A highly contagious viral disease of domestic pigs, ASF manifests itself as a haemorrhagic fever and results in up to 100 percent mortality. The catastrophic effect of this disease on pig production, from household to commercial level, has serious socio-economic consequences and implications for food security. It is a serious transboundary animal disease with the potential for rapid international spread.
First described by Montgomery in 1921 in Kenya, ASF has subsequently been reported from most countries in southern and eastern Africa, where the virus is maintained either in a sylvatic cycle between warthogs (Phacochoerus aethiopicus) and ticks of the Ornithodoros moubata complex or in a domestic cycle that involves pigs of local breeds, with or without tick involvement. Countries where endemicity is confined to the sylvatic cycle include Kenya, Namibia, Botswana, Zimbabwe and northern South Africa. A cycle in domestic pigs apparently occurs in Angola, the Democratic Republic of the Congo, Uganda, Zambia, Malawi, northern Mozambique and probably the Congo (Brazzaville), Rwanda, Burundi and Tanzania. Madagascar experienced ASF for the first time in 1997–98; it caused serious losses and has not yet been eradicated.
The disease spread to Portugal in 1957, almost certainly from Angola. Although it was apparently eradicated, a second introduction in 1959 resulted in spread throughout the Iberian peninsula and to several other countries in Europe, including France, Italy, Malta, Belgium and the Netherlands. ASF became well established in Spain and Portugal, where eradication was only accomplished in the early 1990s and remains endemic on the Italian island of Sardinia. Portugal experienced an outbreak in late 1999, which was evidently rapidly contained.
In 1977, ASF spread to Cuba, where it was eradicated with the loss of some 400 000 pigs. Outbreaks occurred in Brazil and the Dominican Republic in 1978, Haiti in 1979 and Cuba in 1980. Eradication from these countries was achieved only by massive depopulation of pigs. Whether these outbreaks originated in Europe or Africa has never been established.
In West Africa, ASF has been endemic in Cameroon since the first reported outbreaks in 1982. It is endemic in southern Senegal, the Gambia and probably Guinea Bissau and the islands of Santiago and Maio in the Republic of Cabo Verde. The disease has been present in this focus since at least 1958–60. An outbreak of ASF occurred in Nigeria in 1973. In 1996, Côte d'Ivoire experienced a massive outbreak that spread rapidly through the southern parts of the country. The last focus was extinguished by October 1996. In October 1997, ASF was reported in Benin, rapidly followed by Togo and two western provinces of Nigeria. Spread in these countries was rapid. In October 1999, ASF was reported in Ghana. Rapid implementation of control measures has apparently been successful, as no ASF has occurred since February 2000. Because of civil unrest in various regions and lack of disease reporting from some countries, the ASF status of a number of countries in Africa is unknown. All of the countries in sub-Saharan Africa that have significant pig populations must be considered to be infected, potentially infected or at risk from ASF.
The cause of ASF is a unique DNA virus that was formerly classified in the family Iridoviridae because of morphological similarities. Now considered to be more akin to members of the Poxviridae, it is currently the sole member of a family of ASF-like viruses, the Asfaviridae. It is unusual among the DNA viruses in behaving as a true arbovirus, able to multiply in both vertebrate and invertebrate hosts. There is a single serotype. Using restriction length fragment polymorphism and nucleotide sequencing techniques, numerous strains of ASF virus of varying virulence have been detected.
Susceptible species
Only species of the pig family (Suidae) are susceptible to infection with ASF virus.
Domestic pigs are highly susceptible to ASF, which shows no breed, age or sex preference. Certain populations of pigs of local races in central Africa demonstrate a higher-than-expected survival rate during ASF outbreaks. A high proportion of the pigs in these populations are serologically positive for ASF and apparently healthy. This suggests that these pigs, which are derived from pigs introduced into Africa some 400–500 years ago, probably from the Iberian peninsula, may have a degree of genetic resistance to the virus.
All wild African suids are susceptible to infection with the virus but do not develop clinical disease. Warthogs are the major host for ASF virus. Bushpigs (Potamochoerus porcus and P. larvatus) and giant forest hog (Hylochoerus meinertzhageni) have been found to be infected with ASF virus but the extent of infection and their role in the epidemiology of the disease are unknown.
