African swine fever

Pictures

Picture	Title	Caption	Copyright
Title Symptoms Caption Skin erythema on the ear of a pig affected by acute disease. Copyright ©J.M. Sanchez-Vizcaino (CISA)	Symptoms	Skin erythema on the ear of a pig affected by acute disease.	©J.M. Sanchez-Vizcaino (CISA)
Title External symptoms Caption Reddened skin on the extremities is a non-specific lesion associated with a septicemic/viremic condition. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	External symptoms	Reddened skin on the extremities is a non-specific lesion associated with a septicemic/viremic condition.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption A greatly enlarged dark red to black spleen from a pig infected with a highly virulent ASFV isolate. There are petechial haemorrhages in the renal cortex. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	A greatly enlarged dark red to black spleen from a pig infected with a highly virulent ASFV isolate. There are petechial haemorrhages in the renal cortex.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption Very enlarged dark red (haemorrhagic) gastrohepatic lymph nodes from a pig infected with a highly virulent isolate of ASFV. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Very enlarged dark red (haemorrhagic) gastrohepatic lymph nodes from a pig infected with a highly virulent isolate of ASFV.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Lymph nodes Caption Swine lymph nodes: normal (A), affected by acute African swine fever (B), and affected by hog cholera (C). Swine lymph nodes: normal (A), affected by acute ASF (B), and affected by hog cholera (C). Copyright ©J.M. Sanchez-Vizcaino (CISA)	Lymph nodes	Swine lymph nodes: normal (A), affected by acute African swine fever (B), and affected by hog cholera (C). Swine lymph nodes: normal (A), affected by acute ASF (B), and affected by hog cholera (C).	©J.M. Sanchez-Vizcaino (CISA)
Title Pathology Caption Spleen of swines: acute hog cholera (A), acute African swine fever (B), normal (C). Spleen of swines: acute hog cholera (A), acute ASF (B), normal (C). Copyright ©J.M. Sanchez-Vizcaino (CISA)	Pathology	Spleen of swines: acute hog cholera (A), acute African swine fever (B), normal (C). Spleen of swines: acute hog cholera (A), acute ASF (B), normal (C).	©J.M. Sanchez-Vizcaino (CISA)
Title Histology Caption Swine macrophages infected with ASF virus showing hemadsorption. Copyright ©J.M. Sanchez-Vizcaino (CISA)	Histology	Swine macrophages infected with ASF virus showing hemadsorption.	©J.M. Sanchez-Vizcaino (CISA)
Title Pathology Caption Enlarged dark red renal lymph nodes, petechial haemorrhages in the renal cortex and perirenal oedema. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Enlarged dark red renal lymph nodes, petechial haemorrhages in the renal cortex and perirenal oedema.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption Petechial haemorrhages on the serosal surface are indicative of a viremic/septicemic condition. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Petechial haemorrhages on the serosal surface are indicative of a viremic/septicemic condition.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption Oedema of the gall bladder. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Oedema of the gall bladder.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption Chronic ASF; consolidated lobules in the lung. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Chronic ASF; consolidated lobules in the lung.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title Pathology Caption Enlarged bronchial lymph nodes are part of a generalized lymphadenopathy in chronic ASF. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	Pathology	Enlarged bronchial lymph nodes are part of a generalized lymphadenopathy in chronic ASF.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title External symptoms Caption Necrosis of the skin is a frequent lesion in chronic ASF. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	External symptoms	Necrosis of the skin is a frequent lesion in chronic ASF.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)
Title External symptoms Caption Necrosis of the skin in chronic ASF can be focal; the areas begin as raised hyperemic areas and progress to areas of necrosis. Copyright ©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)	External symptoms	Necrosis of the skin in chronic ASF can be focal; the areas begin as raised hyperemic areas and progress to areas of necrosis.	©USDA-2002/Foreign Animal Diseases Training Set/USDA-Animal and Plant Health Inspection Service (APHIS)

Identity

Preferred Scientific Name

• African swine fever

International Common Names

• English: african swine fever - exotic
• Spanish: peste porcina africana
• French: peste porcine africaine

Local Common Names

• Italy: peste suina africana

English acronym

• ASF
• PSA

French acronym

• PPA

Pathogen/s

Overview

African swine fever (ASF) is a highly contagious disease of pigs, first described in Kenya by Montgomery (1921). It is caused by a large DNA virus (170 to 190 kbp), classified as a unique member of the Asfarviridae family, and is considered by many experts to be one of the most complex viral diseases to affect domestic animals.

