H1N1, H3N2 and H1N2 Influenza Viruses in Pigs

The epidemiology of swine influenza (SI) has become increasingly complex over the last decade. Three influenza A virus subtypes – H1N1, H3N2 and H1N2 – are currently circulating in swine worldwide, but the origins and the antigenic and genetic characteristics of these swine influenza virus (SIV) subtypes differ in different continents or regions of the world.The first significant outbreaks of SI on the European mainland occurred in the late 1970s after the transmission of an H1N1 virus from wild ducks to pigs. This ‘avian-like’ H1N1 virus has become established in the European pig population and ultimately became the dominant H1N1 SIV strain. Viruses of human origin, A/Hong Kong/68-like H3N2 formed a stable lineage in European pigs since the early 1970s, but reassortant H3N2 viruses with human haemagglutinin (HA) and neuraminidase (NA) genes and avian-like swine H1N1 internal protein genes have become dominant since the mid 1980s. Finally, ‘triple reassortant’ H1N2 viruses have been isolated frequently from pigs throughout Europe since the mid 1990s. These viruses contain an HA of human influenza virus origin, a NA of swine H3N2 virus origin and internal protein genes of avian-like swine H1N1 virus origin.The HA of these H1N2 viruses shows low antigenic and genetic homology (70·4% amino acid identity in the HA1 region) with avian-like H1N1 viruses and there is no cross-reaction between H1N1 and H1N2 viruses in the HI test. Though most SIV infections are clinically mild or subclinical, all three subtypes have been associated frequently with typical outbreaks of ‘swine flu’ and SIV may be responsible for up to 50% of acute respiratory disease outbreaks in pigs.Viruses of H1N1 and H3N2 SIV subtypes are considered widespread and endemic in pig populations in Austria, Belgium, Denmark, France, Germany, Great Britain, Italy, The Netherlands and Spain. Occasional serological investigations of pigs sampled at slaughter have been performed in most of these countries in the 1980s and early 1990s and revealed seropositivity to H1N1 and H3N2 in >50% of the tested population. However, over the last few years H3N2 activity is believed to be low or absent in France (Brittany) and Great Britain (Brown I., Veterinary Laboratories Agency, Weybridge, UK, unpublished observations; Kuntz-Simon G., Agence Française de Sécurité Sanitaire des Aliments, Ploufragan, France, personal communication). Either or both subtypes have also been reported in Bulgaria, the former Czechoslovakia, Greece, Hungary, Ireland, Macedonia, Poland and Sweden. As for the H1N2 virus, a limited serosurvey in sows in Belgium in 1999 has shown seropositivity in 69% of the 443 sera examined.  There is limited H1N2 seroprevalence data for other European countries and there is little information on the evolution of H1N1 or H3N2 seroprevalence rates in Europe.

References

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