Key Points about African Swine Fever (ASF)

African swine fever (ASF) is one of the most complex and economically devastating viral infectious diseases of swine herds, producing great socio-economic impact in affected countries. ASF is listed as a notifiable disease to the World Organisation for Animal Health (OIE).

African swine fever virus (ASFV) is a very complex and large enveloped DNA virus with a genome of 170 to 190kbp. It is classified as a unique member of the Asfarviridae family, genus Asfavirus (1). This virus presents high genetic and antigenic variability, with 22 different genotypes described, all of them currently circulating on the African continent. The natural hosts of ASFV are wild and domestic pigs of all breeds and ages. The virus also infects different species of soft ticks of the genus Ornithodoros, in which it can persist for periods longer than five years (2). Monocytes and macrophages are the main target cells of ASFV infection. No neutralising antibodies are induced during infection, and no effective vaccine is available. ASFV in the environment and in infected materials is highly resistant to inactivation, surviving more than 15 weeks in putrefied blood or 1,000 days in frozen meat (3).

ASF is a haemorrhagic disease that progresses with different clinical signs depending on isolate virulence, host, dose and route of exposure. Clinical signs in pigs and wild boars are very similar to those of other haemorrhagic diseases such as classical swine fever, salmonellosis or erysipelas; thus, laboratory diagnosis is required for differentiating between them. ASF clinical signs may vary from a hyperacute form with mortality rates of 100 per cent from day 4 to 7 post-infection, to an asymptomatic, chronic form in some cases, which can lead to the presence of carrier animals. The presence of antibodies confers some protection, and it is associated with appearance of the chronic and carrier forms of the disease. European wild boars are usually more resistant than domestic pigs to ASFV infection, although they present a similar pathological and epidemiological pattern (4).

Chronic forms, carrier animals (wild or domestic) and infected ticks play an important role in the persistence and dissemination of the disease in endemic areas (7).

African wild suids such as warthogs (Phacochoerus aethiopicus), bush pigs (Potamochoerus porcus) and giant forest hogs (Hylochoerus meinertzhageni) are also infected by ASFV but they usually do not exhibit clinical signs, allowing them to act as reservoir hosts in Africa (5). In some of these wild suids, ASF infection is characterised by low levels of virus in tissues and low or undetectable viraemia (6). These levels of virus are sufficient for transmission to domestic pigs through tick vectors but usually not sufficient for transmission through direct contact between animals. This wild disease cycle makes it more difficult to eradicate ASF from eastern and southern Africa.

ASFV has been introduced into disease-free areas mainly by feeding contaminated pork products from international airports and seaports to domestic animals. Once established in domestic herds, infected pigs, carriers and pork products become the primary sources of virus dissemination.

Preventing the virus from entering disease–free areas is crucial and must be based mainly on preventing the introduction of potentially infected pigs or pork products, and on properly disposing of pork waste from aircraft and ships as well as other fomites.

Control of the disease is based on early detection with rapid laboratory diagnosis and enforcement of strict sanitary measures (4). Laboratory diagnosis is essential to establish a correct diagnosis of the disease, due to the strong similarity of ASF clinical signs and macroscopic lesions with those of other haemorrhagic diseases of pigs. Several effective tests are available to detect infectious virus, viral antigens, viral DNA or specific antibodies induced by the 22 different ASF genotypes. The simultaneous detection of both antigen and antibodies in parallel is very important for establishing an effective diagnosis and evaluating the progress of the disease control programme (4).

ASF eradication without vaccine is possible but difficult. The eradication of ASF from Portugal and Spain proved that vaccination is not required for eradication of this complex disease, even in endemic countries. However, the establishment of a good eradication programme adapted to each specific scenario is essential (7).

ASF Epidemiology

ASF was discovered by Montgomery in 1921 in Kenya. Since then, many sub-Saharan countries have been affected by the disease. During the 1970s and 1980s, ASFV travelled around the world, affecting various countries in Europe, such as Portugal, Spain, Netherlands, France and Belgium, as well as some parts of the Americas, such as the Dominican Republic and Brazil. As a result of tremendous efforts, the disease was eradicated from all of these territories but it persists on the Italian island of Sardinia and on the African continent, especially in the southeast.

During the 1990s and 2000s, the epidemiology and distribution of the disease changed. ASFV spread to other regions not typically affected by ASF. These included West African countries, where the virus was first reported in Nigeria (1997), Togo (1997) and Ghana (1999), as well as some islands, such as Madagascar (1998) and Mauritius (2007). The most recent change in the epidemiology of the disease occurred in 2007, when ASFV was reintroduced onto the European continent, this time via Georgia.

This significant epidemiological change may have been caused by a combination of several factors, the most important of which is the increasing presence of ASFV on the African continent during the last 15 years, as the virus has invaded previously disease–free territories. This implies increases in the amounts of circulating virus, in the number of infected animals, and therefore, in the amount of contaminated pig products. A second important factor is globalisation. Today, people, animals and products travel around the world within short periods of time. The volume of animals, persons and products being transported increases day by day. The third important factor is the global financial crisis that has forced small farmers to meet their needs in new ways, such as by using swill or garbage to feed their animals. These three factors, together with the resistance of ASFV in the environment and meat products, the presence of asymptomatic carrier animals, and the lack of a vaccine may help to explain how the disease has recently spread to several new territories.

