Epidemiology

Last updated May 25, 2011

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Reservoirs
Anthrax in Animals
Modes of Transmission
Anthrax in Humans—United States
Anthrax in Humans—Global Perspective
Outbreaks of Naturally Occurring Disease
Bibliography

Reservoir

B anthracis is found in soil in many areas of the world. Ecologic factors (such as abundant rainfall following a period of drought) may enhance spore density in soil, although the exact influence of such factors remains poorly understood.

The organism generally exists in the endospore form in nature; germination of spores outside an animal host may occur when the following conditions are met (WHO 1998):

Temperature between 8°C and 45°C
pH between 5 and 9
Relative humidity >95%
Presence of adequate nutrients

Endospores are resistant to drying, heat, ultraviolet light, gamma radiation, and some disinfectants. Spores can persist in soil for decades, as illustrated by biological warfare experiments during World War II on the Scottish island of Gruinard (Manchee 1990). During 1943 and 1944, an estimated 4 x 10¹⁴anthrax spores were dispersed on the island through explosive means. Spores were still detectable more than 40 years later. Disinfection of the island was finally completed in 1987, using a combination of seawater and formaldehyde.

Anthrax in Animals

Most mammals are susceptible to anthrax, but it is predominantly a disease of livestock. Livestock or other herbivores (eg, cattle, sheep, goats, pigs, bison, water buffalo) acquire infection from consuming contaminated soil or feed. Anthrax in animals is hyperendemic or endemic in the following areas of the world (WHOCC):

Most areas of the Middle East
Most areas of equatorial Africa
Mexico and Central America
Chile, Argentina, Peru, and Bolivia
Certain Southeast Asian countries (eg, Myanmar, Vietnam, Cambodia, Thailand)
Papua New Guinea
China
Some Mediterranean countries

In most of the rest of the world, anthrax occurs only sporadically. In the United States, outbreaks in animals have occurred since 1990 in the Midwest (Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota), in the West (California, Nevada), and in Texas and Oklahoma (MBAH 2006, WHOCC). Outbreaks also have recently occurred in Saskatchewan and Manitoba, Canada, affecting more than 800 animals (APHIS 2006).

Anthrax has been reported as the cause of death among chimpanzees in Ivory Coast (Leendertz 2004) and chimpanzees and a gorilla in Cameroon (Leendertz 2006). Investigators postulated that the chimps became ill either from consuming an infected animal or drinking contaminated water. Isolates from the wild apes in both outbreaks showed that the strains were clearly different from those of any previously described. The isolates established a new "forest anthrax cluster," termed "F," suggesting that B anthracis is a far less homogeneous species than currently believed (Leendertz 2006).

Anthrax also has been reported in cheetahs following consumption of infected meat (Good 2008).

Anthrax spores were detected in two of six species of raptors (road-side hawks and chimango caracaras [a bird of prey that is in the falcon family]) in central Argentina, suggesting that scavenger and nonscavenger bird species may influence anthrax epidemiology in some countries (Saggese 2007).

Modes of Transmission

Illness in humans most commonly occurs following exposure to infected animals or contaminated animal products; such exposures include:

Contact with infected tissues of dead animals (eg, butchering, preparing contaminated meat), which generally leads to cutaneous anthrax
Consumption of contaminated undercooked meat, which can lead to gastrointestinal anthrax
Contact with contaminated hair, wool, or hides (particularly during processing) or contact with products made from them, which can lead to either inhalational or cutaneous anthrax. Animal hair from endemic regions continues to represent an occupational risk for modern woolworkers (Wattiau 2008).

Cases following laboratory exposure have been recognized (Brachman 1980, CDC 2002: Suspected cutaneous anthrax in a laboratory worker—Texas, 2002). Person-to-person transmission of B anthracis has been reported rarely with cutaneous anthrax, but has not been recognized with gastrointestinal or inhalational disease (Weber 2001, Weber 2002).

