African Swine Fever

Last updated March 13, 2003

Agent
Hosts
Epidemiology
ASF As a Biological Weapon
Clinical Features
Differential Diagnosis
Laboratory Diagnosis
Treatment
Prevention
Outbreak Control
Public Health Issues
References

Agent

African swine fever (ASF) is a highly contagious, usually fatal viral disease of animals in the pig (Suidae) family. The ASF virus is fairly hardy and has the following characteristics:

Formerly classified as an iridovirus (Iridoviridae)
Recently reclassified into the new family Asfarviridae (from African Swine Fever And Related Viruses) in its own genus, Asfivirus (see References: Pig Disease Information Centre)
Large icosahedral DNA virus (see References: Wilkinson 1992)
Inactivated at pH 3.9 or lower and at 11.5 or higher in serum-free medium
Viable for 7 days at pH 13.4 in presence of 25% serum
Will survive 15 weeks in putrefied blood, 70 days in blood on wooden boards, 11 days in feces at room temperature, 3 hours at 50°C, 18 months in pig blood at 4°C, 150 days in boned meat at 39°C, and 140 days in salted dried hams (see References: Mebus 1998)

Hosts

Only species of the pig family are susceptible to infection, with domestic pigs highly susceptible (usually lethally infected).
Specific hosts include pigs, warthogs, bush pigs, European wild boars, American wild pigs, and giant forest hogs (see References: Mebus 1998; OIE: Technical disease cards database).
Human beings are not hosts (see References: FAO).
Infection is persistent in African warthogs and bushpigs, which are considered natural reservoirs (see References: Vallee 2001).

Epidemiology

Transmission

Direct

Contact between sick and healthy animals

Indirect

Ingestion of garbage containing infected meat (this is the primary means by which the virus spreads from country to country)
Biological vectors: soft ticks of the genus Ornithodoros
Fomites: premises, vehicles, implements, clothing

Sources

Blood, tissue, secretions, and excretions of sick and dead animals
Carrier state exists, especially in African wild swine and domestic pigs in enzootic areas
Soft ticks of genus Ornithodorus

Occurrence

ASF is most prevalent in countries of sub-Saharan Africa.
In Europe, the disease has been reported in Belgium and Holland as well as repeatedly in France and Italy.
Fairly severe epidemics have occurred in Brazil (1978-1981) and Haiti (1978-1984) (see References: Parker 1990).
The disease has been eradicated in South America and the Caribbean. with no cases identified in these regions since the 1980s (see References: OIE: Technical disease cards database).
In 2001, several cases were found in Madagascar in the Anatananarivo province (see References: Roger 2001); epizootics also have been reported in the Democratic Republic of the Congo, Kenya, Zambia, Tanzania, South Africa, Mozambique, and Senegal (see References: Roeder 2002).
ASF is enzootic in warthogs and Ornithodoros porcinus ticks in southern and eastern Africa; it is believed that this sylvatic cycle may be partly responsible for the periodic outbreaks of the disease in domestic animals (see References: Roeder 2002).

ASF As a Biological Weapon

ASF has several characteristics that make it a potential biological weapon:

Infected animals have high morbidity and mortality rates.
The disease is highly communicable both directly and indirectly.
Effective treatments are not available for infected animals, and no vaccines are available to prevent infection.

Clinical Features

ASF is similar to classical swine fever (hog cholera) and porcine dermatitis and nephropathy syndrome in terms of clinical signs and autopsy lesions (see References: Pig Disease Information Centre). Laboratory confirmation is needed to completely differentiate between these diseases. ASF usually presents in one of the four forms outlined in the table below (see References: OIE: Technical disease cards database; FAO).

