Factsheet on swine influenza in humans and pigs
Evidence of person-to-person spread of swine-origin influenza viruses (SIVs) is very limited and most swine-to-human transmissions have been epidemiologically dead-end events, i.e. onward transmissions are very rare. One known exception is the A(H1N1)pdm09 influenza virus. Although this virus is likely of swine origin, it has not (to date) been detected in pigs prior to its appearance in humans. This could simply reflect that, as mentioned above, there is an absence (or limited presence) of programmes of surveillance for SIVs in many parts of the world.
Human infections with swine influenza have been sporadically detected (or at least published in the literature) since the late 1950s. These findings usually occurred in people with direct and indirect exposure to pigs i.e. people working in pig farms, etc. Sporadic transmission events of swine influenza viruses to humans have been reported in Europe and are described in the monthly Communicable Threat Reports (CDTR) as well as in the annual epidemiological zoonotic influenza reports.
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.
There are sporadic reports of human infections with swine influenza viruses, mainly in people who had direct contact with infected pigs. Since swine influenza is not a foodborne illness, it is highly unlikely for humans to contract swine influenza through eating pork, as influenza viruses are often only found in pigs' respiratory tracts and cannot be found in their meat. Transmission occurs through aerosol and close contact with infected pigs as well as through a contaminated environment.
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. The first known introduction of human influenza virus into swine populations occurred after the Spanish flu in 1918 and this lineage was called ‘classical swine’ H1N1 (or lineage 1A). This lineage is still present in pigs in the Americas and Asia, but has not been detected in European pigs in the last two decades.
The first significant outbreak of an avian A(H1N1) influenza virus lineage occurred in 1979 and led to the establishment of an ‘avian-like’ A(H1N1) virus lineage in European pigs. This virus, referred to as Eurasian avian-like 1C lineage, rapidly established itself in Europe and has continued to circulate in swine until the present day.
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 A(H1N2) influenza viruses were established in Europe with the reassortment of swine A(H3N2) viruses and a human-seasonal A(H1N1) virus (H1huN2) and to date are still circulating among European pig populations.
The emergence of the 2009 pandemic A(H1N1) virus (H1N1pdm09) was a result of a reassortment event between a North American ‘triple-reassortant (TRA)’ swine influenza virus and a European ‘avian-like’ A(H1N1) (H1avN1) (lineage 1A.3.3.2/pdm[MV2] ). Since its introduction into the European swine population, it reassorted broadly with the circulating swine A(H1N1), A(H3N2), and A(H1N2) subtypes.
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
To date, the most common 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 different related reassortant genotypes.
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 landscape of circulating swine influenza viruses in the US is quite versatile, due to many reassortment events throughout the years. 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. The first TRA A(H3N2) influenza virus was introduced in the US in 1998, evolving further into defined phylogenetic clades over time, of which some are still dominant to the present day. As a result of further reassortment between the TRA A(H3N2) viruses and classical swine A(H1N1), new TRA A(H1N1) and A(H1N2) viruses were generated and are still present in the US.
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, which produce more than 50% of the worldwide swine population, seems to be the only world region frequently importing pigs (and potentially SIVs) from other continents. The classical swine A(H1N1) viruses were endemic until the 1990s, however European A(H1avN1), A(H3N2), and North American ‘triple reassortant’ lineage viruses were introduced as a result of swine imports from other continents.
Surveillance for swine influenza in European pigs
Swine influenza in pigs is not notifiable at the EU level and no routine surveillance in pig populations is in place. Indeed, worldwide structured surveillance is very weak as swine influenzas are not considered to be a major threat to animal health. Surveillance is dependent on individual national or supranational initiatives to analyse and adapt the available swine influenza vaccines.
The surveillance of influenza in pigs in numerous Member States was conducted in a coordinated way through a project called ESNIP3, which was funded by the European Commission (DG-Research) and ended in 2013. The COST Action project (CA21132) led by the European Swine Influenza Network (ESFLU) aims to broaden the scope of the previous ESNIP2 and ESNIP3 projects towards a One Health approach, with a focus on diagnostic protocols and management capabilities aiming to mitigate swine influenza.
Swine influenza is among the 10 priority zoonotic diseases for which surveillance strategies have been proposed by EFSA.
The 2009 pandemic influenza A(H1N1)
The A(H1N1)pdm09 influenza virus from 2009 contained genes from pig, bird and human influenza viruses, in a combination that had not been reported before in any part of the world. Following the emergence of the 2009 pandemic, this virus was also isolated from pigs in multiple areas of the world (including Europe), and it is currently circulating in pigs in several European countries. The infections in swine were initially due to transmission from infected humans, and then subsequently spread through pig-to-pig transmission.
The pig as a mixing vessel theory
The ‘mixing vessel’ theory supports the hypothesis that swine infected with swine influenza virus could be dually infected with avian or human influenza A viruses, as most swine influenza viruses are reassortants with mixtures of human, avian and swine virus genes. Pigs' ability to serve as a ‘mixing vessel’ depends on the presence of both human-adapted and avian-adapted receptor types in their respiratory tract.
The transmission of these reassortant viruses to humans could then result in the introduction of novel viruses into the human population. On rare occasions, the combination of avian, swine and human influenza virus genes could produce a potential source for a human pandemic strain. 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 derive 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.