Avian Influenza Viruses in Coastal Louisiana Waterfowl: Host and Environmental Factors Affecting Prevalence and Viral Persistence.

Abstract

Cloacal and tracheal swabs were collected from 1,389 hunter-killed ducks in Cameron Parish, Louisiana during the 1986 and 1987 waterfowl seasons. These included 605 blue-winged teal (Anas discors), 75 mottled ducks (A. fulvigula), 375 gadwalls (A. strepera) and 334 green-winged teal (A. crecca). Twenty-eight avian influenza viruses (AIVs) were isolated. Prevalence estimates of AIV in ducks during September, November and December/January were 3.1%, 2.1%, and 0.4%, respectively. Differences in prevalence were detected by season ($P$ =.044) and age class ($P$ =.036). Two isolations from resident mottled ducks indicate local transmission of AIV on these wintering areas. Much subtype diversity was present with 9 of 13 HA and 9 of 9 NA subtypes recovered. Predominant subtypes were typical of AIVs commonly associated with waterfowl. A plaque assay served as an in vitro test of pathogenicity. Results for 18 of 28 AIVs were consistent with expected values for viruses of low-pathogenicity in chickens. Linear regression models for five AIVs demonstrated that at an initial titer of 1 $\times$ 10$\sp6$TCID$\sb{50}$, infectivity would persist in distilled water for up to 207 days at 17 C and 102 days at 28 C. Significant differences in slope were detected between temperature treatments and between AIVs. Combined effects of water temperature, salinity and pH on AIV persistence were evaluated in a model distilled water system using three isolates. Variables were tested within ranges normally associated with surface water. Differences were detected between temperature (17 C and 28 C), pH (6.2, 7.2, 8.2) and salinity (0 ppt and 20 ppt) treatments with a strong interactive effect observed between pH and salinity. The estimated duration of infectivity for 1 $\times$ 10$\sp6$TCID$\sb{50}$ of A/mottled duck/LA/38M/87 (H6N2) under test conditions ranged from 9 to 100 days. Differences in response to these variables were apparent between viruses. The ability of AIV to persist in surface water was evaluated using water samples collected from varied waterfowl habitats in coastal Louisiana. Results were consistent with the model system.