggestive of VHF occur or if a temperature of at least 101°F (38.3°C) is documented by medical staff, ribavirin therapy should be initiated unless another diagnosis is confirmed (or the etiologic agent is known to be a filovirus [Ebola, Marburg] or flavivirus [yellow fever, Kyasanur forest disease, Omsk hemorrhagic fever]).
- Surveillance should be continued for 21 days after the last exposure.
The CDC has not published recent recommendations on antiviral prophylaxis for persons exposed to hemorrhagic fever viruses. The most recent recommendations from the CDC on this issue were published in 1988 and differ from the current recommendations of the Working Group on Civilian Biodefense. The 1988 CDC recommendations state that prophylactic therapy with ribavirin should be given to persons exposed to Lassa virus (500 mg orally every 6 hours for 7 days) (CDC 1988: Management of patients with suspected viral hemorrhagic fever). However, the efficacy of ribavirin in this context has not been documented, and the CDC did not recommend postexposure prophylaxis for persons exposed to an imported case of Lassa fever in 2004 (CDC 2004: Imported Lassa fever—New Jersey).
One group of researchers has suggested that ribavirin be used for postexposure prophylaxis against Lassa fever only following exposure to an acutely ill patient and when high-risk contact has occurred, such as in the following situations (Bausch 2010):
- Penetration of skin by a contaminated sharp instrument (eg, needlestick injury)
- Contamination of mucous membranes or broken skin with blood or bodily secretions (eg, blood splashing in the eyes or mouth)
- Participation in emergency procedures (eg, resuscitation after cardiac arrest, intubation, or suctioning) without use of appropriate PPE
- Prolonged (ie, for hours) and continuous contact in an enclosed space without use of appropriate PPE (eg, a healthcare worker accompanying a patient during medical evacuation)
Several studies have examined the use of postexposure prophylaxis for protection against filovirus infection. This issue is further addressed in the section on vaccines below.
- A recombinant vesicular stomatitis virus–based vaccine was used in Germany to protect a laboratory worker following accidental needlestick exposure to Ebola virus. The worker did not develop illness and apparently was protected by the vaccine (Gunther 2011).
- Another study found that postexposure prophylaxis using concentrated polyclonal IgG antibodies can protect nonhuman primates from infection with Marburg and Ebola viruses (Dye 2012).
Yellow Fever Vaccine
Two yellow fever vaccines are manufactured: the 17DD and 17D-204 yellow fever vaccines. Both are attenuated live-virus vaccines. The yellow fever virus strains in these two vaccines share 99.9% sequence homology (CDC 2010: Yellow fever vaccine).
- The 17DD vaccine is manufactured in Brazil and is used in Brazil and other South American countries.
- The 17D-204 vaccine is manufactured and used outside of Brazil, including in the United States. Currently, yellow fever vaccination is carried out for three primary reasons: to protect populations living in areas subject to endemic and epidemic spread of yellow fever, to protect travelers visiting these areas, and to prevent international spread by minimizing the risk of importation and translocation of the virus by viremic travelers (WHO 2011: Revised recommendations for yellow fever vaccination for international travellers, 2011).
- For US citizens, yellow fever vaccine is recommended only for:
- Travelers (9 months of age and older) to tropical areas of South America and Africa that are endemic for yellow fever (Certain countries require evidence of vaccination from all entering travelers, which includes direct travel from the United States.) Laboratory personnel who might be exposed to virulent yellow fever virus or to concentrated preparations of yellow fever vaccine virus strains by direct or indirect contact or by aerosols
- In the event of a bioterrorist attack, yellow fever vaccine would not be useful for postexposure prophylaxis because the incubation period for yellow fever is short (3 to 6 days) and immunity following vaccination does not develop for approximately 10 days (Borio 2002, Monath 2001).
The CDC recommends a single subcutaneous injection of 0.5 mL of reconstituted vaccine for primary vaccination. Booster doses are recommended every 10 years.
Details about the vaccine, including contraindications, precautions, and adverse reactions, can be found in the 2010 statement from the Advisory Committee on Immunization Practices (CDC 2010: Yellow fever vaccine).
Current yellow fever vaccines have potentially serious and life-threatening adverse reactions, so researchers are exploring potentially safer candidates. One, referred to as XRX-001, is a whole-virus inactivated vaccine. In a 2011 study, a two-dose regimen of XRX-001 induced neutralizing antibodies in a high percentage of subjects (Monath 2011).
Vaccines for Other Hemorrhagic Fever Viruses
- A live-attenuated vaccine against Argentine hemorrhagic fever virus (ie, Junin virus) has shown greater than 95% efficacy and no side effects (Maiztegui 1998). This vaccine has been used in Argentina and has contributed to a decrease in VHF cases in that country (Peters 2010: Lymphocytic choriomeningitis, Lassa virus, and South American hemorrhagic fevers).
