Factsheet on swine influenza in pigs


Swine influenza – animal facts

Influenza in swine is an acute viral infection of the respiratory tract in pigs caused by type A influenza viruses. The mortality rate is low in pigs and recovery usually occurs within 7–10 days. Swine-origin (or swine-lineage) influenza virus (SIV) infections also occur in poultry and humans, but interspecies transmission is considered a rare event. Three subtypes of influenza A virus have been found to circulate in pigs: A(H1N1), A(H1N2) and A(H3N2). They are all swine influenzas, i.e. they are well adapted to their swine hosts. They are different from human influenza viruses with the same nomenclature and can be distinguished from them antigenically.

Current influenza viruses in European pigs

Avian-like swine A(H1N1) influenza viruses (SIVs) of the subtypes A(H1N1), A(H3N2) and A(H1N2) are enzootic and widespread in swine producing regions of Western Europe. In Central Europe, SIV activity is low and the circulation of H3N2 and H1N2 remains to be confirmed. The classical A(H1N1) influenza virus - which was a direct descendant of the 1918 human influenza pandemic virus and is still present in pigs in the Americas and Asia - has not been detected in European pigs for over 15 years.  Contemporary swine A(H3N2) influenza viruses in European pigs descend from early human influenza A(H3N2) pandemic strains, and have diverged substantially genetically and antigenically from contemporary human viruses. The ‘triple re-assortant’ (TRA) swine influenza A(H1) viruses are named like this because they contain genes from avian, human and swine influenza viruses. The TRA influenza viruses seen in North America (including influenza A(H3N2) viruses, also observed in Asia) have not been detected in European pigs to date.

Current Swine influenza viruses circulating in the US and Asia

The epidemiology and virology of SIVs in Europe and the United States seem to be quite different. The TRA viruses of the type S-OtrA(H3N2) seen in pigs and humans in the United States have not been found in Europe to date. The main SIVs circulating in pig herds in the United States in recent years have been swine TRA A(H1N1), A(H3N2) and A(H1N2) viruses. These TRAs have also been found in Asia.

An important point is that there is little, if any, movement of live pigs between Europe and other continents. Indeed, it is not legal for live pigs to be imported from the United States to Europe.

Asia, especially China, has the largest human and pig populations in the world, and seems to be the only world region frequently importing pigs (and potentially SIVs) from other continents. The most commonly circulating influenza viruses in Asia are the classical swine A(H1N1) virus, human-origin A(H3N2) viruses, the American triple reassortant (TRA) A(H1N2) virus, the European avian-like A(H1N1) virus and all their reassortant variants.

Surveillance for swine influenza in European pigs

Swine influenza in pigs is not notifiable at the European Union level. Indeed, worldwide structured surveillance is very weak because swine influenzas are not considered to be a major threat to veterinary health. Through a project called ESNIP3, funded by the European Commission (DG-Research), the surveillance of influenza in pigs in numerous Member States is conducted in a coordinated way and the outputs from it are analyzed. This is supported by largely industry driven activity in a number of countries while formally funded programmes at national level are more limited. However, ECDC is not aware of any integrated animal-human influenza surveillance in Europe.

The pig as mixing vessel theory

Most swine influenza viruses are reassortants with mixtures of human, avian and swine virus genes. This supports the classical theory that pigs are susceptible to both human and avian influenza viruses, and that they can then serve as ‘mixing vessels’ for those viruses in pigs that are infected with two or more viruses. Antigenic ‘drift’ within swine influenza virus lineages does occur but it is less prominent than antigenic ‘drift’ in human influenza viruses. A significant fraction of the currently circulating influenza A(H1N1) viruses in pigs derives from the 2009 pandemic influenza A(H1N1) virus, now officially designated A(H1N1)pdm09 influenza virus. The further reassortment of previously circulating pig influenza viruses such as those of types A(H1N1), A(H1N2) and A(H3N2) with the A(H1N1)pdm09 influenza virus is therefore possible.