Clinical Features of African Swine Fever
*Form*	*Characteristics*
Peracute	—Death without advance warning typical —Before death, recumbency and high fever may be seen
Acute (highly virulent virus)	—Fever (40.5°-42°C) —Early leukopenia and thrombocytopenia (48-72 hr) —Animals huddle together, seeking shade and sometimes water —Animals are reluctant to move —Signs of abdominal pain may occur, including arching of back, abnormal movements, and flank kicking —Reddening of skin (white pigs) on tips of ears, tail, distal extremities, ventral aspects of chest and abdomen —Anorexia, listlessness, cyanosis, and incoordination within 24-48 hr before death —Increased pulse and respiratory rate —Vomiting, diarrhea (sometimes bloody), and mucopurulent eye and nasal discharges may occur —Constipation may develop, with hard, small feces covered in blood and mucus —Difficulty breathing, with possibly bloody froth at mouth and nostrils —Duration of clinical signs generally short (2-7 days) but may be longer, and apparent recovery may be followed by relapse and death —Death within 6-13 days (up to 20 days) —Abortion may occur in pregnant sows —Survivors are virus carriers for life —In domestic swine, mortality rate often approaches 100% —Pigs that do recover from acute infection are generally asymptomatic
Subacute (moderately virulent virus)	—More common in Europe —Less intense symptoms —Fluctuant fever —Weight loss usually occurs —Interstitial pneumonia usually present, resulting in respiratory distress and moist coughing —Secondary bacterial infection may occur —Joints may be painful and swollen —Duration of illness 5-30 days —Cardiac damage may result in death due to acute or congestive heart failure —Abortion may occur in pregnant sows —Death within 15-45 days —Mortality rate lower (30%-70%, varies widely) —Pigs may recover or progress to chronic form of disease
Chronic	—More common in Europe and Angola —Various signs: loss of weight, irregular peaks of temperature, respiratory signs, necrosis in areas of skin, chronic skin ulcers, arthritis, swelling over joints —Subject to secondary bacterial infections —Develops over 2-15 mo —Low mortality —Recovery unlikely

In addition to the signs above, characteristic lesions can be seen on necropsy in affected animals. The type of lesions present depends on the strain of the virus. Descriptions of the lesions follow (see References: Mebus 1998):

Acute

Pronounced hemorrhages in the gastrohepatic and renal lymph nodes
Petechial hemorrhages of the renal cortex, also in medulla and pelvis of kidneys (see Figure 16 from The Gray Book [References: Mebus 1998])
Congestive splenomegaly (see Figure 14 from The Gray Book [References: Mebus 1998])
Edematous areas of cyanosis in hairless parts
Cutaneous ecchymoses on the legs and abdomen
Excess of pleural, pericardial, and/or peritoneal fluid
Petechiae in the mucous membranes of the larynx and bladder and on visceral surfaces of organs (see Figure 14 from The Gray Book [References: Mebus 1998])
Edema in the mesenteric structures of the colon and adjacent to the gallbladder (also in wall of gallbladder)
Reddened skin on the extremities (see Figure 13 from The Gray Book [References: Mebus 1998])

Chronic

Focal caseous necrosis and mineralization of the lungs (may exist)
Enlarged lymph nodes (see References: OIE: Technical disease cards database)
Pericarditis and adhesions of lungs

Differential Diagnosis

ASF is often clinically confused with hog cholera or classical swine fever. In addition, the following diseases should be considered (see References: Mebus 1998; OIE: Technical disease cards database; Roeder 2002):

Erysipelas
Septicemic salmonellosis
Pasteurellosis
Streptococcus suis infection
Eperythrozoonosis
Porcine dermatitis and nephropathy syndrome
Porcine reproductive and respiratory syndrome, especially the "atypical" form
Pseudorabies
Hemophilus suis infection
All septicemic conditions
Coumarin poisoning
Salt poisoning

Laboratory Diagnosis

Because ASF can easily be confused with other important diseases of swine, obtaining samples for laboratory diagnosis is an important step in confirming the illness.

Samples of the following should be sent to the laboratory (they should be kept as cold as possible, without freezing, during transport):

Blood in anticoagulant (heparin or ethylenediaminetetraacetic acid [EDTA])
Spleen
Tonsil
Kidney
Lymph nodes

The following tests are available to identify the ASF virus:

Hemadsorption (HAD) test
Polymerase chain reaction (PCR)
Pig inoculation (no longer recommended)

The following tests can be used to test for antibodies in recovering pigs about 8 to 21 days after infection:

Enzyme-linked immunosorbent assay (ELISA) (preferred test for international trade)
Indirect fluorescent antibody test (FAT)
Immunoblotting test
Counter-immunoelectrophoresis (immunoelectroosmophoresis)

For more information on any of these tests, see References: OIE: Manual of standards for diagnostic tests and vaccines.

