Pathophysiology

The influenza virus infects by binding its haemagglutinin protein to sialic acid-containing molecules on the surfaces of host cells, thereby facilitating entry. The species specificity of influenza viruses is attributable in part to the type of sialic acid and the type of linkage to galactose on host cells. For this reason, avian influenza, which binds with high affinity to alpha-2,3-Gal-terminated sialosaccharides, only infrequently infects human respiratory epithelium, which contains sialic acid (SA)-alpha-2,6-Gal-terminated saccharides. However, pig respiratory epithelium contains both types of sialic acid and linkages, which may explain the ability of swine to serve as a common host for human and avian influenza viruses.

The swine influenza virus is spread by aerosolised droplets or direct contact. It may also be spread by contact with bodily fluids, including diarrhoeal stools. Transmission can occur by person-to-person contact or pig-to-person contact. Eating pork or handling meat does not cause infection. Infection by one H1N1 influenza virus does not confer protection against infection by other H1N1 influenza A viruses, hence the need for annual immunisation against circulating strains.