- A number of Rift Valley fever vaccines have been developed, and additional work in this area is ongoing (Ikegami 2009). An inactivated Rift Valley fever vaccine has been used by the US Army (Pittman 1999).
- A formalin inactivated vaccine against Kyasanur forest disease virus, produced in chick embryo fibroblasts, was shown to be efficacious in field trials in the early 1990s (Dandawate 1994). The vaccine has been licensed and is currently being used routinely in endemic areas (Pattnaik 2006).
- Efforts to develop a vaccine against Lassa fever virus are ongoing (Cleri 2006, Fisher-Hoch 2004, Geisbert 2005, Grant-Klein 2011).
- A number of studies on vaccines for Marburg and Ebola viruses have been published over the last few years.
- Currently, multiple vaccine platforms have provided protection against filovirus infections in nonhuman primates, including DNA, recombinant adenovirus serotype 5, recombinant human parainfluenza virus 3, recombinant vesicular stomatitis virus, and virus-like particles (Falzarano 2011).
- A recombinant vesicular stomatitis virus (rVSV) vaccine that expresses a filovirus glycoprotein has been shown to completely protect nonhuman primates against Marburg virus and three different species of Ebola virus (Geisbert 2011).
- Postexposure vaccination may be useful in accidental exposure situations and could also be used to control secondary transmission during naturally occurring outbreaks or deliberate releases (Feldmann 2007). rVSV-based vaccines have shown utility when administered as a postexposure treatment against filovirus infections. Studies have shown that virus-like particle vaccines can confer complete homologous protection against Ebola virus and Marburg virus in a prophylactic setting in macaques (Warfield 2011).
Transmission within healthcare settings has been noted for a number of hemorrhagic fever viruses, including Ebola, Marburg, Lassa, Machupo, and Crimean-Congo viruses (Weber 2001).
- Nosocomial and household transmission most often has been associated with contact with infected blood or body fluids (Dowell 1999, Monath 1975).
- In some instances, transmission has resulted from reuse of needles or accidental needlesticks (Fisher-Hoch 1995, Guimard 1999).
- In one situation, investigators postulated that a healthcare worker became infected with Ebola virus after touching her eyes with a contaminated glove (Guimard 1999).
- Person-to-person airborne transmission appears to be rare; one patient with Lassa fever who had extensive pulmonary involvement may have transmitted the virus by this route (Carey 1972).
- Airborne transmission of Machupo virus presumably occurred in one situation where a nursing student became infected after watching an instructor change the bed linens of an infected patient; the student had no direct or close contact with the patient or with any associated fomites (Peters 1974).
- Although person-to-person airborne transmission appears unlikely, the potential for airborne transmission of hemorrhagic fever viruses in the healthcare setting cannot be excluded (Borio 2002).
- In some developing countries, inconsistent use of protective gear can contribute to infection of healthcare workers (Borchert 2011). Reasons for poor usage include unavailability of the gear, adherence to traditional explanatory models of disease origin, and bonding with sick colleagues (Borchert 2007).
- Contact with cadavers has also been shown to be a source of exposure during outbreaks of Ebola hemorrhagic fever (CDC 2001: Outbreak of Ebola hemorrhagic fever—Uganda; Roels 1999).
Appropriate isolation precautions for patients with suspected or confirmed VHF include a combination of Airborne and Contact Precautions (Weber 2001). Although airborne transmission of these agents appears to be rare, airborne transmission theoretically may occur; therefore, airborne precautions should be instituted for all patients with suspected VHF. According to the Working Group on Civilian Biodefense, the following precautions should be implemented for such patients (Borio 2002):
- Provide the following PPE for healthcare providers:
- N-95 respirator or powered air-purifying respirator (PAPR)
- Double (leak-proof) gloves
- Impermeable gowns
- Face shields
- Goggles for eye protection
- Leg and shoe coverings
- Place the patient in a private room with:
- Negative air pressure
- 6 to 12 air changes per hour
- Restricted access of nonessential staff and visitors
- Dedicate medical equipment (eg, stethoscopes, glucose monitors, point-of-care analyzers [if available]).
- Assure that healthcare providers adhere strictly to hand hygiene:
- Clean hands prior to donning PPE for patient contact
- After patient care, remove gloves, gown, and leg and shoe coverings and immediately clean hands
- Clean hands prior to the removal of facial protective equipment to minimize exposure of mucous membranes to potentially contaminated hands
- Clean hands again after all PPE is removed
- Place all persons (including medical and laboratory personnel) who have had a close or high-risk contact with a patient suspected of having VHF during the 21 days following onset of symptoms (and before onset of appropriate barrier precautions) under medical surveillance (Borio 2002).
