Abstract:

Natural infections with influenza A viruses have been reported in a variety of animal species. Although viruses of relatively few HA and NA subtype combinations have been isolated from mammalian species, all 15 HA subtypes and all 9 NA subtypes, in most combinations, have been isolated from birds.

In the 20th century the sudden emergence of antigenically different strains transmissible in humans, termed antigenic shift, has occurred on four occasions, each time an influenza pandemic occurred. Genetic analysis of the isolates demonstrated that `new' strains most certainly emerged after reassortment of genes of viruses of avian and human origin in a permissive host. The leading theory is that the pig represents the `mixing vessel' where this genetic reassortment may occur.

Isolations of influenza viruses of avian origin from humans:

• 1996 – H7N7 isolated from a woman with self limiting conjunctivitis.
• 1997 – Hong Kong – H5N1 isolated from 18 patients of which six died.
• 1998 – five isolations of H9N2 from people in mainland China
• 1999 – H9N2 isolated from children with flu like symptoms