FAQ: African swine fever (ASF)

African swine fever is a highly infectious disease of pigs, including pigs, warthogs, bush pigs, European wild boar and American wild pigs. It is clinically indistinguishable from classical swine fever (CSF) and must be differentially diagnosed in the laboratory.

African swine fever virus is a double stranded DNA virus and the only member of the Asfarviridae family (African Swine Fever And related – virus), which shows characteristics of the Poxvirus family (but is structurally distinct).

ASF was first indentified in Kenya in 1921 and is endemic in most sub-Saharan African countries. It has been seen in South America and the Caribbean but has been eradicated. In Europe, cases have been reported in Portugal and Sardinia (Italy).

The clinical signs of ASF can display in either sub-acute, acute, or chronic forms. Clinical signs include: Fever (40.5-42ºC) vomiting, diarrhoea, reddening of the skin at extremities, chest and abdomen, anorexia and abortion. Clinically it is not possible to distinguish between CSFV and ASFV infection, and diagnostic tests must always be carried out to confirm infection. Other differential diagnoses include: Erysipelas, Salmonellosis and Pasteurellosis.

As with CSF, transmission can occur either directly contact of sick and healthy animals, and indirectly through contaminated feed, or on contaminated clothing, vehicles or as other fomites. Animals that have recovered from infection can continue excreting virus throughout their lives. In addition to these routes, which mirror the routes of infection of CSFV, ASFV can also be transmitted via soft ticks of the genus Ornithodoros.

The incubation period for ASF is between 5-15 days, with death occurring 6-13 days post onset of illness. In domestic swine, the mortality rate is very high for swine suffering from the acute form of the disease, often approaching 100%. In the sub-acute, or chronic forms of the disease, mortality is usually much lower (varying between 30-70%).

ASF is clinically indistinguishable to Classical swine fever and must be differentiated with laboratory tests. Diagnosis can be either by identification of the virus by direct antigen detection by immuno-fluorescence, tissue culture inoculation, or detection by polymerase chain reaction (PCR). Serological tests such as an indirect antibody fluorescent test, or ELISA can identify anti-ASFV antibodies in blood samples taken 8-21 days post infection.

As with CSF there is no treatment for ASF infection, but in contrast to the classical form of disease, there is no effective vaccine available to date.

The culling of herds where infected individuals is used as a way of halting the spread of the disease. Carcasses are disposed of either by burning or burial and the premises of the infection is thoroughly disinfected. Setting up a control zone around any infected premises and monitoring movement of pigs within the zone. In African countries where the disease is enzootic it is important that pigs are not brought into contact with ticks that can carry the virus (members of the genus Ornithodoros).

The World Organisation for Animal Health (OIE), is an intergovernmental organisation responsible for improving animal health worldwide. The OIE has a list of approved experts and laboratories.

Defra is the UK’s Department of farming and rural affairs and among other things, deals with outbreaks, surveillance and control of animal diseases.

• African swine fever virus group of the Pirbright Institute
• Virus Taxonomy of the International Committee on Taxonomy of Viruses (ICTV)