The disease is endemic in many African countries south of the Sahara desert. In Europe, it is endemic in Sardinia (Italy), but was successfully eradicated throughout the rest of Europe. However, ASF entered Europe again in 2007, in Georgia, and rapidly spread to neighbouring countries (Armenia, Azerbaijan and Russia), where it is now present without control.

ASF inflicts significant socio-economic impact on affected countries. The ASF virus affects both wild and domesticated pigs of all ages and breeds and is transmitted by soft ticks (Argasidae) of the genus Ornithodoros. Control of the disease is more difficult in outdoor systems than indoors, as this is usually achieved by the control of vectors. No treatment or effective vaccines are available.

This disease is on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's WAHID database on disease occurrence. Please see the AHPC library for further information on this disease from OIE, including the International Animal Health Code and the Manual of Standards for Diagnostic Tests and Vaccines. Also see the website: http://www.oie.int

Host Animals

Systems Affected

Distribution

ASF is present in Africa, particularly south of the Sahara, where the disease is mostly endemic. In Europe, ASF is endemic in Sardinia (Italy). In 2007, ASF entered Eastern Europe. First outbreaks were declared in Georgia, near the port of Poti (Beltrán-Alcrudo et al., 2008) and from there, the disease quickly spread to neighbouring countries (Armenia, Azerbaiyán and Russian Federation). Since the introduction of the disease in the area, hundreds of outbreaks have been reported and thousands of animals have been sacrificed, with huge economic losses associated. ASF is circulating in affected territories without control and this situation implies a risk, specifically for areas close by such as the European Union or China.

During 2011, ASF outbreaks were reported to AU-IBAR by 22 countries (see table below) with a total of 471 affected epidemiological units involving 144,950 cases, 135,712 deaths and a case fatality rate of 93.6%. Significantly, the Democratic Republic of Congo registered the highest number of outbreaks (84) accounting for about 17.8 % of the reported outbreaks and 79.4% of mortalities (AU-IBAR, 2011).

Although ASF was reported throughout the year in Africa, the highest number of outbreaks was recorded in May and January with 57 and 51 outbreaks, respectively.

Countries reporting African swine fever to AU-IBAR in 2011 

NS: Not specified

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Pathology

Post-mortem lesions and histopathological findings in ASFV infections vary widely depending on the virulence of the virus isolate and the species of pig infected; African wild pigs do not normally show lesions (Oura et al., 1988). Lesions are indistinguishable from those of hog cholera.

Three forms of ASF have been described: acute, sub-acute and chronic.

Acute form

The acute form of SAF is characterised by extensive haemorrhages in lymph nodes (mandibular, renal and gastro-hepatic), spleen and kidney and occasionally in the heart. Lymph nodes look like a red dark haematoma with oedema and a friable consistency. The spleen may show congestive splenomegaly when it is dark, enlarged, infarcted and friable. The kidneys usually have petechial haemorrhages of the renal cortex, in the medulla and renal pelvis. An intensive hydropericardium of sero-haemorrhagic liquid, and petechiae in the epicardium and endocardium are also frequently observed. Other lesions include petechiae in the mucous membrane of the urinary bladder, larynx and pleura. Congestion in the liver, fluid in the abdominal cavity and hydrothorax are also frequently observed (Sanchez Botija, 1982; Gomez-Villamandos et al., 1996).

Sub-acute form

The lesions observed in the sub-acute form of ASF are mild versions of those described for the acute form; that is large haemorrhages in lymph nodes and kidneys. An enlarged and haemorrhagic spleen, and congested and oedematous lung and in some cases an interstitial pneumonia are the lesions most frequently observed in the sub-acute form (Mebus et al., 1983).

Chronic form

The chronic form is characterised by enlarged lymph nodes and spleen, pleuritis and fibrous pericarditis. Focal caseous necrosis and mineralization of the lung have also been described (Mebus et al., 1983).

Diagnosis

Introduction

Due to the great similarity of the clinical signs and lesions of ASF and those of other haemorrhagic pig diseases, laboratory diagnosis is an essential prerequisite for correct diagnosis (Sánchez-Vizcaíno, 2006).

Clinical Diagnosis

The clinical presentation varies depending on the virulence of the virus, the route of exposure, dose of virus and the species of pig infected, normally wild boar are more resistant (Sánchez-Vizcaíno, 2006).

