African swine fever is a highly infectious and severe hemorrhagic disease of pigs that produces a wide range of clinical signs and lesions that can closely resemble those of classical swine fever. African swine fever virus (ASFV) has a case fatality rate of up to 100% in domestic pigs and wild boars. The virus is endemic in many African countries, where it is maintained in the environment through a sylvatic cycle involving warthogs and Ornithodoros (soft) ticks. ASFV was transmitted from Africa to Georgia in 2007, and since then, to many countries across central and eastern Europe, Russia, Asia (including China, which produces more than half of the world's pork), and the Americas (Haiti and the Dominican Republic). This outbreak has led to the death and slaughter of millions of pigs, posing a massive threat to the global pig industry. Vaccines with good safety and efficacy have been developed and are being licensed for use in some countries.
African swine fever virus (ASFV) infects domestic swine and other members of the family Suidae, including warthogs (Phacochoerus aethiopicus), bushpigs (Potamochoerus porcus), and wild boars (Sus scrofa ferus). ASFV infection of other animals has never been documented or experimentally induced, and it does not infect humans.
African swine fever (ASF) affects all breeds and types of domestic pigs and European wild boars, and animals of all ages are equally susceptible to the virus.
In Africa, ASFV produces inapparent infection in warthogs, bushpigs, and the soft tick Ornithodoros moubata. When the disease was endemic in southern Spain and Portugal, a different species of soft tick (Ornithodoros erraticus) became infected with the virus. Several other Ornithodoros spp that are not usually associated with pigs or wild swine have been infected with ASFV experimentally.
ASF was considered to be confined to sub-Saharan Africa until 1957, when an outbreak occurred in Portugal as a result of waste from airline flights being fed to pigs near Lisbon airport. Another introduction of ASF to Portugal was reported in 1960. ASF remained endemic on the Iberian Peninsula until the mid-1990s.
Outbreaks of ASF were reported in several other European countries during the 20th century, including Malta (1978), Italy (1967, 1980), France (1964, 1967, 1977), Belgium (1985), and the Netherlands (1986). The virus was eradicated by slaughter from each of these countries; however, it has remained endemic in Sardinia since its introduction in 1978.
During the 1970s and 1980s, ASFV spread to the Caribbean and Brazil, where outbreaks were stamped out after they had devastated the swine industries in the affected countries.
ASFV appeared again in continental Europe in 2007, initially in Georgia, from where it spread to western parts of Russia, Armenia, Azerbaijan, Ukraine, Belarus, Lithuania, Latvia, Poland, and Estonia.
In 2018, ASFV spread to China, from where it rapidly spread to many neighboring Asian countries, including Mongolia, Cambodia, India, Laos, Vietnam, the Philippines, North and South Korea, Myanmar, Timor-Leste (East Timor), and Papua New Guinea.
ASFV has also spread across much of Europe, infecting domestic pigs and wild boars in most countries in central and eastern Europe, with countries as far west and north as Germany and Sweden reporting cases in wild boars and countries as far south as Italy reporting cases in wild boars and domestic pigs in 2023.
After an outbreak of ASF in wild boars in Belgium, ASFV was successfully eradicated. The virus remains endemic in wild boar populations across most of eastern and central Europe and Asia, and it poses a high risk to the remaining uninfected countries across the world.
As of March 2024, reports from the UN Food and Agriculture Organization (FAO) and the OIE indicate that many millions of pigs have either died or been culled as a direct result of the ASF outbreak across Europe, Asia, and the Caribbean, having a large impact on global pork supplies.
Etiology and Pathogenesis of African Swine Fever
African swine fever virus is a large, enveloped, double-stranded DNA virus that is the sole member of the genus Asfivirus within the family Asfarviridae. ASFV is distantly related to virus families of lower eukaryotes, including the family Faustoviridae and genus Kaumoebavirus. Together with other viruses that replicate in the cytoplasm, the Asfarviridae have been classified in the phylum Nucleocytoviricota.
The primary route of infection by ASFV is the upper respiratory tract, where the virus replicates in the tonsils and lymph nodes draining the head and neck. Generalized infection is transmitted rapidly via the bloodstream, leading to high concentrations of the virus in all tissues.
