Swine influenza

Swine influenza is a highly contagious viral infection of pigs. The disease in swine occurs within a herd either as an epizootic or enzootic form. In the epizootic form, the virus quickly moves through all phases of a swine unit with rapid recovery, provided there are not complicating factors such as secondary bacterial infections. In the enzootic form, clinical signs may be less obvious and not all pigs may demonstrate traditional clinical signs of infection. Morbidity rates can reach 100% with swine influenza infections, while mortality rates are generally low. The primary economic impact is related to retarded weight gain resulting in an increase in the number of days to reach market weight. Swine influenza is caused by influenza A viruses in the family Orthomyxoviridae. Influenza A viruses are further characterised by subtype by the two major surface glycoproteins, haemagglutinin and neuraminidase. One relatively stable subtype, H1N1, was the etiologic agent of most swine influenza until the mid-1990s, and has been the strain historically most commonly associated with “classical swine influenza”. Since that time, established swine influenza viruses comprise various subtypes and variants, many of which are the result of substantial reassortment between influenza A viruses of several hosts. Currently circulating influenza viruses infecting swine also include genetic components, or entire viruses, of avian and human influenza viruses. The most common subtypes of influenza virus in swine are H1N1, H1N2, and H3N2. Despite the same subtype classification, swine influenza viruses in Europe and the United States of America (USA) are genetically distinct. H3N1 influenza viruses have also been isolated from pigs in the USA and Korea (Rep. of), and H2N3 influenza viruses were detected in pigs in the USA but have not apparently become established in the pig population.