European wild boar (Sus scrofa) are fully susceptible to ASF, with a mortality rate similar to that of domestic pigs. Feral pigs in the American region, probably partially derived from European wild boar, have been shown to be highly susceptible to experimental infection, as have farmed descendants of European wild boar and domestic pigs in South Africa. The susceptibility of other wild suids in areas where ASF does not occur has not been investigated, with the exception of the collared peccary (Tayassu tajaccu), which proved completely resistant.
Human beings are not susceptible to ASF.
Virus survival
In the environment. ASF virus, in a suitable protein environment, is stable over a wide range of temperature and pH. It has been shown to survive in serum at room temperature for 18 months, in refrigerated blood for six years and in blood at 37°C for a month. Heating at 60°C for 30 minutes will inactivate the virus. In the laboratory, ASF virus remains infective indefinitely at - 70°C but may be inactivated if stored at - 20°C. In the absence of a protein medium, viability is greatly reduced. ASF virus is generally stable over a pH range of 4–10 but in a suitable medium (serum) has been shown to remain active at lower and higher values for between a few hours and three days. Putrefaction does not necessarily inactivate the virus, which may remain viable in faeces for at least 11 days, in decomposed serum for 15 weeks and in bone marrow for months. On the other hand, culture of virus from decomposed samples is frequently unsuccessful.
As a result of its tolerance to a wide range of environmental factors, only certain disinfectants are effective in the control of ASF.
In the host. After infection with ASF virus, domestic pigs may shed infective amounts of virus for 24–48 hours before clinical signs appear. During the acute stage of disease, enormous amounts of virus are shed in all secretions and excretions and high levels of virus are present in tissues and blood. Pigs that survive the acute disease remain infected for several months but do not readily shed virus for more than 30 days. As in wild suids, infective levels of virus are found only in lymph nodes; other tissues are unlikely to contain infective levels of virus for more than two months after infection. The exact length of time over which infective levels of virus are maintained in lymphoid tissues in either wild suids or domestic pigs is unknown and is probably subject to considerable individual variation.
In animal products. The ability of ASF virus to remain infective in edible products such as chilled meat (at least 15 weeks) and three to six months in processed hams and sausages that have not been cooked or smoked at a high temperature has important implications for spread of ASF. Undercooked pork, dried and smoked pork and carcass meal derived from pigs must be regarded as potentially dangerous if fed to pigs.
Disease transmission. In the sylvatic cycle between warthogs and argasid ticks of the Ornithodoros moubata complex, transmission occurs between ticks and neonatal warthogs, among ticks and between ticks and domestic pigs. Adult warthogs, even if they have infective levels of ASF virus in lymph nodes, do not shed virus or develop viraemia sufficient to permit infection of ticks that feed on their blood. It has been demonstrated that transmission of ASF virus between ticks and warthogs probably occurs exclusively during the first four to six weeks of life, when the young warthogs spend most of their time in the burrows, which are inhabited by large numbers of ticks. Infected ticks feeding on baby warthogs transmit infective levels of virus in their saliva, which acts as an anticoagulant, to cause viraemia in the warthogs sufficient to infect other ticks. Even at this stage, the young warthogs show no signs of disease. Among ticks, ASF virus is transmitted transovarially, transstadially and sexually from males to females via the spermotheca. Although Ornithodoros spp. generally feed rapidly and drop off the host, relatively large numbers of particularly nymphal ticks have been found on warthogs shot far from their burrows. Transmission to domestic pigs from warthogs is believed to occur mostly via infected ticks that drop off warthogs in their vicinity. Since warthogs inhabit savannah regions that are generally fairly dry, they are attracted to the food and water sources available to domestic pigs. Transmission via feeding the remains of warthogs to pigs is frequently suggested as the source of an outbreak but this has proved difficult experimentally. The sylvatic cycle predominates in eastern and southern African countries where warthogs and Ornithodoros moubata occur and where pig production is at a low level or of a modern, intensive nature. Although warthogs occur widely in the savanna areas of West Africa, the presence of Ornithodoros ticks has not been demonstrated and the outbreaks have occurred outside the warthog distribution areas. Because of excessive hunting, bushpigs are regarded as virtually extinct and therefore an unlikely reservoir for ASF virus.
