[Influenza virus epidemiology and ecology, with special reference to bird species associated with water. Literature review and observations]

Only a limited number of A-subtypes of influenza virus so far caused disease in human subjects, pigs and horses; this occurred in more or less defined areas which occasionally showed epidemic aggravations, becoming apparent as rapidly spreading epidemics or otherwise in even the form of pandemics. However this number of antigenic subtypes was found to be fairly constant and host-specific. Earlier studies were done in domesticated fowl and birds, though particularly in water birds in recent years, and numerous subtypes were detected, only a small number of these subtypes also being found to occur in man, pigs and horses. It became increasingly apparent that particularly mallards, but also other water birds play an extremely important role in the maintenance as well as in the distribution and circulation of these orthomyxoviruses in nature. These infections in water birds were not merely caused by a single subtype but occasionally by two or more antigenically different subtypes. This could be conducive to the appearance of recombinants as a result of genetic rearrangement in the cells lining the alimentary tracts of birds. Occasionally, subtypes observed in man were also found to occur in birds, which gave rise to the question of the extent to which birds are the origin or sources of infections of human epidemics caused by these subtypes. This also holds good for the subtypes in pigs. In addition to a number of oecological and ornithological considerations, reference was also made to systematic facts and routes along which further investigations on the presence of influenza viruses in the world of birds could be taken up, particular attention being paid to migratory birds. As birds of passage pass over and find their way into isolated areas as well as human population centres, these birds play a role which is yet unknown both in the distribution and in the overwintering of influenza viruses. Conditions in which wild and domesticated (water) birds, pigs, horses and man form a chain of close contact, and the areas in which new influenza viruses pathogenic for man are most likely to appear. Studies on the transgression of these barriers of species by subtypes of influenza virus still are entirely separate matter. The fact that a multidisciplinary approach is essential admits of no discussion.