Swine influenza, or swine flu (hog flu, pig flu), is one of the most common viral diseases of the respiratory tract in pigs. It is caused by the influenza A virus (IAV), an Orthomyxovirus. IAV have a broad host spectrum but their primary hosts are waterfowl. The principal mammalian hosts include humans, pigs and horses. A distinction is therefore made between avian, human, porcine and equine influenza viruses, although they are capable of exchanging genetic segments.
Influenza A viruses are coated spherical or pleomorphic particles measuring 80-120 nm. The negative-strand RNA genome of influenza A viruses consists of 8 segments coding for 11 proteins. Among these proteins, a distinction is made between surface proteins and internal proteins inside the virion.
Further classification into different subtypes is based on the surface antigens HA (haemagglutinin) and NA (neuraminidase) in or on the cell membrane, which also contain the virus's most important antigenic structures.
Haemagglutinin and neuraminidase are the most important biological markers and are essential for virus entry into the cell and virus excretion.
18 HA subtypes and 11 NA subtypes have been identified to date, allowing many different combinations.