Influenza: Emergence and Control

The natural reservoirs of influenza A viruses are the aquatic birds of the world (91), in which the viruses appear to be in evolutionary stasis—they are in equilibrium with their natural host and cause no disease. Periodically, the influenza viruses are transmitted to other hosts, including mammals, and cause transitory infections and occasionally deaths. Less frequently, influenza viruses are transmitted to other species and establish continual infections in those hosts. Thus, permanent lineages of influenza A viruses occur in humans, swine, horses, and domestic poultry. What is the difference between transitory infection occasionally causing death and the establishment of permanent lineages in the host? The molecular bases of a virus's ability to spread among a range of hosts and of the pathogenicity of influenza viruses are still unresolved. Since 1997, when H5N1 influenza virus was transmitted to humans and killed 6 of 18 infected persons, there have been multiple transmissions of avian influenza viruses to mammals. Either the whole virus is transmitted directly (12, 81) or gene segments from the avian influenza virus are acquired by mammalian strains (e.g., H3N2 triple reassortants in pigs in the United States) (37, 38, 97, 98). Widespread infections of poultry with H5N1 viruses in Asia have caused increasing concern that this subtype may achieve human-to-human spread and establish interspecies transmission. In this minireview, we consider recent interspecies transmissions of influenza A viruses and examine our limited knowledge of the contributors to the success of these viruses. Finally, we briefly consider control measures.