The endemic cycle in domestic pigs that occurs over large parts of central Africa has not been fully elucidated. In some areas, particularly in Malawi, ticks of the Ornithodoros moubata complex, which inhabit human dwellings and the shelters in which pigs are kept at night, are involved in a cycle of transmission to domestic pigs that have a higher than expected survival rate. In other areas, the occurrence of Ornithodoros has not been proven. In the Iberian peninsula, Ornithodoros erraticus contributed significantly to ASF endemicity. This species occurs in North Africa as far south as Dakar but has not been shown to occur in the Gambia, southern Senegal, Cabo Verde or Sardinia. A number of species of Ornithodoros that occur in the Caribbean and North America are capable of maintaining and transmitting ASF virus but ticks were apparently not involved in the Caribbean outbreaks of ASF. Although carrier pigs apparently shed virus for only short periods after infection and although transmission by feeding of tissues from chronically infected animals is apparently short-lived, some mechanism must exist for the maintenance and transmission of disease in domestic pigs where the tick vectors do not occur. Investigation of large numbers of ectoparasites-including pig lice, mange mites and ticks other than Ornithodoros that feed on pigs, such as Rhipicephalus- has revealed their inability either to maintain ASF virus or transmit it mechanically. Only stable flies of the genus Stomoxys have been shown to maintain and transmit infective levels of virus for 24–48 hours.
During an epizootic, transmission is by direct contact between infected pigs and their secretions and excretions. Infection generally occurs via the oronasal route. Aerosol transmission has been shown to occur only over very short distances. Spread via fomites - contaminated vehicles, equipment and clothing - is likely when there are high levels of environmental contamination. Iatrogenic spread via contaminated needles is likely, as attempts may be made to vaccinate against classical swine fever (CSF) or to treat for bacterial diseases such as erysipelas without inadequate sterilization or replacement of needles. Although waste disposal is often via rivers and other bodies of water, waterborne transmission is most unlikely because of dilution of the virus. When waterways are used for disposal of carcasses, however, transmission through carrion feeding is highly likely. It has been shown that pig sties in tropical countries do not remain infective for more than three to four days, even in the absence of cleaning and disinfection, but high levels of ASF virus may persist in protein-rich, most environments such as slurry.
FIGURE 1
Scavenging animals
Scavenging animals are major concern during an emergency disease outbreak, because they
can catch and/or spread disease easily.
Swill feeding, in particular swill originating from aircraft and ships, has been incriminated as a major source of infection. Swill that consists of or contains large amounts of infected pork has a high potential for spreading infection and has probably contributed to many of the outbreaks that have occurred. Scavenging of offal and remnants of infected pork discarded during preparation for human consumption is probably more significant in areas where national dishes are subjected to lengthy cooking. When an outbreak occurs, large amounts of infected pork become available as pigs die. Surplus meat may be dried or subjected to other processes that do not inactivate the virus and pigs are moved rapidly in attempts to avoid disease and evade uncompensated compulsory slaughter. It is likely that in Africa, at least, the potential to move live pigs infected over long distances is grossly underestimated. The incubation period varies from 5 to 15 days. Clinical disease is usually peracute or acute. Subacute or chronic manifestations of ASF may occur, particularly when less virulent strains are involved, but have rarely been described in Africa.
High mortality among pigs of all ages is a major indicator for ASF.
Peracute ASF
Pigs are usually found dead without premonitory signs. Recumbency, accompanied by high fever, indicated by flushing of the ventral area and extremities in white-skinned pigs, shade seeking, huddling together and rapid shallow breathing may be observed in some animals before death.
Acute ASF
Pigs develop a persistent fever of up to 42°C. They become listless and anorexic, huddle together, seek shade and sometimes water and are reluctant to move. White-skinned pigs become flushed to cyanotic, particularly the ears, lower legs, and ventral abdomen. Mucopurulent ocular and nasal discharges may be evident. Signs of abdominal pain such as arching of the back, uncomfortable movements and flank kicking may occur. Vomiting is common and pigs may develop either constipation, with hard small faeces covered in blood and mucus, or bloody diarrhoea, with soiling of the tail and perineum. Ataxia due to hind-limb weakness usually develops. Difficult breathing, sometimes with froth that may be bloody at the mouth and nostrils, often occurs and is indicative of the lung oedema that is often the primary cause of death. Pigs that survive longer may develop nervous signs, including convulsions. Pinpoint to larger haemorrhages may be visible on the mucosa and skin. Abortions may occur at any stage of pregnancy. Duration of clinical signs is generally short - two to seven days - but may be longer and apparent recovery may be followed by relapse and death. Mortality approaches 100 percent. Pigs that do recover from acute infection are generally asymptomatic. Subacute and chronic forms of ASF were common in Europe and the Caribbean but are rarely seen in Africa, although there are early descriptions of chronic disease in Angola.
