Swine influenza (SI) is an acute respiratory disease of swine caused by type A and type C influenza viruses. Influenza A viruses are segmented negative-strand RNA viruses and can be isolated from a number of other animal host species, including birds, humans, horses, whales, and mink. Although whole influenza viruses rarely cross the species barrier, gene segments can cross this barrier through the process of genetic reassortment, or genetic shift. Pigs support the replication of both human and avian influenza A viruses and have been postulated to play an important role in interspecies transmission by acting as a “mixing vessel” for reassortment between viruses specific to different host species (Scholtissek, Eur. J. Epidemiol. (1994) 10:455-458). This may lead to the generation of influenza viruses capable of crossing the species barrier to humans.
Influenza virions include an internal ribonucleoprotein core (a helical nucleocapsid) containing the single-stranded RNA genome, and an outer lipoprotein envelope lined inside by a matrix protein (M1). The genome of influenza A virus consists of eight segmented negative sense single-stranded RNA molecules. Each segment possesses segment-specific RNA packaging signals which are composed of both the noncoding regions and short coding regions at both 5′ and 3′ ends. The eight segmented RNAs encode 11 viral proteins, including RNA-dependent RNA polymerase proteins (PB2, PB1 and PA) and nucleoprotein (NP) which form the nucleocapsid; the matrix membrane proteins (M1, M2); hemagglutinin (HA) and neuraminidase (NA), both surface glycoproteins which project from the lipid-containing envelope; the nonstructural protein (NS1), nuclear export protein (NEP, also termed NS2), the proapoptotic factor PB1-F2. HA is critical for virus binding and entry to the cells, and is the major neutralizing antibody target, whereas NA plays a role in progeny virus release and is essential for virus propagation. Transcription and replication of the genome take place in the nucleus and assembly occurs via budding on the plasma membrane. The viruses can reassort genes during mixed infections.
Multiple swine influenza virus (SIV) subtypes continue to circulate in swine populations despite available vaccines. Currently, H1N1, H3N2, and H1N2 are the dominant subtypes that cause disease in the North American swine population. SIVs of the subtype H3N2 were generated by reassortment between human, avian and classical swine viruses, are undergoing rapid evolution and in general cause more severe disease than classical H1N1 SIV. Current SIV vaccines do not provide cross-protection against multiple antigenic SIV variants.
Thus, there remains a need for the development of effective strategies for the treatment and prevention of swine influenza infection.