African swine fever (ASF), a devastating pig disease which causes major economic losses in many African countries, has spread to Georgia, south-east Europe. This incursion has serious consequences for Georgia and potentially surrounding countries. An increase in mortality amongst domestic pigs was observed in disease outbreaks in April but attempts to diagnose the cause were inconclusive.

On 2nd June samples from affected pigs were received by the Institute for Animal Health’s Pirbright Laboratory. By the following day the Institute’s scientists had identified the presence of African swine fever virus (ASFV; illustrated, right), using three tests, described below.

ASFV is highly lethal to domestic pigs and has the potential to spread rapidly. As at early June, outbreaks had been reported at 10 sites distributed throughout Georgia, and 20,000 pigs had been slaughtered in attempts to contain the outbreak.

Dr Linda Dixon, Head of the ASFV Research Group in the Institute for Animal Health said “Our genetic fingerprinting indicates that the source of the infection is the eastern side of southern Africa, rather than west or central Africa or Sardinia.”

The virus is spread by contact between pigs, by eating of infected pig meat, contamination of the environment and by biting flies. The virus also grows in a species of soft tick (illustrated, below) and, in areas where these are present, bites from infected ticks can spread virus to pigs. There is no vaccine and no effective treatment. The virus does not cause disease in humans.
This is the second emergence of an exotic disease in Europe within a year. Last summer bluetongue virus was introduced into northern Europe. It was then spread to ruminants e.g. cattle and sheep, in Belgium, France, Germany, Luxembourg and the Netherlands by biting midges. Genetic fingerprinting by scientists at the Institute for Animal Health identified the causative bluetongue virus as type 8, which had never previously been detected in Europe. These incursions again highlight the potential for movement of exotic disease over long distances and highlight the need for assistance to developing countries to monitor and control important infectious diseases.

About the disease

African swine fever is caused by a virus, African swine fever virus (ASFV). It infects a range of swine species: domestic and wild pigs, warthogs and bushpigs. The latter two are the natural hosts of the virus in Africa and in these species no signs of disease are seen. However, in domestic and wild pigs it is very serious. Most strains of the virus are highly virulent with very high mortality rates. Infected pigs develop an acute fever with bleeding (haemorrhages) in many parts of the body. The virus grows in some cells of the immune system, and destroys cells lining blood vessels. This results in the bleeding that is observed. Death results within 8 to 12 days. Some strains have been identified which cause a milder disease. Some of the pigs infected with the mild strains continue to harbour the virus for a long time: they become carriers. There is no vaccine and no effective treatment. The virus does not cause disease in humans.

How the Institute for Animal Health made the diagnosis

The Georgian authorities sent parts of infected pigs to the Institute for Animal Health where scientists used three diagnostic techniques. In one they were able to grow the virus in pig cells (primary pig bone marrow cultures) in the laboratory. When they added red blood cells to the pig cells, they stuck to the surface of the infected cells (illustrated, below). This ‘haemadsorption’, which could be seen easily down a microscope, is diagnostic for ASFV.

They were able to detect the genetic material of the virus using a polymerase chain reaction test (PCR) for the gene called p72. In the third test they detected virus particles and protein components of the virus in the infected pig samples using an antigen-detection enzyme-linked immunosorbent assay (ELISA).

The figure on the left shows a pig macrophage cell (blue) with ASFV growing in it (yellow), to which red blood cells have adhered. ASFV has a specialised protein at its surface to enable it to attach to cells, including red blood cells. This protein also goes to the surface of infected cells. That is why the red blood cells stuck to the surface of the infected cells.

Why the samples were sent to the Institute for Animal Health

The Institute for Animal Health is an official Reference Laboratory for African swine fever on behalf of the World Organisation for Animal Health (Office International Epizooties, OIE) and the United Nations’ Food and Agriculture Organisation. A Reference Laboratory is one which holds virus and serum samples from previous outbreaks, and has the detailed knowledge and expertise to deal with new outbreaks. The Institute also does testing on behalf of the UK’s Department of the Environment, Food and Rural Affairs (Defra) and the European Union. The Institute has a research programme on African swine fever, headed by Dr Linda Dixon. Dr Dixon’s objectives are to identify the genes that make the virus cause disease, and to use this information to develop a safe vaccine.

Where did the virus come from?

As its name implies, the natural home of African swine fever virus is in Africa and disease is endemic in many African countries. The virus has been long established in a wildlife cycle in east and southern Africa involving transmission between warthogs and a species of soft tick, Ornithodoros moubata. In these hosts no disease is observed. Following its introduction into domestic pigs in Kenya in the 1920s African swine fever spread to most sub-Saharan African countries. In 1957 and again in 1960 it was introduced into Portugal via swill feeding of infected meat from air flights. It remained endemic in Spain and Portugal from that time until the mid 1990s and from there spread to other European countries (Italy, France, Holland, Belgium, Malta) and to South America and the Caribbean (illustrated). It was eradicated from all of these countries apart from Sardinia where it has remained endemic since its introduction in 1982.

The African swine fever viruses of different countries are not identical. Genetic fingerprinting has shown that the viruses of west and central Africa, and those that spread to Europe, South Africa and the Caribbean, are very similar to each other, but substantially different from those in 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. By what means/route the virus got to Georgia is not known. The United Nations’ Food and Agriculture Organisation (FAO) has suggested that the virus probably entered Georgia through imported frozen or processed pig meat. In some previous outbreaks elsewhere swill feeding has been incriminated as a source of infection.

How African swine fever virus spreads

African swine fever virus is spread in a number of ways. Virus is spread when infected meat is fed to pigs and this has often been the route by which virus has spread over long distances. Another route of transmission is by ticks when they suck blood from infected pigs and then feed on other pigs. The virus grows in the ticks, and can survive in the ticks for years. Consequently infected ticks are a reservoir for the virus, enabling the virus to remain in an area even when infected pigs have been killed. The virus also spreads when the pigs come into contact with each other, by the environment becoming contaminated, and by biting flies. The virus does not grow in these flies but they spread the virus as their mouth parts become contaminated with the virus. Wild boars can spread the virus. In addition to domestic and wild swine the virus infects, but does not cause disease in, warthogs and bushpigs.

For more information and diagrams on African Swine Fever click here.

June 2007