Anthrax in Humans—United States

Approximately 130 human cases of anthrax occurred annually in the United States during the early 1900s. The incidence has gradually declined over time, with typically fewer than 10 cases reported each year since the early 1960s (CDC 1995).
About 95% of naturally occurring cases in the United States are cutaneous and 5% are inhalational. Only one case of gastrointestinal infection has been recognized in this country (Brachman 1980, Ramer 2010).
Only 18 cases of naturally occurring inhalational anthrax were reported in the United States during the 20th century (Brachman 1980). All but three were associated with industrial exposures; two of the remaining cases were laboratory-acquired, and the source of exposure for the third case remains unknown.
Between 1990 and 2000, only two cases of naturally occurring anthrax were reported in the United States (one in 1992 and one in 2000); both patients had cutaneous disease. The latter case occurred in North Dakota and resulted from agricultural exposure (CDC 2001: Human anthrax associated with an epizootic among livestock—North Dakota, 2000).
Since 2006, three cases of anthrax have resulted from direct occupational association with djembe drums made from untreated West African animal hides. Two cases were of cutaneous anthrax (CDC 2008), and one case was inhalational (CDC 2006, Walsh 2007). This was the first new case of naturally occurring inhalational anthrax in the United States since 1976. In response to this case, the Centers for Disease Control and Prevention (CDC) developed a document on safety issues related to anthrax and animal hides (CDC: Q & A: anthrax and animal hide drums).
In December 2009, a New Hampshire woman developed gastrointestinal anthrax after attending a drum circle gathering. Two animal hide drums and environmental samples from the building later tested positive for the same strain of B anthracis that infected the woman. She is believed to have swallowed airborne anthrax spores released from the drums during the gathering (CDC 2010: Gastrointestinal anthrax after an animal-hide drumming event—New Hampshire and Massachusetts, 2009).

Anthrax in Humans—Global Perspective

An estimated 2,000 to 20,000 human cases of anthrax occur globally each year (Brachman 1984).
Human cases generally follow disease occurrence in ruminants and are most prevalent in Africa, the Middle East, and parts of Southeast Asia.
Most cases are cutaneous.
Viable anthrax spores recently were detected in goat hair fibers, airborne dust, and unprocessed wastewater in a Belgian factory that scours wool and goat hair. Although no definitive clinical cases of anthrax were recorded among this unvaccinated workforce, evidence of asymptomatic B anthracis infection was found in approximately 10% of the employees (Wattiau 2009).
Two fatal cases of inhalational anthrax recently occurred in the United Kingdom, one in Scotland in 2006 and one in England in 2008. Both patients were drum makers (Anaraki 2008).
A recent systematic review of MEDLINE (1996-2005) and selected journal indexes (1900-1966) for inhalational anthrax cases worldwide between 1900 and 2005 identified 82 cases of inhalational anthrax (Holty 2006: Systematic review: a century of inhalational anthrax cases from 1900 to 2005). Cases from the 1979 Sverdlovsk outbreak in the former Soviet Union (see section Use of Anthrax as a Biological Weapon) were excluded from analysis because symptoms, treatment, and disease progression were not described.

Seventy-one of the 82 identified cases were naturally occurring, and 11 were part of the 2001 bioterrorism outbreak in the United States.
Among naturally occurring cases, most involved exposure to contaminated wool, goat hair, or animal hides.

An outbreak of anthrax was first observed among injection-drug users in Scotland in December 2009 (Ramsay 2010). Since then, 31 confirmed cases and 11 deaths among heroin users have been reported from three countries (Scotland, England, and Germany) (Christie 2010). The patients reported injecting, smoking, and/or snorting heroin. The symptoms at presentation varied greatly, were inconsistent, and were not typical of cutaneous, inhalational, or gastrointestinal anthrax (Booth 2010).

Outbreaks of Naturally Occurring Disease

Outbreaks have been reported in industrial settings where animal products are processed and in agricultural settings following consumption of or exposure to ill animals. Notable examples of outbreaks include the following:

A major outbreak involving nearly 10,000 cases and 182 deaths (most of them cutaneous infection) occurred in Zimbabwe during the late 1970s and early 1980s (Davies 1982). An epizootic in cattle occurred at that time in the same area.
An outbreak involving 9 cases (5 inhalational and 4 cutaneous) occurred in 1957 in the United States in a New Hampshire goat-hair processing plant (Brachman 1960, Plotkin 1960). This was the last recognized outbreak of naturally occurring infection in this country.
An outbreak of oropharyngeal anthrax involving 24 cases occurred in Thailand in 1982 following consumption of contaminated meat (Sirisanthana 1984). Oropharyngeal disease is an unusual manifestation of infection, which makes this outbreak of particular interest.


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