| Summary: | Influenza A virus (IAV) has been associated with multiple unusual mortality events (UMEs) in North Atlantic pinnipeds, frequently attributed to spillover of virus from wild-bird reservoirs. To determine if endemic infection persists outside of UMEs, we undertook a multiyear investigation of IAV in healthy, live-captured Northwest Atlantic gray seals (Halichoerus grypus). From 2013 to 2015, we sampled 345 pups and 57 adults from Cape Cod, MA, USA and Nova Scotia, Canada consistently detecting IAV infection across all groups. There was an overall viral prevalence of 9.0% (95% confidence interval (CI): 6.4%–12.5%) in weaned pups and 5.3% (CI: 1.2%–14.6%) in adults, with seroprevalences of 19.3% (CI: 15.0%–24.5%) and 50% (CI: 33.7%–66.4%), respectively. Positive sera showed a broad reactivity to diverse influenza subtypes. IAV status did not correlate with measures of animal health nor impact animal movement or foraging. This study demonstrated that Northwest Atlantic gray seals are both permissive to and tolerant of diverse IAV, possibly representing an endemically infected wild reservoir population. National Institute of Allergy and Infectious Diseases (U.S.) Massachusetts Institute of Technology. Sea Grant College Program (Project 2013-DOH- 45-LEV) Natural Sciences and Engineering Research Council of Canada United States. National Marine Fisheries Service (Grants NMFS #17670-01, NMFS #10080-95 and FWS #53514-13003) National Institute of Allergy and Infectious Diseases (U.S). Centers of Excellence for Influenza Research and Surveillance Program (Grant HHSN272201400008C)
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