Three forms of ASF have been described: acute, sub-acute and chronic.

The isolate currently circulating in the Russian Federation and Caucasus region belongs to genotype II (Rowlands et al., 2008) and only induces acute forms of the disease. Other forms of the disease can be observed in Sardinia (Italy) where ASF virus genotype I is circulating or in Africa where all the 22 ASF virus genotypes are circulating (Sánchez-Vizcaíno, 2006).

Acute form

Acute infections are characterised by high mortality (90-100%), fever (40-42ºC), leucopaenia and thrombocytopaenia. Reddening of the skin at the tips of ears, and chest and abdominal areas are frequently observed in white pigs. Vomiting and haemorrhagic diarrhoea may be observed. Abortion in pregnant sows is frequently described. One or two days before dying, the affected animals usually present anorexia, listlessness, cyanosis and incoordination (Mebus et al., 1983).

Sub-acute form

Sub-acute infections are produced by moderately virulent virus isolates, which present similar but less intense symptoms than those in the acute form. The mortality of the sub-acute form is approximately 30-70%, depending on the virus isolate. Illness and abortion is frequently observed.

Chronic form

Chronic infections result in low mortality (2-10%). Symptoms include weight loss, respiratory problems, arthritis, chronic skin ulcers or necrosis.

Lesions

See Pathology.

Differential Diagnosis

Differential diagnosis should consider the following diseases:

• classical swine fever or hog cholera


• erysipelas


• salmonellosis


• pasteurellosis

Laboratory Diagnosis

The samples that should be collected for ASF laboratory diagnosis are lymph nodes, kidneys, spleen, lung, blood and serum. Tissues are used for virus isolation (HA test) and viral antigen detection (PCR techniques and DIF test), while blood is used for virus isolation and PCR. Tissue exudates and serum are used for antibody detection by: IIF, ELISA or IB.

A wide variety of laboratory tests are available for ASF viral DNA and antibody detection (Sánchez-Vizcaíno, 2006). The most convenient, safe and specific tests for virus detection are: PCR techniques (both real time and conventional) (King et al., 2003; Agüero et al., 2004), direct immunofluorescence (DIF) (Bool et al., 1969) and haemadsortion (Malmquist and Hay, 1960).

The detection of the ASF virus genome by PCR has been developed with the use of sets of primers from a highly conserved region of the viral DNA, to detect all range of ASF isolates belonging to all the known virus genotypes including both non-haemadsorbing viruses and low virulence ones. This test is particularly useful for viral DNA identification from tissues which are poorly conserved, even if they have undergone putrefaction or the virus has been inactivated. It is an excellent and relatively rapid technique (results could be obtained in 5 hours) for ASF diagnosis, and is the most frequently technique used in worldwide laboratories for ASF virus detection. It needs good training and good laboratory practices to avoid contaminations and false positive results . Two types of PCR are validated by OIE for ASF diagnosis, conventional PCR (Agüero et al., 2003) and real-time PCR (King et al., 2003).

Direct immunofluorescence (DIF) is based on the detection of viral antigen in impression smears or frozen tissue sections with a fluorescein labelled immunoglobulin directed against the ASF virus. It is a very rapid (one hour) and economic test with high sensitivity to the acute form of ASF. However, for sub-acute or chronic infections, the DIF test has a sensitivity of only 40%. This decrease in sensitivity seems to be related to the formation of antigen-antibody complexes, which do not facilitate the reaction with the ASF conjugate (Sánchez-Vizcaíno, 1986). The use of DIF together with an indirect immunofluorescence test (IIF) makes it possible to detect 85 to 95% of all ASF cases (acute, sub-acute and chronic) in less than three hours (Sánchez-Vizcaíno, 1986).

The haemadsorption test (HA) is a technique used as the gold standard test for ASF virus identification due to its sensitivity and specificity. This test is usually only performed in ASF reference laboratories in order to confirm any new outbreak and when other tests have yielded negatives. HA is based on the haemadsorption characteristics that most ASF virus isolates induce when pig macrophages are infected in the presence of erythrocytes. A characteristic rosette around the infected macrophages develops before the cytopathic effect appears. A small number of field strains have shown cytopathic effect without producing the haemadsorption phenomenon (Sanchez Botija, 1982); these strains are identified using the DIF test on the sediments of these cell cultures. HA is relatively economical but access to ASF-free pigs and sterile facilities are also needed.

