A recent study led by scientists from The Raptor Center (TRC) at the University of Minnesota College of Veterinary Medicine (CVM) offers important new insight into the impact of highly pathogenic avian influenza (H5N1) on raptors in the upper Midwest, revealing that some species, including bald eagles, have been exposed to the virus and survived. The research provides critical insights into the ongoing avian influenza outbreak and its implications for wild bird populations.

The H5N1 strain of avian influenza has caused widespread infections in wild birds, including large-scale mortality events documented in numerous species. The persistence of the current H5N1 virus in wild bird populations over multiple migration seasons has significant implications for disease transmission, as infected birds contribute to viral spread across vast geographic regions. This circulation increases the risk of spillover to other wildlife and domestic species and also raises concerns about broader ecological disruption.

Raptors are highly susceptible to severe disease from H5N1, but the long-term population impacts have not been evaluated. For this study, researchers analyzed serum and plasma samples from wild raptors to assess previous exposure to H5N1 avian influenza. Samples were collected from two primary sources: injured or sick birds admitted to TRC and healthy wild raptors captured for banding at the Hawk Ridge Bird Observatory during migration.  

The study found that 27 percent of raptors sampled tested positive for antibodies to the influenza A virus (IAV), with 19 percent showing antibodies specific to the H5 and N1 subtypes. Notably, 69 percent of bald eagles (67 out of 97 samples) tested positive for IAV antibodies, with 78 percent (52 out of 67 samples) of those birds carrying antibodies to H5 and N1. These findings suggest that survival rates post-infection may be higher than previously believed, challenging assumptions about the virus’s fatality in raptor species.

“At The Raptor Center, we received more than 200 birds that were infected and sick with HPAI from March of 2022 through December 2023, and only one survived to release,” reports Kelsey Rayment, CVM veterinary resident and author of the study. “However, we only saw the sickest raptors that could be caught and brought in. The findings of our study show that there are raptors in the wild who were infected and survived. The number of eagles in our study with antibodies to HPAI was significantly higher than we expected and also higher than has been documented in previous studies.”

The research compared current antibody prevalence with historical data from a 2012 serosurvey at The Raptor Center, which found significantly lower antibody rates (5.1 percent in bald eagles). The dramatic increase in seroprevalence underscores the widespread exposure of raptors to the virus during the ongoing global outbreak of the H5N1 strain.

While the study provides evidence of survival, the researchers caution that the long-term effects of H5N1 on raptor health and population dynamics remain uncertain. Additional research is needed to determine whether immunity from past infections offers long-term protection and how continued exposure may affect different age groups within raptor populations.

The study also underscores the value of wildlife rehabilitation centers and observatories in tracking emerging infectious diseases.  

“At The Raptor Center, we see about 1,000 birds of prey a year, which gives us a unique view on the health of our native wild raptors,” Rayment says. “You can imagine that going out and catching a large number of adult raptors for disease monitoring would be incredibly challenging. We see raptors from a large part of the state, and we can leverage that to get information on not just the health of the individual bird but also understand the bigger picture of what is going on in their environment.”

The research highlights the need for ongoing surveillance and conservation efforts to protect vulnerable raptor species as H5N1 continues to circulate globally, as well as further studies to evaluate transmission dynamics, long-term immunity, and potential impacts on raptor reproductive success.

The study was conducted in collaboration with researchers from the University of Minnesota-Duluth, the University of Georgia Southeastern Cooperative Wildlife Disease Study (SCWDS), The Pennsylvania State University, and the Hawk Ridge Bird Observatory. Funding was partially supported by the National Institute of Allergy and Infectious Diseases.

Find the full study in Nature Scientific Reports.