Wild or feral pigs are increasing in number in Europe and are posing a health risk to domestic herds.
The pig industry’s focus over the past two decades has shifted away from long-known conditions such as Aujeskzy’s (pseudorabies) to the newer viruses of PRRS and PCV2. However, a serious threat still remains from African swine fever, classical swine fever (hog cholera) and foot-and-mouth disease discussed at The 6th International Symposiums on Emerging and Re-emerging Pig Diseases held in Barcelona, June 12-12, 2011.
In the definition agreed by the World Health Organization, a disease agent is considered emerging if it has appeared in a population for the first time. The description of re-emerging is applied if the agent existed previously, but has now shown a rapid rise in incidence or geographic distribution. Candidates to be included in each of these categories are now arriving almost daily, delegates at the symposium were told. In particular, it seems almost to be raining viruses because of the number of viral agents being researched—some apparently new, others with a long history behind them.
Pig virus lineupProfessor X.J. Meng of Virginia Tech College of Veterinary Medicine, USA, created a list as shown in Table 1, dividing the emerging viruses into those known to be pathogenic in pigs and others for which a clinical effect had not been demonstrated or determined. Most of the names are unfamiliar to anyone without a veterinary qualification, but the list underlined that many of them could hardly be called new when they have been studied for 10 years or more.
Another aspect revealed by the presentations, was that several of these viral agents have already been identified as infecting wild pigs as well as domesticated ones. It proved to be a recurring theme at the symposium, with specific implications for the European pig sector, because commercial herds in Europe are at direct risk from viruses old and new because of the increasing presence of wild pigs in close proximity to farms.
African swine fever One virus receiving special priority is African swine fever. The warning about its potential to be spread by wild pigs took on even more significance after a call from veterinary specialists with the FAO food/agriculture organization of the United Nations. International action is needed to stop ASF sweeping westwards from new east-Europe epicenters in Georgia, Armenia and the southern part of the Russian Federation.
The FAO reported that African swine fever entered Georgia in 2007 when local pigs ate infected food waste discharged at the Black Sea port of Poti by a ship from southern Africa. This viral strain proved to be particularly aggressive and able to create outbreaks long distances apart, with 2011 bringing a further increase in the number of cases. World animal health agency OIE produced a map (see, Figure 1) of ASF hot spots. The map also highlighted the connection to populations of so-called wild boar.
To quote from various presentations in Barcelona, we can be sure from long experience that both domestic and wild The pig industry’s focus over the past two decades has shifted away from long-known conditions such as Aujeskzy’s (pseudorabies) to the newer viruses of PRRS and PCV2. In the definition agreed by the World Health Organization, a disease agent is considered emerging if it has appeared in a population for the first time. The description of re-emerging is applied if the agent existed previously, but has now shown a rapid rise in incidence or geographic distribution.
Candidates to be included in each of these categories are now arriving almost daily, delegates at the symposium were told. In particular, it seems almost to be raining viruses because of the number of viral agents being researched—some apparently new, others with a long history behind them.
Based on a presentation by X.J. Meng MD PhD, Professor of Molecular Virology, Virginia Tech, USA, this list includes the Torque teno sus virus or TTSuV, which has been found in pigs affected by PMWS. Although the virus PCV2 is known to be at the heart of this disease, there are suspicions that symptoms may be worse where TTSuV is also present.
Expanding feral populations The role played by wild pig populations in spreading the virus is widely speculated although less well documented. Beyond any doubt, however, is that these populations has grown sharply in recent years throughout the eastern European area as well as in Switzerland, Austria, Germany, France and the Netherlands. The latest French accounts speak of a doubling nationally to more than 1 million animals within the past 10 years, while German reports say the number locally is 2.5 million and growing fast.
The reasons given for such a rapid increase range from higher pig survival rates with the onset of warmer winters, to larger food supplies with the cultivation of more maize (corn) and oilseed rape (colza) by European farmers as the raw material for producing biofuel. However, it also owes something to the attitude of townspeople who regard wild pigs as part of the natural wildlife and therefore needing to be protected. Some people close to urban areas are known to feed them.
Throughout Europe, according to a Barcelona report, Eurasian wild boar (Sus scrofa) populations have expanded both geographically and numerically in the last decades and are known reservoirs for African swine fever and other viruses that are transmissible to domestic pigs. According to commentators in Europe, however, the introduction of measures to reduce population numbers would be easier politically if the targets were referred to as feral pigs instead of by the common yet erroneous term of wild boar.
Virus behavior changes Whichever name is used, international cooperation to achieve stricter control is a European necessity, given that African swine fever is devastating to commercial pig industries and no effective vaccine is available to stop it. The symposium was reminded that the history of the disease shows how its behavior or epidemiology can change, creating new challenges for the veterinary agencies trying to beat it.
Although ASF was discovered in Kenya 90 years ago, it did not reach Western Europe or other places such as Brazil until the 1970s and 1980s. Within another 10 years or so, its distribution changed again. This time it moved across Africa into regions that had stayed free before, especially the western African states of Nigeria, Togo and Ghana with the islands of Madagascar and Mauritius. Meanwhile, the virus has persisted on the Italian island of Sardinia in the Mediterranean, but all other parts of the European continent were ASF-free until the outbreak in Georgia in 2007.
Virologists told the symposium that this latest epidemiological change may have been assisted by globalization, but the increasing presence of the ASF virus on the African continent during the last 15 years also was discussed. The invasion of previously disease-free territories in Africa implies increases in the amount of circulating virus and in the number of infected animals adding to the contamination of pork products. A third factor has been the way in which the global financial crisis has forced more small farmers to switch to feeding their animals waste food.
The virus responsible for the Georgia 2007 case was of a genotype circulating in Mozambique, Madagascar and Zambia, see Figure 2. OIE has been notified of more than 260 outbreaks since that introduction into the Caucasus region. In the Russian Federation alone, the cost of the disease and associated losses has been calculated at 30 billion Rubles, or about US$1 billion.
Russia will struggle to stop ASF from becoming endemic and spreading to nearby unaffected areas for a variety of reasons, but not least due to the fact that the local wild boar (feral pig) populations are infected. Russia’s chief veterinary officer has already predicted a spread of the disease northwards, a situation that would increase the risk of ASF in the European Union.
On a more positive note, new animal health initiatives fostered by the European Commission attach great importance to the principle of achieving prevention rather than waiting for a problem to happen and then trying to stop it. This recognizes that an outbreak of any notifiable disease of farm livestock would jeopardize the international trade in meat products, besides its obvious effect on local farms.
An additional consolation in Europe is that ASF has been contained so far by the measures already in place. The hope now is that these barriers will continue to work, both against a transfer of the virus in pork and to prevent a cross-border spread by domesticated herds coming into contact with feral pigs.
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African swine fever has been documented in parts of Africa previously free of the disease as well as establishing an infection hotspot in Eastern Europe.
A strain of African swine fever virus originating in east Africa was introduced to the Caucasus region at the edge of Europe in 2007 when contaminated food waste reached pigs close to a sea port.
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