The emergence of Highly Pathogenic Avian Influenza (HPAI) A/H5N1 clade 2.3.4.4b has led to an unprecedented intercontinental spread, affecting a broad spectrum of avian and mammalian species. Recent outbreaks in US dairy farms, underscore the urgent need to understand the transmission pathways. The study aimed to evaluate the modes of introduction and transmission to dairy farms, through geospatial and exposure analyses. Our findings favour a singular introduction over multiple independent introductions, with non-waterfowl species exhibiting the highest dairy farm exposure, which is a major shift from historical waterfowl spread. Moreover, bidirectional spread between cattle and poultry highlights the intricate nature of disease transmission within the agricultural ecosystem. Additional factors such as livestock trade, poultry litter feed and contaminated milking machinery likely contributed to the amplification of the outbreaks throughout the United States. As large-scale outbreaks persist in the United States, the likelihood of a human pandemic increases, making it imperative to enact and sustain heightened surveillance measures across all potentially impacted species.

The authors have declared no competing interest.

Funding was provided by The Balvi Filantropic Fund for Haley Stone, Mehak Jindal, Samsung Lim, Rebecca Dawson, and Ashley Quigley. Haley Stone would like to acknowledge the support through a University International Postgraduate Award from University of New South Wales. Mehak Jindal would like to acknowledge the support through a Tuition Fee Scholarship Award from the University of New South Wales. Raina MacIntyre is supported by a NHMRC Investigator Grant, 2016907.

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