By Dr Peter Evans, SAPPO’s health liaison officer
African Swine Fever (ASF) was first recognised in East Africa as a disease which clinically resembles Classical Swine Fever (CSF). Studies conducted between 1910 to 1917 demonstrated that it was caused by a virus which was different in epidemiology and immunology to CSF. Outbreaks were related to association of free-ranging pigs with wild pigs.In South Africa from 1900 to 1918, both CSF and ASF apparently occurred but CSF has not been recorded after 1918. Periods of apparent freedom from ASF outbreaks include 1918 to 1926, 1939 to 1951 and 1962 to 1973. The disease has been described from most countries in Central and Southern Africa as well as West Africa. ASF also reached Portugal in 1957 and 1960 and outbreaks have been reported in several European countries as well as Cuba, Haiti and Brazil. In 2007 Russia experienced a period with ASF outbreaks and this is still continuing.
The ASF virus is unrelated to any other known virus and is well adapted to long term survival in a cycle involving arthropods and wild pigs indigenous to Africa. The virus does not cause any obvious clinical signs in its indigenous hosts and therefore its existence has gone unnoticed by livestock keepers until domestic pigs were first introduced to sub-Saharan Africa about 500 years ago.
In Africa, the ASF virus persists in at least three different cycles:
1. A sylvatic cycle between wild pigs, particularly warthogs and Ornithodoros sp. ticks.
• Young warthogs confined to burrows in which they were born develop viraemias high enough to infect ticks that feed on them. This high viraemia may last for up to three weeks but is thereafter intermittent and at levels that are too low to infect ticks. In a study where ticks were experimentally infected it was found that these ticks have the ability to transmit the disease for up to three years.
2. A cycle involving domestic pigs and Ornithodoros sp. ticks that live in pig houses in Malawi.
3. Maintenance of the virus in domestic pig populations independent of wild pigs or ticks.
It is often difficult to discover the source of infection. Infected warthogs do not transmit the disease to pigs housed in close contact with them, although bush pigs may do so. In Southern Africa the most likely explanation is that live warthogs carry infected ticks which could come into contact with domestic pigs. These ticks can then transmit the infection when engorging on domestic pigs.
With the ASF that appeared in Portugal in 1957 the virus was imported with contaminated pig meat products from Angola, where the disease is endemic in free-ranging pigs. It is well known that once ASF establishes in a pig population, further spread can occur in three ways:
1. Transmission by direct contact between infected and susceptible animals
2. Indirect transmission – vehicles that have not been decontaminated or even mechanical transmission by stable flies.
3. Pigs fed meat products from animals slaughtered in the acute stage of the infection.
The acute form of ASF is the most common in African outbreaks. Very few animals recover with the exception of animals in the endemic areas. The incubation period (following contact with infected pigs), varies from five to 15 days according to the strain and intensity of exposure. Following a tick bite the incubation may vary between four to five days to 13 days. After ingestion of contaminated meat incubation may as short as 48 hours extending to seven to eight days with minimal doses.
Fever is a common finding and reaches a plateau within 48 hours after the initial rise. Listlessness, partial or complete anorexia, incoordination, convulsions and muscular tremors have all been recorded. There is an increasing congestion of the skin of the tail, limbs, ears , snout and abdomen. This is easy to observe in white-skinned pigs. Haemorrhages often develop in the skin. Breathing becomes shallow and rapid. Nasal discharges are sometimes observed, followed by a profuse watery or frothy, blood-stained nasal discharge in animals with severe lung oedema. Lung oedema is often the immediate cause of death for many animals.
Vomiting is common and some animals may become constipated. Others may develop a watery diarrhoea with fresh or blackened blood. Pregnant sows abort soon after infection and at all stages of pregnancy. Towards the end of the disease the rectal temperature falls rapidly, followed by coma and death.
Survival time for virulent African strains vary between two and nine days in more than 90% of the cases. Clinical signs in animals that last for three to four weeks include anorexia, loss of condition and pneumonia.
Acute ASF is characterised by reddening to purplish cyanosis of extremities and the stomach in white-skinned pigs. Mucous membranes of the eyes and mouth are usually congested and may even have haemorrhages on the surface. Nasal discharges may be observed and there may be a bloody froth at the nostrils.
Organs are generally congested and the spleen may sometimes be markedly enlarged. Lymph nodes, particularly those of the head and gastrointestinal tract are markedly swollen and haemorrhagic. These may sometimes even resemble blood clots. There may be haemorrhages on the kidneys as well as the epi- and endocardium of the heart. The lungs do not collapse on opening the chest cavity. Lungs are usually congested and haemorrgagic. On cutting the lungs, frothy fluid may ooze from the organ. The trachea may be filled with blood-stained froth.The appearance of the gastrointestinal tract may be variable. It is common to find haemorrhages in the stomach.
Outbreaks of acute ASF should not be difficult to recognise when factors such as epidemiology, clinical signs and post mortem lesions are taken into account. There are few diseases which cause such high number of deaths in all ages of pigs. ASF should be confirmed in the laboratory and there are many techniques available.
There is no treatment or vaccine available and control of ASF depends on preventing contact between domestic pigs and sources of the virus. Pig production systems in which pigs do not come into contact with infected pigs, their products or other sources of the virus are the most successful defence against ASF.
Infectious Diseases of Livestock, Chapter 98, p 1088, 2nd Edition.