- High risk is defined as having mucous membrane contact or having percutaneous injury involving contact with secretions, excretions, or blood from a patient with VHF.
- Close contact is defined as those who live with, shake hands with, hug, process laboratory specimens from, or care for a patient with VHF.
- If a filovirus or arenavirus infection is confirmed for the index patient, then medical surveillance should be continued until 21 days after the last exposure.
- If the index patient has Rift Valley fever or a flavivirus infection, then medical surveillance needs to be continued until 21 days after the last exposure only for those who processed laboratory specimens from the infected patient prior to initiation of appropriate precautions (since these conditions are transmitted in the laboratory setting but not via person-to-person transmission).
- If multiple patients with suspected VHF are admitted to one healthcare facility, they can be cohorted in the same part of the hospital to minimize exposure to other patients and healthcare workers.
Hemorrhagic fever viruses have lipid envelopes and are not environmentally stable; therefore, these viruses would not be expected to persist in the environment following a bioterrorist attack. However, such viruses can persistent on inanimate surfaces for up to several weeks (Percy 2010).
According to the Working Group on Civilian Biodefense, decisions about decontamination of the environment following an intentional release would depend upon the specific events surrounding the attack and should be made by experts who are familiar with the situation (Borio 2002).
The Working Group on Civilian Biodefense and the CDC make the following recommendations for environmental decontamination in the hospital setting (Borio 2002; CDC 1995: Update: Management of patients with suspected viral hemorrhagic fever—United States).
- Environmental surfaces, inanimate contaminated objects, or contaminated equipment should be disinfected with an Environmental Protection Agency–registered hospital disinfectant or a 1:100 dilution of household bleach using standard procedures.
- Contaminated linens should be incinerated, autoclaved, or placed in double (ie, leak-proof) bags at the site of use and washed without sorting in a normal hot water cycle with bleach.
- Hospital housekeeping staff and linen handlers should wear appropriate PPE (as outlined in the section on isolation practices above) when handling or cleaning potentially contaminated material or surfaces.
The 1995 CDC guidelines for management of patients with VHF indicate that efforts should be made to decontaminate stool, fluids, and secretions before disposal. According to the CDC, such fluids should be autoclaved, processed in a chemical toilet, or treated with several ounces of household bleach for 5 or more minutes before flushing or disposal (CDC 1995: Update: Management of patients with suspected viral hemorrhagic fever—United States). However, the Working Group on Civilian Biodefense has stated that since hemorrhagic fever viruses are not likely to survive standard US sewage treatment, such practices are unnecessary (Borio 2002).
- Guidelines from the CDC indicate that Standard Precautions should be used for postmortem care. These include using a surgical scrub suit, surgical cap, impervious gown or apron with full sleeve coverage, a form of eye protection (eg, goggles or face shield), shoe covers, and double surgical gloves with an interposed layer of cut-proof synthetic mesh (CDC 2004: Medical examiners, coroners, and biologic terrorism).
- In addition, autopsy personnel should wear N-95 respirators during all autopsies, regardless of suspected or known pathogens. PAPRs equipped with N-95 or HEPA filters should be considered.
- Other experts have recommended that aerosol-generating procedures (such as bone sawing) should be avoided during autopsies if possible. If such procedures are necessary, then HEPA-filtered masks and negative-pressure rooms should be used (Inglesby 2000: Plague as a biological weapon; Weber 2001).
- Contact with corpses should be limited to trained personnel, and routine precautions should be implemented when transporting corpses.
- According to CDC guidelines, "Bodies contaminated with hemorrhagic fever viruses should be cremated without embalming. If cremation is not an option, the body should be properly secured in a sealed container (eg, a Zigler case or other hermetically sealed casket) to reduce the potential risk of pathogen transmission" (CDC 2004: Medical examiners, coroners, and biologic terrorism).
- The Working Group on Civilian Biodefense has made the following recommendations for postmortem care of patients with VHF (Borio 2002).
- Trained personnel should perform autopsies using appropriate barrier precautions and HEPA-filtered respirators (N-95 masks or PAPRs).
- Autopsies should be performed in negative-pressure rooms.
- Postmortem examinations should be performed only if absolutely indicated.
An example of the type of system that can be used to seal remains prior to placing them in a casket for burial is the BioSeal Facility System, produced by Barrier Products. This system utilizes a poly-aluminum foil–extruded laminate material that when used with a heat sealer will provide Level 1 containment for all gases, fluids, vapors, and odors associated with the transport and storage of human and animal remains.
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