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African Swine Fever (ASF)

(534) African swine fever (ASF) resembles classical swine fever (CSF) (hog cholera) so closely that laboratory tests are required to differentiate them. The clinical signs and post-mortem lesions of the two diseases are almost indistinguishable. ASF is caused by a unique virus which is distinct from that of CSF and which infects only domestic and wild pigs and a variety of soft bodied ticks. The virus is endemic in Africa south of the equator, in warthogs and bush pigs, but the infection in them produces no clinical disease. It circulates between warthogs and the soft bodied ticks which inhabit their burrows. The ticks transmit it through all stages of their life cycle and perpetuate it. It is also endemic in the domestic pigs of some African countries.

ASF virus is relatively tough and can survive in the environment and in pig carcasses for a long time. Curing and smoking pork products does not destroy it. Its main method of spread from country to country is via waste uncooked pork products fed to pigs. Mostly, the waste products have come off aeroplanes or ships but in one case at least it was waste food given to a boar by a returning tourist which resulted in twelve herds being infected and slaughtered. Its spread between herds within a country is by direct and, to a lesser extent, indirect contact between pigs. Indirect contact usually involves contamination from dead pig tissues and secretions.

Importance of ASF

When virulent strains cross into domestic pigs they cause very serious disease. Virtually all the pigs in the infected herd become ill and the majority die. It is not surprising that all countries regard it as important. There is no vaccine for it and those European, South American and Caribbean countries which have been infected have adopted a slaughter policy to eradicate it. Mild strains of the virus also occur which cause a milder but still serious disease in domestic pig herds.

It was carried from Africa to Portugal in 1957 and again in 1960. It spread through Portugal and into Spain. Outbreaks then appeared in France, Italy, Belgium, the Netherlands, Malta, Cuba, Brazil, Haiti and the Dominican Republic. It has been eradicated by slaughter from all of these except the Italian island of Sardinia where pigs are kept free-ranging to forage for food. There are no ticks that transmit it there and although it is a virulent strain which causes severe disease in wild boars on the island they are not a major factor in its spread. At the time of writing there are a few cases still occurring in Sardinia and by the time you read this it may have been eradicated.

Should you be concerned about ASF?

Outbreaks occur from time to time in domestic pig herds in Africa even in the large control areas in South Africa where a slaughter policy exists. Herds usually become infected by eating warthog flesh and blood or from tick infestations or from other infected domestic pigs. If your pig farm is in an infected region of Africa it is clearly at risk. To keep it out you should adopt biosecurity measures described. The central aim of your biosecurity must be to keep your herd well away from live and dead warthogs, their ticks and other domestic pigs. The disease is still present in Sardinia so if your farm is in Sardinia or to a lesser degree mainland Italy there is some risk. It spread from Sardinia to the mainland in 1983 in boar meat and could do so again. Pig farms in Sardinia could be infected from direct or indirect contact with infected pigs.

In indoor herds reasonable biosecurity should prevent contamination. If your pigs run outside the herd should be double ring fenced. Soft bodied ticks are not a risk in Italy. While the disease is still present in Sardinia, there is a small chance of it appearing in pigs in Western Europe which have been illegally given infected uncooked pork products from Sardinia. Such products would most likely be brought back by holiday-makers ignorant of the risks to domestic pigs and be fed to back-yard pigs or pet pigs. You have a duty to warn anyone you know with back-yard pigs or pet pigs against feeding any household waste

The main threat to pig herds outside Africa is the introduction of infected pork products in waste food from aeroplanes and ships arriving from the southern half of Africa. This is unlikely to occur in the EU, North America or the Antipodes because of the strict rules governing the disposal of such waste but the rules may not be so strict in some other countries. There is a small theoretical risk of seagulls carrying scraps of such waste from ships to coastal pig farms. Alert your veterinarian promptly if several of your pigs become ill with a high fever, and if he suspects ASF or CSF, (he would not be able to distinguish between them) he will have to notify the veterinary authorities who will close your herd and carry out laboratory tests. If the diagnosis of ASF is confirmed the herd will be slaughtered.

Clinical signs

In the acute form of the disease caused by highly virulent strains, several pigs develop a high fever 40-42°C but may not show any other very noticeable signs for a couple of days. They then gradually lose their appetites and become depressed. If they are white skinned pigs their extremities (nose, ears, tail and lower legs) become cyanotic (blue-purple colour) and discrete haemorrhages appear in the skin particularly on the ears and flanks. They lie down huddled together shivering, breathing abnormally and perhaps coughing and they do not want to get up. If you make them get up they are unsteady on their legs. Within a few days they become comatosed and die. Pregnant sows abort. The disease spreads through the herd over several days or sometimes more slowly over several weeks and many pigs die. Some may die very soon after they become ill.

In Sardinia the virus has remained fairly virulent, but elsewhere less virulent strains have become evident. Some outbreaks of the disease in African countries, in which the virus is endemic in the domestic pig population, are milder, run a longer course and spread more slowly through a herd with fewer deaths. Some affected pigs become very thin and stop growing and develop signs of pneumonia, skin ulcers, and swollen joints.

Milder strains may also be introduced accidentally into pig herds in countries outside Africa in waste food from ships ,aeroplanes or returning travellers. The disease may then go unrecognised for some time.

Diagnosis

Pigs that die early in an outbreak may have no very noticeable lesions but as the disease progresses the lesions then are striking. Bright red haemorrhages in the lymph nodes, kidneys, heart and linings of the body cavities are common findings. There may also be excess haemorrhagic fluid in the body cavities and gelatinous fluid in the lungs. The spleen may be enlarged, darkened and crumble on slight pressure.

The veterinarian will have to send samples to a laboratory which specialises in CSF and ASF diagnosis. The best samples to send are blood, lymph nodes, spleen and in chronic cases, serum for serology. In case it is CSF and not ASF the tonsils might also be sent. The veterinarian should consult the appropriate veterinary authorities on how best to send these.

The tonsils of the pig are very easy to find. Laying the dead pig on its back, cut away the skin and flesh under and between its lower jaw bone and also tongue. The pair of tonsils are two large red patches each about the size of the end half of your thumb or perhaps slightly bigger. Their surfaces are covered with small pits or depressions.

In South Africa and countries outside Africa it is essential to isolate and identify the virus. Only about six laboratories in the world can do this. In African countries in which the disease is endemic in the domestic pig population the veterinarian may only send serum samples for antibody detection.

Virus may be isolated in primary cultures of pig bone marrow or peripheral blood leucocytes. Infected cells haemadsorb i.e. pig red cells will adhere to them. Virus can also be detected in infected cells by fluorescent antibody tests. ELISA tests are also used to detect antibodies. In doubtful cases samples can be injected into experimental pigs.

Serum antibody titres may be tested in a number of ways. The indirect immunofluorescence (IIF) and the ELISA tests seem to be the most favoured.

Note that porcine dermatitis and nephropathy syndrome, which occurs from time to time in most pig rearing areas can resemble ASF and CSF clinically and at post mortem examination. Laboratory examination may be necessary to eliminate them form the diagnosis.

Treatment

  • There is no treatment.
Management control and prevention
  • No effective attenuated or inactivated vaccines have been developed and so none are available.
  • Prevention in countries outside Africa has to be on a national basis by restrictions on incoming pigs and pork products, compulsory boiling of waste animal products under licence before feeding to pigs and the application of a slaughter policy when the disease is diagnosed.
  • Prevention in Africa is based on measures to keep warthogs and materials contaminated by warthogs away from the herd.

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