58       Asfarviridae

 

African swine fever virus (ASFV) is the sole member of the genus Asfivirus in the family Asfarviridae. Virions are 175 to 215 nm in diameter and consist of a membrane-bound nucleoprotein core inside an icosahedral capsid, surrounded by an outer lipid-containing envelope. The genome consists of a single molecule of linear double-stranded DNA. Following replication in the cytoplasm of host cells, virus is released either by budding through the plasma membrane or following cellular disintegration. African swine fever virus is stable in the environment over a wide range of temperature (4 to 20°C) and pH values. The virus may persist for months in meat.

African swine fever

African swine fever (ASF) is an economically important viral disease of pigs, characterized by fever, haemorrhages in many tissues and a high mortality rate. It is endemic in sub-Saharan Africa, Madagascar and Sardinia. A large outbreak beginning in Georgia in 2007 has spread across Russia, Belarus and Ukraine. Domestic and wild pigs are the only species susceptible to infection. In Africa, ASFV is maintained in a sylvatic cycle involving soft ticks of the genus Ornithodorus and inapparent infection of warthogs and bushpigs. Replication of virus occurs in the ticks and both transovarial and trans-stadial transmission have been described. Soft ticks feed for short periods on hosts before dropping off and sheltering in crevices in walls or cracks in the ground. The presence of infected ticks in a particular region makes the eradication of ASF difficult. Virulent strains of ASFV, producing high mortality in infected animals, are widely distributed in Africa. Many isolates from other parts of the world are less virulent and mortality rates are usually below 50%.

Feeding uncooked swill is an important mechanism of spread of ASF internationally, with outbreaks often starting in herds close to airports and harbours. Pigs which have recovered from clinical disease may remain infected for long periods. Carrier pigs are considered to be important sources of virus dissemination.

Infection in domestic pigs is usually acquired via the oronasal route. The virus replicates primarily in cells of the lymphoreticular system. Lesions include splenic enlargement, swollen haemorrhagic gastrohepatic and renal lymph nodes, subcapsular petechiation in the kidneys, petechial and ecchymotic haemorrhages on serosal surfaces, oedema of the lungs and hydrothorax. The widespread haemorrhages result from disseminated intravascular coagulation, endothelial damage and destruction of megakaryocytes. The clinical signs of ASF, which range from inapparent to peracute, relate to the challenge dose and virulence of the virus and to the route of infection. The incubation period is typically five to seven days in acute cases. Mortality rates, which are variable, depend on the age and general health of infected pigs. Animals may recover and appear clinically normal or may develop a chronic form of the disease, which usually occurs in regions where ASFV is endemic.

Laboratory confirmation of ASF is based on detection of ASFV using tests such as PCR, direct immunofluorescence and haemadsorption. Suitable samples include blood, serum, tonsil, spleen and lymph nodes. Antibodies persist for long periods in recovered animals and serological testing may be the only means of detecting animals infected with strains of low virulence.

Restriction of pig movement, serological monitoring of carrier pigs, and prevention of contact between domestic pigs and warthogs or ticks are important control measures in countries where the disease is endemic. Eradication of tick species which act as vectors of ASFV is an essential part of a control programme. An effective vaccine is not yet available. The occurrence of strains of low virulence renders eradication difficult.