Subacute ASF
Pigs that survive longer, usually after infection with less virulent strains, may have a fluctant fever and usually lose condition. An interstitial pneumonia is usually present, which may result in respiratory distress and moist coughing. Secondary bacterial infection may occur. Joints may be painful and swollen. Death may occur after a variable period of weeks to months, or the pigs may recover or progress to the chronic from of the disease. Cardiac damage may result in death from acute or congestive heart failure.
Chronic ASF
Chronically infected pigs are usually severely emaciated and stunted, with a long dull hair coat. Signs of pneumonia may be present, as well as lameness and ulcers over bony points. These pigs are subject to secondary bacterial infections. They may survive for several months but recovery is unlikely.
Gross Pathology
Pigs that die of peracute ASF may show few gross lesions, apart from the blood splashing and mild accumulation of fluid in body cavities that usually accompany sudden death.
In acute ASF, the carcass is often in good condition. In white-skinned pigs, the extremities and the ventral surface may be cyanotic and subcutaneous haemorrhage may be evident. Mucosa are often congested to haemorrhagic. When the carcass is opened, straw-coloured to blood-coloured fluid may be present in body cavities. Organs are generally congested and haemorrhages may be evident over serosal surfaces. Pinpoint haemorrhages are often present in the renal cortex, over the splenic capsule and in the lungs, with larger haemorrhages often occurring on the epi-and endocardium and on the gastro-intestinal serosa. The spleen is slightly to considerably enlarged, soft and dark, with rounded edges. Peripheral infarcts may be present; in these cases the spleen is generally only moderately enlarged. Lymph nodes, particularly the gastrohepatic, mesenteric, renal and submandibular lymph nodes, are enlarged and severely haemorrhagic; they often resemble blood clots. The mucosa of the stomach is often deeply congested to haemorrhagic and sometimes necrotic; haemorrhage may be present in the gall bladder and the urinary bladder. The lungs do not collapse and are enlarged due to the accumulation of fluid, so that interlobular septa are prominent. Fluid and froth ooze on cut surfaces and the trachea is often filled with froth, which may be bloody.
The main features of subacute and chronic ASF are loss of condition to emaciation, interstitial pneumonia and enlarged lymph nodes, which may be firm and fibrous in the chrnic form of the disease.
Histopathology
Pathological changes are ascribed to the effects of the virus on macrophages, which result in massive destruction of these cells accompanied by release of cytokines.
The most striking histopathological feature of ASF is massive karyorrhexis in lymphoid tissues, often accompanied by haemorrhage. The S-S (Schweiger-Seidel) sheaths of the spleen are virtually obliterated. Blood vessel walls, especially in the lymphoid tissues, often exhibit fibrinoid change resulting from necrosis of the endothelium and leakage of inflammatory mediators. Other changes include interstitial pneumonia with accumulation of fibrin and macrophages, renal tubular degeneration with hyaline droplet absorption, infiltration of portal tracts in the liver with macrophages and lymphocytic meningoencephalitis.
Antibodies against ASF are detectable in serum 7–12 days after clinical signs appear and persist for long periods, possibly for life, in both warthogs and domestic pigs. They do not protect fully against subsequent infection in domestic pigs, although a degree of immunity to infection with homologous strains of virus has been reported. Serologically positive sows transmit antibodies to piglets in colostrum. In subacutely and chronically infected pigs, virus replication continues in the presence of antibodies. The deposition of immune complexes in tissue may account for many of the lesions observed in these forms of disease.
Since no vaccine is available for ASF, the detection of antibodies in pigs can be confidentally attributed to exposures to natural infection. There are no known serological cross-reactions with other viruses.
Field diagnosis
Unusually high mortality among pigs of all age groups should lead to a strong suspicion of ASF. Additional indicators are the typical clinical signs and lesions of ASF, failure to respond to antibiotic treatment and the fact that no other livestock species are involved. There are relatively few serious diseases of pigs that affect other species and the omnivorous habits of pigs make it likely that they will be the first victims of malicious or accidental poisoning. Laboratory confirmation of ASF is essential.
Differential diagnosis
Hog cholera, or CSF, is the most important differential diagnosis for ASF. Clinical signs and gross lesions may be identical and such minor differences as have been described are not pathognomic or consistent. Lesions such as button ulcers at the ileocaecal junction described in CSF are far from frequent and spelenic infarction possibly has a similar incidence in both diseases. Laboratory diagnosis is therefore absolutely essential in any case of suspected swine fever.
