The ecology of influenza viruses in ducks and analysis of influenza viruses with monoclonal antibodies

Abstract

It has been suggested that influenza A viruses from lower animals may be involved in the origin of human pandemic strains. To test this hypothesis, we have been analyzing the influenza A viruses present in lower mammals and birds. Over the past few years, it has become increasingly evident that a major reservoir of influenza viruses exists in both domestic and feral avian species, particularly ducks. For example, studies on migratory ducks in Canada in 1978 showed that 50% of these healthy birds were infected with influenza viruses. These viruses included 16 different combinations of hemagglutinin and neuraminidase subtypes, including those related to human strains as hemagglutinins, Hav7, H2, and Hsw1 and neuraminidases, N1 and N2. The duck isolates which were antigenically related to the Hsw1N1 viruses from pigs and man were evaluated by in vivo pathogenicity tests in ferrets, ducks, pigs, and chickens; the results indicated that these avian Hsw1N1 viruses replicated in both avian and mammalian species, underlining their potential for interspecies transmission. Since the reassortment of genetic information between human and animal influenza A viruses has been suggested as the mechanism for the origin of new human strains, we also examined the migratory ducks for evidence of mixed infections, i.e. infection with two antigenically distinct viruses to determine if this phenomenon occurs in nature. Antisera to the original isolate was used to select for antigenically different viruses in the original cloacal sample. Mixtures of two and, on rare occasions, three distinct subtypes (e.g., Hav7N2, Hav6N2, and Hav7Neq2) were isolated from one bird. Analysis of the RNAs of these viruses by polyacrylamide gel electrophoresis suggested that genetic reassortment between viruses in the mixedly infected birds was occurring. These findings agree with laboratory studies in which recombinant viruses were isolated from mixedly infected ducks. These data indicate that mixed infections do occur in the natural setting, thus establishing the situation where genetic reassortment between viruses could occur. The avian species, particularly ducks, continue to represent an impressive reservoir of influenza A viruses and may well contribute to the appearance of related influenza viruses in other species, including humans.