Currently, three subtypes circulate in swine: H1N1,
H1N2 and H3N2. In contrast to human influenza, the properties of
swine influenza differ from region to region. The predominant subtype
in Europe is of avian origin, most likely introduced into pigs in
1979 from wild aquatic birds, such as ducks. In contrast, there are two
distinct subtypes circulating in North America, the classical H1N1
subtype introduced into pigs shortly after the 1918 human pandemic,and the second a reassortment between H1N1 with either H3N2 or
H1N1 viruses.