Several other diseases that may be confused clinically with ASF are given below.
FIGURE 2
Sampling
Taking blood samples, seen here in Cabo Verde in 1999, is a routine necessary in ASF
diagnosis and surveillance.
Cases of subacute and chronic ASF are difficult to distinguish from other causes of pigs failing to thrive; if these are caused by strains of lower virulence, diagnosis may be very difficult. If they are simply manifestations of disease in surviving pigs, the herd history should indicate that high mortality with appropriate clinical signs and lesions occurred in the herd.
Laboratory diagnosis
Laboratory confirmation of a presumptive diagnosis of ASF depends upon detection of the virus or detection of antibodies. Since most pigs die of acute ASF before antibodies are produced, detection of the virus is the most important method of diagnosis.
Detailed instructions for laboratory diagnostic procedures for ASF are to be found in the OIE Manual of standards for diagnostic tests and vaccines. The following is a summary, with the emphasis on tests that are usually used.
Collection and transport of diagnostic specimens. Preferred samples for virus isolation/antigen detection are:
To detect antibodies, blood samples should be collected in red-topped tubes (i.e. without anticoagulant). Various methods of collecting blood using filter paper strips or capillary tubes are available but in practice these samples are difficult to handle correctly.
A range of tissues - spleen, lymph nodes, lung, liver, kidney and brain- may be collected in 10 percent buffered formalin for histopathological examination and detection of virus by immuno-peroxidase.
Whole blood and unpreserved tissue samples should be chilled and transported on water ice or frozen gel packs. If a break in the cold chain is likely or chilling is primarily impossible, the addition of 50 percent glycerosaline will provide adequate preservation while enabling viral culture. The addition of antibiotics - 200 units of penicillin and 200μg/ml streptomycin - will prevent bacterial growth. The addition of formol-glycerosaline will permit detection of viral DNA but will not permit culture. Freezing is not recommended if culture is intended, as ASF virus may be inactivated at - 20°C.
Serum samples should be centrifuged if possible or the clot removed before transport. After collection, blood samples destined for serology should be allowed to stand at room temperature for at least sufficient time for clotting before refrigeration. If the tubes are placed stopper-down, the blood clot can easily be removed with the stopper, and the stopper then replaced. The samples are then submitted on ice as described for tissue samples or they may be frozen.
Unpreserved diagnostic samples should be placed in a strong watertight container, generally a plastic screw-topped jar or, in the case of blood or serum, a vacutainer. This is wrapped in absorbent material, placed in a strong leakproof secondary container, usually a plastic or styrofoam cold-box, and finally in a solid outer covering. The package is then labelled with waterproof ink for despatch to a national or international reference laboratory. If samples are being transported in hot climate conditions from the field to a national laboratory, it is advisable to acquire a cold-box. Ice may be available in villages; if ice is not available, bottled refrigerated drinks or chilled plastic bags of water for drinking can be used to provide additional cooling in the box during transport. When samples are sent by air, International Air Transport Autority (IATA) rules should be followed. Information about the carrier, airway bill number and time of arrival should be sent ahead to the laboratory.
All specimens should be accompanied by basic information: name of owner, locality, brief history (number and dates of pig deaths, ages of pigs, clinical signs), date of collection, disease suspected and tests required. If several samples are submitted, each should be labelled or given a number in waterproof ink referring to the accompanying information,.
Laboratory diagnosis should only be attempted by trained personnel in well equipped laboratories.
FIGURE 3
Diagnostic capacity
It is essential to have appropriate diagnostic capacity during emergency disease outbreaks, in
order to identify the causative agent(s) immediately.
Virus isolation. Isolation should only be attempted in well equipped laboratories in countries where pigs guaranteed free of ASF are readily available, in order to obtain the materials and resources to maintain capacity in the absence of field samples.
ASF virus may be isolated by the methods given below.
Antigen detection. The following tests may be used:
Detection of viral genetic material. A polymerase chain reaction (PCR) test is available for ASF. PCR is a highly sensitive and specific technique but because of the possibility of cross-contamination, its use is in practice confined to laboratories that have this capacity for other diseases and a considerable degree of sophistication. Expense is another factor.
Antibody detection. Serological tests for ASF include:
