COINFECTION OF WILD DUCKS BY INFLUENZA-A VIRUSES - DISTRIBUTION PATTERNS AND BIOLOGICAL SIGNIFICANCE

Coinfection of wild birds by influenza A viruses is though to be an im portant mechanism for the diversification of viral phenotypes by gener ation of reassortants. However, it is not known whether coinfection is a random event or follows discernible patterns with biological signif icance. In the present study, conducted with viruses collected through out 15 years from a wild-duck population in Alberta, Canada, we identi fied three discrete distributions of coinfections. In about one-third of the events, which involved subtypes of viruses that appear to be ma intained in this duck reservoir, coinfection occurred at rates either close to or significantly lower than one would predict from rates of s ingle-virus infection. Apparently, the better adapted an influenza A v irus is to an avian population, the greater is its ability to prevent coinfections. Conversely, poorly adapted, nonmaintained viruses were s ignificantly overrepresented as coinfectants. Rarely encountered subty pes appear to represent viruses whose chances of successfully infiltra ting avian reservoirs are increased by coinfection. Mallards (Anas pla tyrhynchos) and pintails (A. acuta) were significantly more likely to be infected by a single influenza A virus than were the other species sampled, but no species was significantly more likely to be coninfecte d. These observations provide the first evidence of nonrandom coinfect ion of wild birds by influenza A viruses, suggesting that reassortment of these viruses in a natural population does not occur randomly. The se results suggest that even though infections may occur in a species, all subtypes are not maintained by all avian species. They also sugge st that specific influenza A virus subtypes are differentially adapted to different avian hosts and that the fact that a particular subtype is isolated from a particular avian species does not mean that the vir us is maintained by that species.