Treatment

There is no treatment for ASF (see References: OIE: Technical disease cards database).

Prevention

There is no vaccine available to prevent ASF.

There have been reports of a limited number of pigs showing clinical signs but not dying after inoculation with an infected spleen treated with nonionic detergent in Freund's Complete Adjuvant. Also, pigs given passive antibodies or piglets given colostral antibodies show reduced levels of virus in their blood and little to no clinical signs of the disease (see References: Wilkinson 1992).
Because different strains of the virus do not give cross-protection, it is not likely a vaccine will be developed in the future (see References: Roeder 2002).

Avoid using garbage as feed if possible. If garbage must be fed, it should be cooked (see References: US Animal Health Association).
Import only ASF disease–free pigs (see References: US Animal Health Association).
Waste food from aircraft or ships coming from infected countries must be properly destroyed (see References: OIE: Technical disease cards database).
General disease prevention protocols need to be followed (see References: Roeder 2002):

Isolation of new introductions for at least 30 days
Locating herds a considerable distance apart
Shower-in/shower-out required for farm employees and visitors
Restricted entry to farm of people, equipment, instruments, and vehicles
Total confinement of animals, including all-in/all-out flow
Prevention of wild or domestic animal entry

Outbreak Control

The following measures should be taken in the case of an outbreak:

Rapid slaughtering of all pigs and proper disposal of cadavers and litter
Thorough cleaning and disinfection of equipment and facilities
Designation of an infected zone and control of animal movement
Detailed epidemiologic investigation, with tracing of possible sources (upstream) and possible spread (downstream) of infection
Surveillance of the infected zone and surrounding area

In countries where ASF is present, the following measures should be taken:

Avoid contact between pigs and soft tick vectors (Africa) by preventing pigs from wandering (see References: OIE: Technical disease cards database).
Avoid contact between domestic and feral swine.

Public Health Issues

ASF virus does not cause infection in humans; therefore, there are no public health issues to be considered.

References

FAO (Food & Agriculture Organization of the United Nations). African swine fever resources for contingency planning [Web page]

Mebus CA. African swine fever. In: US Animal Health Association, Committee on Foreign Animal Disease. Foreign animal diseases: the gray book. Ed 6. Part IV. Richmond, Va: US Animal Health Assoc, 1998 [Full text]

OIE (Office International des Epizooties/World Organization for Animal Health). African swine fever. In: Manual of standards for diagnostic tests and vaccines. Ed 4. Chap 2.1.12. Paris: OIE, 2000 [Full text]

OIE (Office International des Epizooties/World Organization for Animal Health). African swine fever. Technical disease cards database. 2002 [Full text]

Parker MT, Collier LH. Topley and Wilson's principles of bacteriology, virology, and immunity. Vol 1. Kent, England: Edward Arnold, Hodder & Sloughton, 1990:41-3

Pig Disease Information Centre. African swine fever (ASF), 2000-01 [Web page]

Roeder PL,Taylor WP. Swine fever: classical swine fever and African swine fever. Vet Clin North Am Food Anim Pract 2002 Nov;18(3):431-51

Roger F, Ratovonjato J, Vola P, et al. Ornithodoros porcinus ticks, bushpigs, and African swine fever in Madagascar. Exp Appl Acarol 2001;25(3):263-9 [Abstract]

Vallee I, Tait SWG, Powell PP. African swine fever virus infection of porcine aortic endothelial cells leads to inhibition of inflammatory responses, activation of the thrombotic state, and apoptosis. J Virol 2001 Nov;75(21):10372-82 [Full text]

Wilkinson PJ, Denyer, MS. African swine fever. In: Roitt IM, DeWes PJ. eds. Encyclopedia of immunology. Vol 1. London: Academic Press, 1992:41-3


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