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About the Authors:
Viviane Hénaux
* E-mail: [email protected]
Affiliation: Department of Forest and Wildlife Ecology, University of Wisconsin, Madison, Wisconsin, United States of America
Michael D. Samuel
Affiliation: Wisconsin Cooperative Wildlife Research Unit, U.S. Geological Survey, Madison, Wisconsin, United States of America
Robert J. Dusek
Affiliation: National Wildlife Health Center, U.S. Geological Survey, Madison, Wisconsin, United States of America
Joseph P. Fleskes
Affiliation: Western Ecological Research Center, U.S. Geological Survey, Dixon, California, United States of America
Hon S. Ip
Affiliation: National Wildlife Health Center, U.S. Geological Survey, Madison, Wisconsin, United States of America
Introduction
Wild birds (orders Anseriformes and Charadriiformes) are capable of maintaining and spreading most subtypes of low pathogenic avian influenza viruses (LPAIv) [1]. LPAIv replicate primarily in the intestinal tract of infected birds, with large amounts of virus shed through feces into the environment [2]. Based on experimental studies, Hénaux and Samuel [3] estimated that virus excreted during the infectious period represented about 1,500 times the median bird infectious dose (BID50) for LPAIv. This level of contamination implies that the environment is critical to AIv transmission through the fecal/oral route [4]. Accordingly, recent modeling of LPAIv dynamics in wild waterfowl suggested that disease cannot be maintained in many populations without environmental transmission [5]–[6].
The role of the environment as a reservoir for AIv is also supported by the ability of LPAIv to persist in water for extended periods [7]–[9]. Experimental studies demonstrated that temperature greatly influences viral persistence, with an exponential decay of viral infectivity as temperature increases [7]. In addition, AIv are most stable in freshwater (i.e., low salinity) with pH between 7.4 and 8.2 [8], [10]–[11]. Prolonged infectivity in cold freshwater (≤4°C [2], [7], [9]) suggests that in the northern hemisphere (implied hereafter) AIv may persist longer in northern than southern waterfowl habitats, and infect migratory birds returning to breeding areas during spring [12]–[13]. In contrast, decreased survival in warmer water implies limited LPAIv persistence and transmission among non-migratory waterfowl during summer on southern wetland areas [7].
Although the transmission of AIv was documented in resident waterfowl in southern areas during winter [14], the role of local populations in the maintenance of AIv during summer is still unknown. Identifying the sources of AIv affecting wintering waterfowl (i.e., AIv...