Sacramento refuge study finds H5N1 in three sampled waterbirds
The story at a glance
Researchers tested 384 swab samples and 204 serum samples from waterfowl and other waterbirds at California’s Sacramento National Wildlife Refuge Complex between October 2022 and July 2025. H5N1 was confirmed in three birds: two Snow Geese and one Greater White-fronted Goose. All three appeared asymptomatic when captured. 123
Antibodies to influenza were widespread in sampled ducks and geese, suggesting previous exposure. The findings do not establish population-level immunity, or show that all antibody responses were caused by highly pathogenic H5N1. The researchers describe serological monitoring as a potential tool to inform future disease management. 12
What you need to know
- The three confirmed infections were found among 384 birds tested by swab. The infected geese showed no apparent symptoms at capture, so their condition at that point did not indicate whether they had H5N1. 12
- Among sampled ducks, antibodies to both H5 and N1 were detected in 64% of birds sampled in 2023 and 80% in 2024. These results are evidence of substantial previous exposure, not a direct measure of how many birds in the wider population are protected. 12
- The antibody tests cannot reliably distinguish exposure to highly pathogenic H5 viruses from exposure to low-pathogenic H5 viruses. The results therefore cannot establish that all detected antibodies followed H5N1 infection. 12
- American Coots had H5 antibodies, but antibody prevalence for influenza A and H5 was lower in coots and Black-necked Stilts than in the waterfowl sampled alongside them. The study could not determine why, and notes limited sampling, particularly for stilts. 12
- The authors present serological surveillance as a supplementary way to inform future H5N1 risk management for wild birds. The study reports monitoring results, not an intervention or evidence that a management action has improved outcomes. 12
Why this matters
The study shows how antibody testing can add information about previous influenza exposure alongside tests for active infection. For managers, the distinction matters: antibody findings may help inform surveillance, but they cannot on their own show which virus circulated or establish immunity across a population. 12
What happens next
The authors identify broader, repeated sampling across seasons and years as a way to investigate changes in infection and antibody prevalence. Further work would also be needed to clarify which viruses account for the antibody results and why prevalence was lower in coots and stilts. 12
What we know
- The samples were collected at the Sacramento National Wildlife Refuge Complex from October 2022 to July 2025. 123
- The study analysed 384 swab samples and 204 serum samples from waterfowl and other waterbirds. 123
- California Department of Fish and Wildlife surveillance recorded H5N1 detections in 117 wild birds across 29 counties between October 2023 and April 2024. This provides statewide context, not an independent test of the refuge study. 4
What we don't know
- The results do not establish how infection or antibody prevalence changes across seasons and years; the authors identify broader repeated sampling as an open need. 12
- The antibody tests cannot reliably determine whether detected H5 antibodies followed exposure to highly pathogenic H5N1 or to low-pathogenic H5 viruses. 12
- The study cannot explain the lower antibody prevalence in coots and stilts or establish how well that finding reflects their actual infection risk. Sampling, particularly of stilts, was limited. 12
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