Revisiting the Environmental Persistence and Airborne Spread of HPAI H5

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Not quite two weeks ago, in Preprint: Surveillance on California Dairy Farms Reveals Multiple Sources of H5N1 Transmission, we looked at a (yet-to-be-peer-reviewed) paper that found evidence of extensive environmental (air, water & milking equipment) contamination on HPAI H5 infected dairy farms.

That report - when combined with a recent study (see Dairy Cows Infected with Influenza A(H5N1) Reveals Low Infectious Dose and Transmission Barriers) - would seem to challenge the popular assumption that cow-to-cow transmission of HPAI was primarily due to contaminated milking machines.

Two days ago the Journal Nature took note of the California study:

NATURE BRIEFING
12 August 2025

Daily briefing: Bird flu is ‘everywhere’ on dairy farms

H5N1 avian influenza might be airborne, helping it to spread rapidly in dairy cows

Yesterday UNMC's Global Center for Health Security - which quoted Dr. Richard Webby as saying  “It’s a ridiculously contaminated environment” -  published:

Bird Flu on Dairy Farms May Be Airborne After All

These airborne concerns go far beyond just`exhaled' breath from infected cattle in milking parlors, or `milk spray', as contaminated milk and manure from infected cows must be safely handled and disposed of (along with farm wastewater); none of which are trivial tasks.  

While the USDA has issued biosecurity guidelines (link), the details (and enforcement) are left up to local officials and the producers. 

Any way you slice it, HPAI infected poultry and dairy farms must deal with extensive environmental contamination issues. A concern because we've seen environmental persistence studies showing that - under the right conditions - HPAI H5 can survive for days, weeks, or even months outside of a living host. 

• In 2012's EID Journal: Persistence Of H5N1 In Soil, we looked at several studies that found H5N1 could remain viable on various surfaces, and in different types of soil, for up to 13 days (depending upon temperature, relative humidity, and UV exposure).
• In 2017, researchers showed that - when refrigerated - H5N1 infected poultry could remain infectious for months (see Appl Environ Microbiol: Survival of HPAI H5N1 In Infected Poultry Tissues).

• In 2020 we looked at a study from researchers at the USGS (see Proc. Royal Society B: Influenza A Viruses Remain Viable For Months In Northern Wetlands - USGS), which found long-term (up to 7 months) survival of influenza A viruses in wetlands in both Alaska and Minnesota.

How long avian flu viruses may remain viable, and how far they might be carried (by personnel, vehicles, peridomestic mammals, birds, flies, or even the wind), continues to be poorly understood.

As recently as last February - in Preprint: Genetic & Meteorological Data Supporting Windborne Transmission of HPAI H5N1 - we saw a study that strongly suggested that windborne spread of HPAI H5 virus particles may have spread the virus as far as 8 km between poultry farms in the Czech Republic.

Separation of Farms In Study

We can go back more than a dozen years to find other studies which came to similar conclusions, including. 

• In December of 2012 (see Barnstorming Avian Flu Viruses?) we looked at a study called Genetic data provide evidence for wind-mediated transmission of highly pathogenic avian influenza that found patterns that suggested farm-to-farm spread of the 2003 H7N7 in the Netherlands due to the prevailing wind.

• Another study of the same outbreak, Modelling the Wind-Borne Spread of Highly Pathogenic Avian Influenza Virus between Farms (PloS One 2012), found that windborne transmission could have accounted for up to 24% of the transmission over distances up to 25 km.

• In the spring of 2015 during the North American H5Nx epizootic, the idea of farm-to-farm spread via infected dust was openly discussed by the USDA (see Bird Flu’s Airborne `Division’).

• In 2018, in Frontiers: Two Studies On The Epidemiology of Avian Influenza Viruses, we looked at a study that detected airborne HPAI viruses during the 2016-17 H5N8 epizootic in France, which saw more than 400 farms affected.

• And in 2019, in Nature: Airborne Transmission May Have Played A Role In Spread Of U.S. 2015 HPAI Epizootic, we saw a study that looked at air movement trajectories and viral concentrations during the epizootic and the probability of airborne transmission for the 77 HPAI cases in Iowa. While not definitive, long-range airborne spread was considered plausible.

In 2022's Zoonoses & Public Health: Aerosol Exposure of Live Bird Market Workers to Viable Influenza A/H5N1 and A/H9N2 Viruses, Cambodia, researchers were able to extract viable avian flu viruses from the air in and around live bird markets in Cambodia.

And last January, in Osterholm Podcast: The Potential Environmental (Airborne) Spread of H5N1, Dr. Mike Osterholm discussed the real possibility that the H5N1 virus may be carried by contaminated `dust' from poultry farms, infecting other nearby farms, animals, and potentially even humans.

As we discussed yesterday, a big concern is the potential introduction of HPAI to swine (see Frontiers Vet. Sci (Review): Emerging Threats of HPAI H5N1 Clade 2.3.4.4b in Swine).  Airborne spread between farms is one plausible way that could happen.

Whether H5N1 has the ability to spark a pandemic remains to be seen - but even if it can't - it can still do tremendous damage to agricultural interests and to the economy.  

Which is why a fuller understanding of its abilities (both existing and evolving) is crucial if we hope to avoid a larger crisis.