The lack of an effective vaccine against ASF virus and the long duration of the specific ASF-IgG reaction in infected pigs (detectable in blood on days 6-10 post-inoculation and subsequently for protracted periods, even years) has made the study of ASF antibody reactions an important prerequisite for the detection of sub-acute and chronic forms of ASF, and for ASF eradication programs (Arias and Sánchez-Vizcaíno, 2002). Several techniques have been adapted for ASF antibody detection, but the most common, practical and inexpensive tests are ELISA (Sánchez-Vizcaíno et al.,1986), immunoblotting (IB) (Pastor et al., 1987) and IIF (Bool et al., 1969).

The IIF test is a fast technique and has high sensitivity and specificity for the detection of ASF antibodies from either sera or tissue exudates (Sanchez Botija et al., 1970). It is based on the detection of ASF antibodies that bind to a monolayer of cell lines (by MS) infected with an adapted ASF virus. The antibody-antigen reaction is detected by a fluorescein labelled protein A. ELISA testing is the most useful method for large-scale serological studies. It is highly sensitive and specific, simple, rapid and economic and commerical kits are available. It is based on the detection of ASF antibodies bound to the viral proteins which are attached to a solid phase by addition of protein A-conjugated with an enzyme that produces a visible colour reaction when it reacts with the appropriate substrate.

The IB test is a highly specific, sensitive and easy to interpret technique which has been successfully used as a confirmatory method to IIF for low or doubtful ELISA sera (Sánchez-Vizcaíno, 2006).

Immunology

The immune response to ASF virus infection is still poorly understood. The main difficulty encountered has been the lack of neutralising antibodies and the great variability of the virus isolates. These characteristics made the production of an effective vaccine impossible to date.

Monocytes and macrophages are the main target cells for ASF virus replication; no evidence of virus replication in T or B lymphocytes has been observed (Minguez et al., 1988; Gomez-Villamandos et al., 1995). However, a lymphopenia, due to apoptosis of lymphocytes, mainly in the T area of the lymphatic organs, have been described (Carrasco et al., 1996).

Another characteristic of the response to ASFV is the lack of viral neutralisation. ASF virus-specific neutralised antibodies have never been demonstrated to entirely fulfil the classic definition of antibody neutralisation. It has proved impossible to neutralise 100% of the homologous virus, even with sera from recovered animals infected with attenuated ASF isolate; in this case a 10% fraction of non-neutralising virus is observed to persist (Ruiz et al., 1986). In contrast, animals that have recovered from ASFV infections can produce neutralised antibodies to foot and mouth virus (De Boer, 1967), and T cytotoxic lymphocytes (CD 8+) from recovered pigs are able to destroy macrophages infected with ASFV (Martin and Leitao, 1994).

ASF virus is very antigenic, inducing antibodies to a great number of viral proteins. IgM is detectable in animal sera at 4-6 days post inoculation and IgG is detectable at 6-9 days post inoculation with low or medium virulent ASF isolates (Sánchez-Vizcaíno, 2006).

Humoral and cell-mediated immunity does not seem to be affected in pigs infected with ASF virus.

List of Symptoms/Signs

Disease Course

ASF virus normally infects the pig through the oral or nasal passages, but infection may also occur by cutaneous scarification or by intramuscular, subcutaneous, intraperitoneal or intravenous injections and by the bite of an infected tick (Colgrove et al., 1969; Plowright et al., 1969; McVicar, 1984). Primary virus replication begins in monocytes and macrophages of the lymph nodes near the site of infection; in oral infection, replication starts in the tonsils and mandibular lymph nodes. After initial replication the virus spreads through the blood (associated with the erythrocyte cell membrane) and/or lymphatic vessels to reach the target organs, where secondary replication takes place. The organs most usually affected are the lymph nodes, bone marrow, spleen, kidney, lungs and liver.

The incubation periods of natural or experimental infection vary widely depending on the virus isolate, route of infection and quantity of virus inoculated. This period will vary in natural infections between 4-8 days for the shortest incubation period and 15-19 days for the longest. In experimental infections, the incubation period is usually shorter than in natural infections, varying from 2 to 5 days.

Viremia in ASF infection generally starts 6-8 days post-infection, and due to a shortage of neutralizing antibodies may remain for a long time, even several months. Antibodies are detectable in sera and tissue exudates 7-10 days post-infection. In sera, they may be present for long periods, sometimes for more than a year post-infection.

