The North American H5N1 epizootic: Unraveling the Ecological Spread
Since late 2021, a devastating panzootic of highly pathogenic H5N1 has wreaked havoc on wild birds, agriculture, and mammals. This analysis delves into the intricate web of its spread, revealing a fascinating yet concerning story. Imagine a complex puzzle where each piece represents a different species and their interconnected journeys.
Our investigation, published in Nature (https://www.nature.com/articles/s41586-025-09737-x), analyzed 1,818 haemagglutinin sequences from a diverse range of sources: wild birds, domestic birds, and mammals. The findings paint a vivid picture of how this panzootic took hold.
The North American panzootic, it turns out, was fueled by approximately nine introductions into the Atlantic and Pacific flyways. These introductions were then rapidly disseminated through the migratory patterns of wild birds. Here's where it gets intriguing: Anseriformes, a specific group of birds, played a pivotal role in driving transmission. Meanwhile, non-canonical species acted as dead-end hosts, unable to sustain the virus for long periods.
Comparing this outbreak to the one in 2015 (refs. 1, 2), we found a key difference. Outbreaks in domestic birds were triggered by around 46-113 independent introductions from wild birds, each lasting up to 6 months. Backyard birds were infected approximately 9 days earlier on average than commercial poultry, providing a potential early-warning system for transmission spikes.
This study highlights the critical role of wild birds in the epizootic. It emphasizes the importance of enhanced surveillance in these populations and implementing strategies to reduce transmission at the wild-agriculture interface. By understanding these intricate ecological dynamics, we can better prepare for future outbreaks and protect both wildlife and human health.
