Q: Why is there new interest in African swine fever?
A: Although it occurs in at least 15 countries of central/southern Africa and also has been endemic in the Mediterranean island of Sardinia since 1982, African swine fever (ASF) hit parts of eastern Europe for the first time in 2007. Previously it had not been seen in any part of Europe since an outbreak in Portugal in 1999 and before that it had occurred sporadically only in southern European countries, over the past 50 years.
From May onwards this year, its presence has been confirmed officially in Georgia, Armenia and Russia. There were unconfirmed reports that it had also entered an enclave of Azerbaijan. Georgia has recorded more than 60 outbreaks. Armenia has described 15. The outbreak in Russia in November 2007 was the country's first for 30 years. It affected a border zone between Georgia and Chechnya.
Q: Has the origin of these outbreaks been explained?
A: The incursion onto Russian territory is attributed to a migration of wild boar across the border. These animals are said to move freely along river valleys in this area. They pose an obvious threat of spreading the causal virus to any domestic pigs with which they have contact.
Q: Is this of concern internationally?
A: Although the local area has few commercial pig units, there have been local suggestions that African swine fever could already be more widespread because of unreported cases in backyard pigs. Warnings also have been voiced about a possible further spread of the virus both within and beyond the Caucasus region.
The pig populations in each neighbouring country would be entirely susceptible to infection as there has been no previous exposure of the animals.
Q: What causes ASF?
A: African swine fever is so-called because the first recorded case was in Kenya, in 1921. Investigations uncovered a highly contagious virus categorised as the Asfivirus genus of a new viral family termed the Asfarviridae. The natural hosts of this virus are African warthogs and bushpigs, neither of which show disease signs even though they are infected.
The particular point about ASF is that it can be tick-borne. The bite from a soft tick known as Ornithodoros is capable of transmitting the virus to pigs in Africa and Europe. Direct transmission more commonly occurs by contact with infected pigs or their secretions. The virus also may be transmitted on contaminated implements and instruments as well as on clothing and in feed and water. In local zones it is transmitted by biting flies. It can persist in infected meat and manure for long periods, but is susceptible to sunlight, heat, acidity and dryness. Some pigs become carriers, harbouring the virus for a long time after being infected with a mild strain of the virus.
Virus identification with the outbreak in Georgia was done by a reference laboratory at the Institute of Animal Health in the UK, which notes that the ASF viruses of different countries are not identical. Genetic fingerprinting has shown that the ones which spread in the past to Europe, South Africa and the Caribbean were similar to the viruses found in west and central Africa, but these differ substantially from the forms affecting the eastern part of southern Africa, including Mozambique and Zambia, and the island of Madagascar. Fingerprinting of the Georgia virus showed that it was most closely related to those present in the eastern part of southern Africa.
Q: What are the disease signs?
A: Unfortunately, they are non-specific. ASF is easily confused with classical swine fever (hog cholera) although the viruses are not related. A differential diagnosis also is required against other conditions such as erysipelas and Salmonellosis. Recently, veterinary authorities have added a warning that the circovirus-associated porcine dermatitis and nephropathy syndrome (PDNS) can be mistaken for either ASF or classical swine fever. Differentiation in each instance is possible only by laboratory testing.
On the farm, strain or isolate differences for virulence have been described. The most acute cases lead to a high death loss, 100% mortality being common from highly virulent isolates and 30-50% from the moderately virulent. Effects may be minor or subclinical at the other extreme. The incubation time between infection and disease onset varies from 3 days to 5 days and the resulting mortality becomes evident within another 8-12 days.
In the acute form, infected pigs develop a high temperature (40.5°C) and lose their appetite before starting to suffer from haemorrhages in many parts of the body. A bulletin from the Institute of Animal Health explains that the bleeding results from the virus growing in some cells of the immune system and destroying cells lining blood vessels.
Some pigs at the start of infection will vomit or show diarrhoea. Many develop a reddening or darkening of the skin, particularly around the ears and snout, and a sticky residue may form across their eyes. A swelling of joints may be observed. Laboured breathing and coughing are other common indicators. Affected sows may abort. Post-mortem may reveal haermorrhages of internal tissues as well as the skin an
Q: Can pigs be treated or vaccinated?
A: There is no known treatment and no vaccine is available yet. Moreover, most countries prohibit vaccination because it would interfere with detection of the disease agent.
Q: Can the virus be kept out?
A: The main risk of entry into a country is in infected meat. Normal biosecurity measures are successful in excluding the ASF virus at farm level. However, the virus is persistent so vigilance is vital, especially in the current situation of increased outbreaks. PIGI