Changing Prevalence of Influenza A Virus in Wild Birds Across the United States

Influenza A viruses, specifically highly contagious avian influenza viruses, pose many risks to certain avian species, the agricultural industry, and public health. Humans can be infected with the virus by coming in contact with infected birds or their surrounding environments. Influenza A viruses (IAV) threaten poultry production in the United States, as well as human health when spillover (transmission from avian species to humans) occurs.

In order to evaluate IAV risk to wild bird populations, poultry production, and human health effects, scientists have been focusing on IAV prevalence among different wild waterfowl species throughout the year in different areas of the United States (spatiotemporal patterns). Wild waterfowl are birds such as geese, ducks, and swans that have not been domesticated. This study focused on wild waterfowl populations as they are the primary host for many viruses, such as influenza A virus. These wild waterfowl often spread disease to domestic poultry, which can cause outbreaks of illness on poultry farms. 

CEID’s Justin Bahl, worked with infectious disease researchers across the country to explore these spatiotemporal patterns and variation across species. 

The research team utilized two available data sets from the U.S. Department of Agriculture and National Institutes of Health to examine prevalence and trends in wild waterfowl across the country. Of the birds observed in these datasets, 11.8% tested positive for influenza A viruses. 

The study results found that certain species of wild waterfowl experienced greater IAV prevalence rates during different months of the year, though some overall patterns across species were observed. Overall, most species experienced an increase in positive IAV cases starting in late summer and fall. This is consistent with previous research conducted in this field, and is believed to be due to many young birds being born around this time. These juvenile birds possess weakened immune systems and IAV prevalence lessens in the winter months as these birds gain immunity. Many species also were observed to have another, smaller peak in positive cases around February or March. This smaller peak in IAV prevalence may be linked with emergence of different virus strains, spring migration, or a weaker immune response after the larger fall peak.

Each month, there was variation in IAV prevalence across different areas of the United States. From August through October, there was a higher IAV prevalence in the north-central portion of the country. During fall, the highest prevalence of positive cases moved southward, into the Pacific Northwest and Great Lakes regions. The lowest prevalence of influenza A virus for wild birds was in the Southeastern United States throughout the year. Areas of high IAV prevalence are typically correlated with a high density of ducks in an area. Presumably, the prevalence shift between areas is due to migration patterns.

These findings provide a greater understanding of variations of IAV prevalence across species, time, and regions of the United States. The results of this study can be utilized to assess influenza A virus risk for wild birds, domestic birds, and the public health industry. This data can be used  to guide wildlife management practices to prevent risks for influenza A virus outbreaks.  

The stratified approach used by researchers in this study allowed different IAV prevalence patterns to be identified. Future research examining the underlying contributors to these prevalence patterns and more comprehensive sampling of wild waterfowl throughout the year would be beneficial. 

To read more about this study, click here.

By: Brenna Daly