The factors that produce the hemorrhagic lesions of ASF are not fully defined; however, severe disruption to the blood clotting mechanism plays a major role. ASFV is excreted mainly from the upper respiratory tract and is present in all secretions and excretions that contain blood.
Epidemiology and Transmission of African Swine Fever
The prolonged period during which ASF has been endemic in Africa is likely to have led to the selection of viruses of varying virulence. Distinct ASFV genotypes have been differentiated by sequence analysis of viral genomes obtained from different geographical areas over a long period of time.
ASFV is highly resistant to a wide pH range and to a freeze-thaw cycle, and it can remain infectious for many months at room temperature or when stored at 4°C. Virus in body fluids and serum is inactivated at 60°C in 30 minutes; however, virus in unprocessed pig meat, where it can remain viable for several weeks or months, can be inactivated only by heating to 70°C for 30 minutes.
ASFV has distinct patterns of transmission: a sylvatic cycle in warthogs and ticks in Africa, and epidemic and endemic cycles in domestic swine and wild boars. Ornithodoros ticks are biological vectors for the virus.
The virus is maintained in Africa by a sylvatic cycle of transmission between warthogs and the soft tick vector O moubata, which inhabits warthog burrows and from which it is unlikely ever to be eliminated.
The virus can be transmitted from wildlife reservoirs to domestic pigs via the bite of an infected soft tick or via the ingestion of warthog tissues.
Once ASFV has been introduced in domestic pigs or wild boars, infected animals are the most important source of the virus for susceptible swine. Pigs usually become infected via the oronasal route, by direct contact with infected pigs or by the ingestion of waste food containing unprocessed pig meat or pig meat products.
In the current ASF outbreak in Europe and Asia, the virus has survived and spread in wild boar populations independently of outbreaks in domestic pigs. In addition, long-distance jumps by the virus are most likely due to the movement of contaminated meat or meat products by humans.
In pigs acutely infected by ASFV, all body fluids and tissues contain large amounts of infectious virus from the onset of clinical signs until death. Pigs infected with less virulent isolates can transmit virus to susceptible pigs for up to 1 month after infection; blood is infectious for up to 6 weeks, and transmission can occur if blood is shed.
Pigs that survive ASFV infection may be persistently infected; however, their role in the epidemiology of the disease is not fully understood.
Clinical Signs and Lesions of African Swine Fever
African swine fever occurs in peracute, acute, subacute, and chronic forms, and mortality rates vary from 0% to 100%, depending on the virulence of the infecting virus. The acute and peracute forms of ASF are characterized by a severe hemorrhagic disease that has an almost 100% mortality rate.
After an incubation period of 3–7 days, swine with ASF develop a high fever (up to 42°C), followed rapidly by inappetence, incoordination, and prostration. Swine may die at this stage without exhibiting other clinical signs. Surviving swine next exhibit reddening or cyanosis of the ears and snout (see ear hyperemia image), followed rapidly by more generalized reddening of the body (see generalized hyperemia image) and bleeding from the nose and anus. Infected pigs may also show dyspnea, vomiting, and if pregnant, abortion.
Courtesy of Dr. C. A. L. Oura.
Courtesy of Dr. C. A. L. Oura.
Swine that survive acute ASF infection may appear healthy or chronically diseased. Some isolated viruses of low virulence produce nonspecific clinical signs and lesions that often are characterized by emaciation, swollen joints, and respiratory problems.
The severity and distribution of the lesions due to ASFV vary according to the virulence of the virus. In acutely fatal cases, gross lesions are most prominent in the lymphoid and vascular systems.
Hemorrhages occur predominantly in lymph nodes, which may resemble blood clots (see hemorrhagic gastrohepatic lymph node image), and in the kidneys (usually as petechiae) and heart.
Courtesy of Dr. C. A. L. Oura.
The spleen is often large and friable.
There may be straw-colored or blood-stained fluid in pleural, pericardial, and peritoneal cavities, as well as congestion of the lungs.
Diagnosis of African Swine Fever
Laboratory testing
Rapid, early laboratory diagnosis of ASF is essential for timely control. The clinical signs can be similar to those of several other diseases, including bacterial septicemia (eg, erysipelas and acute salmonellosis); however, the major diagnostic problem is to distinguish ASF from classical swine fever (hog cholera).
