Recent zoonoses caused by influenza A viruses.

Influenza is a highly contagious, acute illness which has afflicted humans and animals since ancient times. Influenza viruses are part of the Orthomyxoviridae family and are grouped into types A, B and C according to antigenic characteristics of the core proteins. Influenza A viruses infect a large variety of animal species, including humans, pigs, horses, sea mammals and birds, occasionally producing devastating pandemics in humans, such as in 1918, when over twenty million deaths occurred world-wide. The two surface glycoproteins of the virus, haemagglutinin (HA) and neuraminidase (NA), are the most important antigens for inducing protective immunity in the host and therefore show the greatest variation. For influenza A viruses, fifteen antigenically distinct HA subtypes and nine NA subtypes are recognised at present; a virus possesses one HA and one NA subtype, apparently in any combination. Although viruses of relatively few subtype combinations have been isolated from mammalian species, all subtypes, in most combinations, have been isolated from birds. In the 20th Century, the sudden emergence of antigenically different strains in humans, termed antigenic shift, has occurred on four occasions, as follows, in 1918 (H1N1), 1957 (H2N2), 1968 (H3N2) and 1977 (H1N1), each resulting in a pandemic. Frequent epidemics have occurred between the pandemics as a result of gradual antigenic change in the prevalent virus, termed antigenic drift. Currently, epidemics occur throughout the world in the human population due to infection with influenza A viruses of subtypes H1N1 and H3N2 or with influenza B virus. The impact of these epidemics is most effectively measured by monitoring excess mortality due to pneumonia and influenza. Phylogenetic studies suggest that aquatic birds could be the source of all influenza A viruses in other species. Human pandemic strains are thought to have emerged through one of the following three mechanisms: genetic reassortment (occurring as a result of the segmented genome of the virus) of avian and human influenza A viruses infecting the same host direct transfer of whole virus from another species the re-emergence of a virus which may have caused an epidemic many years earlier. Since 1996, the viruses H7N7, H5N1 and H9N2 have been transmitted from birds to humans but have apparently failed to spread in the human population. Such incidents are rare, but transmission between humans and other animals has also been demonstrated. This has led to the suggestion that the proposed reassortment of human and avian viruses occurs in an intermediate animal with subsequent transference to the human population. Pigs have been considered the leading contender for the role of intermediary because these animals may serve as hosts for productive infections of both avian and human viruses and, in addition, the evidence strongly suggests that pigs have been involved in interspecies transmission of influenza viruses, particularly the spread of H1N1 viruses to humans. Global surveillance of influenza is maintained by a network of laboratories sponsored by the World Health Organization. The main control measure for influenza in human populations is immunoprophylaxis, aimed at the epidemics occurring between pandemics.