AFRICAN SWINE FEVER
Aetiology Epidemiology Diagnosis Prevention and control References
AETIOLOGY
Classification of the causative agent
DNA virus not classified to date. Has characteristics of an Iridovirus and a Poxvirus
Resistance to physical and chemical action
Temperature: Highly resistant to low temperatures. Heat inactivated by 56°C/70 min; 60°C/20 min pH: Inactivated by pH <3.9 or >11.5 in serum-free medium. Serum increases the resistance of the virus, e.g. at pH 13.4 - resistance lasts up to 21 hours without serum, and 7 days with serum Chemicals: Susceptible to ether and chloroform Disinfectants: Inactivated by 8/1,000 sodium hydroxide (30 min), hypochlorites - 2.3% chlorine (30 min), 3/1,000 formalin (30 min), 3% ortho-phenylphenol (30 min) and iodine compounds Survival: Remains viable for long periods in blood, faeces and tissues. Can multiply in vectors EPIDEMIOLOGY
Hosts
- Pigs, warthogs, bush pigs, European wild boar, American wild pigs. Species-linked predisposition: African wild swine (warthogs and bush pigs) are usually inapparently infected
Transmission
- Direct transmission:
- contact between sick and healthy animals
- Indirect transmission
- feeding with garbage containing infected meat
- biological vectors: soft ticks of the genus Ornithodoros
- fomites: premises, vehicles, implements, clothes
Sources of virus
- Blood, tissues, secretions and excretions of sick and dead animals
- A carrier state exists, especially in African wild swine, and in domestic pigs in enzootic areas
- Soft ticks of genus Ornithodorus
Occurrence
African swine fever is enzootic in most countries of Sub-Saharan Africa. In Europe it has been reported in the Iberian Peninsula and in Sardinia. It was present in four South American and Caribbean countries, but has been eradicated
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Bulletin
DIAGNOSIS
Incubation period is 5-15 days
Clinical diagnosis
Acute form (highly virulent virus)
- Fever (40.5-42°C)
- Early leucopaenia and thrombocytopaenia (48-72 hours)
- Reddening of the skin (white pigs) - tips of ears, tail, distal extremities, ventral aspects of chest and abdomen
- Anorexia, listlessness, cyanosis and incoordination within 24-48 hours before death
- Increased pulse and respiratory rate
- Vomiting, diarrhoea (sometimes bloody) and eye discharges may exist
- Death within 6-13 days, or up to 20 days
- Abortion may occur in pregnant sow
- Survivors are virus carriers for life
- In domestic swine, the mortality rate often approaches 100%
Subacute form (moderately virulent virus)
- Less intense symptoms
- Duration of illness is 5-30 days
- Abortion in pregnant sows
- Death within 15-45 days
- Mortality rate is lower (e.g. 30-70%, varies widely)
Chronic form
- Various signs: loss of weight, irregular peaks of temperature, respiratory signs, necrosis in areas of skin, chronic skin ulcers, arthritis
- Pericarditis, adhesions of lungs, swellings over joints
- Develops over 2-15 months
- Low mortality
Lesions
Acute form (not all lesions are seen; this depends on the isolate)
- Pronounced haemorrhages in the gastrohepatic and renal lymph nodes
- Petechial haemorrhages of the renal cortex, also in medulla and pelvis of kidneys
- Congestive splenomegaly
- Oedematous 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
- Oedema in the mesenteric structures of the colon and adjacent to the gall bladder; also wall of gall bladder
Chronic form
- Focal caseous necrosis and mineralisation of the lungs may exist
- Lymph nodes enlarged
Differential diagnosis
- Classical swine fever (CSF) (hog cholera). It is not possible to differentiate ASF and CSF by clinical or post-mortem examination. It is essential to send samples for laboratory examination
- Erysipelas
- Salmonellosis
- Pasteurellosis
- All septicaemic conditions
Laboratory diagnosis
Procedures
Identification of the agent
- Isolation:
- Cell culture inoculation (primary cultures of pig monocytes or bone marrow cells - most isolates produce haemadsorption)
- Pig inoculation - unvaccinated and vaccinated against classical swine fever (hog cholera)
- Antigen detection by direct immuno-fluorescence
- Detection of virus genome by polymerase chain reaction (PCR)
Serological tests
- Group-specific tests
- ELISA (prescribed test in the Manual)
- Indirect fluorescent antibody test
- Immunoblotting (confirmatory test)
- Counter immunoelectrophoresis test (only for screening of large groups)
- Type-specific tests - none available for routine use
Samples
Identification of the agent
- Blood collected during the early febrile stage in heparin (10 IU/ml) or EDTA (0.5%)
- Small pieces (2-5 g) of spleen, kidney and lymph nodes kept at 4°C
Serological tests
- Serum collected within 8-21 days after infection in convalescent animals
PREVENTION AND CONTROL
- No treatment
- No vaccine to date
Sanitary prophylaxis
Free countries
- Careful import policy for animals and animal products
- Proper disposal of waste food from aircraft or ships coming from infected countries
- Efficient sterilisation of garbage
In outbreaks
- Rapid slaughtering of all pigs and proper disposal of cadavers and litter is essential
- Thorough cleaning and disinfection
- Designation of infected zone, with control of pig movements
- Detailed epidemiological investigation, with tracing of possible sources (up-stream) and possible spread (down-stream) of infection
- Surveillance of infected zone, and surrounding area
Infected countries
- Avoid contact between pigs and soft tick vectors (Africa) - i.e. prevent pigs from wandering
REFERENCES AND OTHER INFORMATION
- Reference experts and laboratories
- Classified as an OIE List A disease (A120)
- Chapter 2.1.12. in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
- Terrestrial Animal Health Code
- Other references - see the Index
- World Animal Health .
- Current Animal Health Status (Disease Information)
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Contact : scientific.dept@oie.int