ASF Status of the Caucasus Region and Russian Federation

In April 2007, a new outbreak of ASF genotype II, compatible with the virus circulating in Mozambique, Madagascar and Zambia, reached the European continent via Georgia. This ASFV is thought to have come from international ships that contained infected swills used to feed pigs near the port of Poti(8). After this introduction, the disease spread very quickly, affecting four different countries: Georgia, Armenia, Azerbaijan and the Russian Federation. Since the introduction of the virus in the Caucasus region, the OIE has been notified of more than 260 outbreaks, in which 76,000 animals have died. The disease and economic losses in the Russian Federation have been estimated at 25 to 30 billion rubles (RUB; US$800 million to $1 billion)(9).

All the ASFV isolates found in the Caucasus region and Russian Federation since virus introduction show identical sequences, suggesting only one virus introduction in 2007(10). Two recent outbreaks, in October 2009 and December 2010, occurred in areas very near European Union borders, less than 150km away from Estonia and Finland(11).

The likelihood that ASF will become endemic and spread to nearby unaffected areas of the Russian Federation has been estimated as very high(12), due to the presence of key factors in the area: demonstrated infection of ASFV in wild boar populations, extremely high volume of illegal trade of pigs and pork products within the country, a traditional custom of swill feeding, absence of adequate veterinary services and lack of pig production infrastructures and traceability(13).

All these factors make control and eradication of the disease from this area very difficult, and increase the risk of spread to neighbouring countries, especially those with commercial and socio–cultural relations with the Russian Federation. These observations are supported by comments of the Russian Chief Veterinary Officer, who recently predicted a spread of the disease toward the northern and northwestern regions(14). This situation could increase the risk of introduction into the EU.

What is Happening in the EU and What is the Risk of ASFV Introduction?

The European Union (EU) is aware of the potential risk of ASFV introduction within its borders. The risk assessment(12) estimates the risk of ASFV introduction into the EU as moderate. However, this estimation, according with the evolution of the disease in Russia and the recent outbreaks near the EU border, should be reconsidered.

The same risk assessment highlights the risk of introduction through contaminated products used for swill feeding. Historically, this was the most frequent route of ASFV introduction into disease–free countries, e.g. Spain, Netherlands, Belgium, Cuba and, more recently, Georgia. This risk assessment further predicts that once the disease enters the EU, the risk that it will persist there is low, given the relatively high biosecurity of the pork production industry.

More detailed and complete analyses are being developed within the European project, ASFRISK (EC, FP7-KBBE-2007-1, Project #211691) to estimate the most likely pathways, countries and months for ASFV introduction into the EU. Preliminary results of this analysis place the likelihood of ASFV introduction into the EU by legal import of live pigs as low(15). This risk is mainly concentrated in Poland during the months of November and December, with the Russian Federation being the primary source of contaminated material. Methods and results obtained by this risk assessment may help to allocate financial and human resources in areas and periods at higher risk, helping to reduce the chance that ASFV will enter the EU.

Conclusions

• Circulation of the ASF virus has increased for the last 15 years on the African continent
• The possibility that ASF will become endemic in the Caucasus region and the Russian Federation is very high
• The risk of ASF virus introduction into the EU is increasing
• An effective vaccine is not predicted to be available in the near future
• An effective control and eradication programme should be established for different scenarios and with the participation of all actors, such as veterinarians, farmers and administrators.

Acknowledgment

This paper was funded by the EU ASFRISK project (EC, FP7-KBBE-2007-1, Project #211691). Lina Mur holds a scholarship of from the FPU Programme, (Ministry of Education and Science, Spain).

References

1. Dixon L, et al. 2005. Asfarviridae. Eighth report of the International Committee on Taxonomy of Viruses.
2. Oleaga-Pérez A et al. 1990. Vet. Rec. 126:32-37.
3. EFSA (2007). Scientific review on African swine fever.
4. Sánchez-Vizcaíno J.M. 2006. African swine fever. In: Diseases of Swine.
5. De Tray D.E. 1957. J Am Vet Med Assoc. 130:137-140.
6. Plowright W. 1981. African swine fever. In: Infectious Diseases of Wild Mammals.
7. Arias M., Sánchez-Vizcaíno J.M. 2002. African swine fever. In: Trends in Emerging Virus Infection.
8. Beltrán-Alcrudo D. et al. 2008. African swine fever in Caucasus region.
9. United States Department of Agriculture 2010. Foreign Agriculture Services. Russian Federation. Global Agricultural Information Network, Report RS1017.
10. Gallardo C. et al. 2009. 14th International Symposium of the World Association of Veterinary Laboratory Diagnosticians.
11. World Organisation for Animal Health. 2009, 2010. WAHID database.
12. EFSA. 2010. Scientific opinion on African swine fever.
13. Beltrán-Alcrudo D. et al. 2009. African swine fever spread in the Russian Federation and the risk for the region.
14. Vlasov N. 2011. Expert meeting on African Swine Fever. Berlin, January 2011.
15. Mur et al. 2010. Proc. IPVS Congress. O188, 222.

Reference

Sánchez-Vizcaíno J.M., L. Mur and B. Martínez-López. 2012. African Swine Fever Update. Proceedings of the 6th International Symposium of on Emerging and Re-Emerging Pig Diseases. Barcelona, Spain. June 2011. p29-31.

Further Reading

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March 2012