Epidemiology

The ASF virus is found only in wild and domestic pigs and a number of soft ticks, mainly Ornithodoros moubata in Africa (Plowright et al., 1970) and Ornithodoros erraticus in the Iberian peninsula (Sanchez Botija, 1963). Ornithodoros corinaceus, a tick indigenous to the USA, has also been found to harbour and transmit ASF virus in experimental settings (Groocock et al., 1980), as has Ornithodoros savignyi which is present in Africa (Mellor and Wilkinson, 1985).

Some epidemiological differences have been observed between the transmission of ASF virus in Africa and Europe. In East and South Africa, ASF virus usually induces a non-apparent infection in three wild boar species: warthog (Phacochoerus aethiopicus), giant forest hog (Hylochoerus meinertzhageni) and bushpig (Potamochoerus porcus). Infection is characterised by low levels of virus in the tissues and low or undetectable levels of viremia. In the case of P. porcus, viremia has been observed between 35 and 91 days following infection and the virus can persist in lymphatic tissues for 34 weeks (Anderson et al., 1998). Viral infection normally moves from these animals to domestic pigs through a biological vector, Ornithodoros moubata, and not by direct transmission. In East and South Africa, ASF virus infection is maintained by a cycle of infection between wild boars and ticks, and only when domestic pigs are present is disease observed. In contrast, the European reservoir hosts is the wild boar (Sus scrofa), which is susceptible to ASF infection, exhibiting clinical symptoms and mortality similar to those observed in domestic pigs (Contini et al., 1982; Sanchez Botija, 1982). Similar results were observed in an experimental infection by ASF virus in feral pigs in Florida, USA (McVicar et al., 1981). Direct transmission by contact between sick and healthy animal is the most common mode of transmission, but in Europe indirect transmission by biological vectors, such as Ornithodoros erraticus, has been described in the Iberian Peninsula, especially in outdoor-reared pigs (Arias and Sánchez-Vizcaíno, 2002).

ASF virus infections in the African vector Ornithodoros moubata are transmitted by transovarial and transtadial routes, whilst only transtadial transmission has been observed in the European vector Ornithodoros erraticus.

Impact: Economic

ASF is classified on OIE list A, which by definition contains only diseases that have the potential for very serious and rapid spread, producing serious socio-economic consequences (OIE, 1999). As there is no vaccine or treatment available for ASF, the identification and slaughter of sick and carrier animals is crucial to the control of the disease. The last five years (1985-1990) of the ASF Spanish eradication programme cost approximately US $92 million.

Disease Treatment

No effective treatment or vaccine against ASF virus is yet available. Live-attenuated vaccines that have been widely used experimentally, protect some animals against challenge infection with a homologous virus, but not with a heterologous one and most of them become carriers with ASF virus in several lymph nodes. Inactivated vaccine or viral protein vaccine does not appear to induce any protection. The role that some ASF virus genes may play in the modulation of protection is already being researched. These studies are opening new opportunities for the production of an ASF virus vaccine, but until now the only treatment for ASF is eradication, based on the control of animal transport and vectors as well as the early detection and slaughter of infected and carrier animals.

Prevention and Control

As no vaccine for ASF is available, the control of this disease is based on rapid laboratory diagnosis and the enforcement of strict sanitary measures. Depending on the epidemiological status of disease in a particular region, different measures are recommended.

Epizootiological studies have shown that the most frequent source of ASF contamination in infection-free countries is refuse from international airports or ports. All leftover food from aeroplanes and ships should be routinely incinerated or efficiently sterilised. Import policy for animals and animal products should consider the disease status of the exporting nation. In infected European areas such as Sardinia (Italy) where the disease is enzootic and where mild or non-apparent clinical signs can be observed, the most important aspects of ASF prevention are the control of animal movement and the use of extensive serological surveys to detect carrier pigs. In endemic areas of Africa, the most important factor is to control the natural tick vectors and wild pig reservoirs, and/or limit their contact with domestic pigs.

During disease outbreaks, the rapid and efficient slaughtering of all pigs and the proper disposal of carcases and all waste material is critical. Other important aspects to consider are the cleaning and disinfecting of affected farms, designation of the infected area and increased control of animal movements. Serological surveys should be undertaken in the surrounding area. In the presence of any suspicious haemorrhagic pig disease, a differential laboratory diagnosis should be undertaken; as low virulence ASF strains do not produce significant lesions.

References

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