Avian Influenza Prevalence Correlated to Mercury Concentrations in Wild Waterfowl Active

Wild waterfowl, serve as natural reservoirs for low pathogenic avian influenza viruses.  Previous studies by U.S. Geological Survey (USGS) researchers have improved our understanding of viral transmission, including environmental persistence and infection prevalence across space and time. 

In general, low pathogenic viruses present limited risk to wild waterfowl; however, when introduced into domestic poultry facilities, there is a risk of viruses mutating into highly pathogenic forms that can have dramatic economic effects on the poultry industry. For example, highly pathogenic avian influenza was recently introduced into North America in December 2021, likely from wild birds, and resulted in several outbreaks and large domestic bird population losses. 

Although the transmission of avian influenza is regularly under investigation, few studies have investigated how vulnerability to infection can be affected by exposure to environmental contaminants such as mercury in wetland habitats inhabited by waterfowl.  This gap in information limits our understanding of how contaminants and pathogens interact to influence wildlife health. 

To improve the understanding of how mercury contamination across the landscape may affect avian influenza transmission, USGS scientists and their collaborators collected approximately 750 samples from 11 species of wild waterfowl in the Central Valley and San Francisco Bay Estuary of California. These two areas have documented mercury contamination from atmospheric deposition and from industrial outputs or gold mining and are important sites for overwintering waterfowl.

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Approximately 60 percent of the migratory waterfowl in the Pacific Flyway overwinter in this region annually. The combination of mercury contamination, high waterfowl abundance, and intensive poultry production in this region makes northern California an important area for understanding if mercury contamination affects influenza prevalence and wildlife health. 

In this study, researchers reported that active and prior (as indicated by the presence of antibodies) avian influenza infections were greater in wild ducks with higher average blood mercury concentrations. The positive relation between influenza infection and mercury concentrations was stronger when measuring antibodies indicative of prior infection than of active infection. Furthermore, influenza antibody prevalence was related to average mercury concentration across all species, with some differences among individual species likely owing to habitat use, diet, and immunology. 

The results of this study highlight the importance of understanding the interactions among contaminants, infection, and wildlife health. Together these results provide an indication that exposure to mercury may be related to immunotoxicity. More generally, these results indicate that the effects of mercury and potentially other contaminant exposures could extend beyond the geographic area of contamination for highly mobile avian species.   

Although the results indicate contaminant-induced susceptibility to avian influenza infection in wild waterfowl, questions remain about the mechanisms of toxicity and the species that could be affected. Answers to these questions can provide information valuable for selecting mitigation steps that interrupt the infection cycle and help stop the spread not only among wildlife but also to domestic species. 

This study was supported, in part, by the USGS Ecosystem Mission Area, through the Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology Program) Ecologically Driven Environmental Pathways Team; and the Biological Threats and Invasive Species Research Program; and the California Department of Water Resources; and by the National Institute of Health (NIAID grants HHSN272201500008C and HHSN26200700010C).