Avian Influenza Viruses in Minnesota Ducks During 1998–2000

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Abstract
Although wild ducks are known to be a major reservoir for avian influenza viruses (AIV), there are few recent published reports of surveillance directed at this group. Predominant AIV hemagglutinin (HA) subtypes reported in previous studies of ducks in North America include H3, H4, and H6, with the H5, H7, and H9 subtypes not well represented in these host populations. The objective of this study was to determine whether these subtype patterns have persisted. Each September from 1998 to 2000, cloacal swabs were collected from wild ducks banded in Roseau and Marshall counties, MN. Mallards (Anas platyrhynchos) were sampled all years, and northern pintails (A. acuta) were sampled only in 1999. Influenza viruses were isolated from 11%, 14%, and 8% of birds during 1998, 1999, and 2000, respectively. Prevalence, as expected, was highest in juveniles, ranging from 11% to 23% in mallards. Viruses representative of the HA subtypes 2, 3, 4, 5, 6, 7, 9, 10, 11, and 12 were isolated. Viruses in the H5, H7, and H9 subtypes, which are associated with high-pathogenicity influenza in poultry or recent infections in humans, were not uncommon, and each of these subtypes was isolated in 2 out of the 3 years of surveillance.