Any febrile disease in swine associated with disseminated hemorrhage and high mortality rates should raise suspicion of ASF.
Diagnosis of chronic ASF infections is problematic because the clinical signs and lesions in chronically infected pigs are highly variable. Laboratory confirmation is essential, and samples of blood, spleen, kidney, lymph nodes, and tonsils, in particular, should be collected for virus isolation, detection of antigen, or PCR testing for viral DNA.
Virus isolation is carried out in swine bone marrow or peripheral blood leukocyte cultures, in which hemadsorption of red blood cells on the surface of the infected cells can be demonstrated (see hemadsorption image). (There are nonhemadsorbing viral strains that still produce a cytopathic effect in pig leukocytes.)
Courtesy of Dr. C. A. L. Oura.
Antigen detection can also be achieved by immunofluorescence staining of tissue smears, immunodiffusion using tissue suspensions as the source of antigen, or antigen-detection ELISA.
Antibodies can be detected by ELISA and indirect immunofluorescence; in acute cases of ASF, however, the pigs may die before antibodies are produced.
Treatment, Control, and Prevention of African Swine Fever
No effective treatment
Vaccines being trialed and licensed in some countries
There is no treatment for ASF. Many laboratories around the world are working to develop safe and effective vaccines, and they have had some promising results; however, more work is required to ensure safety and efficacy. Two commercially produced modified live virus vaccines are currently under field evaluation in some countries and have been licensed for field use in Vietnam.
The prevention and control of ASF is complicated by several factors, including the transmission of virus in fresh meat and some cured pork products, the possible persistent infection in some swine, the presence of wildlife reservoirs of virus (including wild suids and soft ticks) in some parts of the world, and diagnostic confusion with agents that lead to similar clinical signs, such as classical swine fever (hog cholera).
Prevention of ASF depends on ensuring that neither infected live domestic or wild swine nor swine meat products are introduced into areas free of ASF.
The presence of ASFV in ticks and warthogs in many countries of sub-Saharan Africa makes it difficult, if not impossible, to break the sylvatic cycle of the virus. However, domestic swine can be reared in Africa if the management system excludes the feeding of uncooked waste food scraps and prevents the access of ticks and contact with warthogs, usually by double fencing, with a wire mesh perimeter fence extending beneath the ground.
Countries free of ASF maintain their virus-free status by prohibiting the importation of live swine and swine products from infected countries and by monitoring the destruction of all waste food scraps from ships and aircraft involved in international routings.
If ASF occurs in a previously uninfected country, control depends first on early recognition and rapid laboratory diagnosis. After ASF is confirmed in a country that has been free of the disease, prompt action is required to control and then eradicate the infection. The strategy for eradication is slaughter, or sometimes selective slaughter, of infected swine and swine in contact with those infected animals, and the safe disposal of carcasses.
The presence of infected wild boars substantially complicates ASF control programs. Direct contact between wild boars and domestic pigs must be avoided, through enhanced biosecurity in backyard farms.
Pearls & Pitfalls
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Movement of swine between farms is controlled, and feeding of waste food is prohibited. Where soft ticks are known to occur, infested buildings are sprayed with acaricides, and restocking of farms should be allowed only if sentinel swine do not become infected.
The rapid removal of wild boar carcasses from the environment has proved to be important in controlling ASFV transmission in wild boar populations. In all cases, the movement of infected meat and meat products by humans must be avoided.
Key Points
African swine fever (ASF) is a highly infectious, often rapidly fatal viral disease of pigs.
The virus (ASFV) is endemic to sub-Saharan Africa, where it is transmitted from warthogs and bushpigs by soft ticks.
An outbreak of ASF that began in Georgia in 2007 has since expanded to numerous countries in Asia, Europe, and the Caribbean.
Vaccines are becoming available, with promising candidates being tested in field trials and licensed for field use.
For More Information
World Organisation for Animal Health: African Swine Fever
Food and Agriculture Organization of the United Nations: African Swine Fever Prevention, Detection and Control in Resource-Limited Settings
Animal and Plant Health Inspection Service, USDA: African Swine Fever
