Credit EID Journal
#18,821
Six weeks ago, in Dairy Cows Infected with Influenza A(H5N1) Reveals Low Infectious Dose and Transmission Barriers, we looked at a preprint which challenged the popular assumption that cow-to-cow transmission of HPAI was primarily due to contaminated milking machines.
While they confirmed that Bovine (B3.13) H5N1 efficiently infected the bovine mammary gland at very low infectious doses - and produced clinical disease and reduced milk production - they were unable to duplicate the spread of the virus via contaminated milking equipment under controlled experimental conditions.
While contaminated milking equipment is strongly hypothesized to be the primary route of exposure on-farm5,6, our experimental findings did not confirm this, falling short of an explanation for the apparently frequent on-farm transmission.
While small sample sizes in experimental settings may limit the ability to observe transmission, this also raises important questions about whether real-world farm conditions present higher risks than can be reproduced in high-containment facilities.
Other agent, host, and environmental cofactors that might contribute to transmission cannot be ruled out and must be explored as this study demonstrates critical gaps in our understanding of transmission.
We've previously looked at a number of other plausible vectors, including insects (see H5N1 in California: The Return of the Fly), rodents (see Emer. Microbe & Inf.: HPAI Virus H5N1 clade 2.3.4.4b in Wild Rats in Egypt during 2023), and even contaminated dust (see Preprint: Genetic & Meteorological Data Supporting Windborne Transmission of HPAI H5N1).
Today we have another preprint (not yet peer reviewed) that - despite relying on fairly limited testing - provides evidence of other potential avenues of on-farm transmission.
The researchers collected and tested air, wastewater, and milk samples from multiple California farms and milking parlors to identify transmission sources, and found evidence of extensive environmental (air, water & milking equipment) contamination.
Key findings include:
- detection of infectious H5N1 virus in milking parlor air and farm wastewater
- evidence of airborne transmission from exhaled cow breath
- H5N1 infection without mastitis symptoms, suggesting subclinical milk producing cows may be going undetected
- the detection of an N189D HA mutation (which may affect the receptor binding domain) in at least one air sample
- and heterogeneous patterns of viral infection across individual udder quarters that call into question the assumed `primary role' played by milking equipment
Running 31 pages, this preprint has a lot to unpack, and most of my readers will want to follow the link to read it in its entirety. I've only posted the abstract a a few short excerpts below.
I'll have a brief postscript after the break.
Surveillance on California dairy farms reveals multiple sources of H5N1 transmission
A.J. Campbell, Meredith Shephard, Abigail P Paulos, Matthew Pauly, Michelle Vu, Chloe Stenkamp-Strahm, Kaitlyn Bushfield, Betsy Hunter-Binns, Orlando Sablon, Emily E Bendall, William J Fitzimmons, Kayla Brizuela, Gracie Quirk, Nirmal Kumar, Brian McCluskey, Nishit Shetty, Linsey C Marr, Jenna Guthmiller, Kevin Abernathy, Adam S Lauring, Blaine T Melody, Marlene Wolfe, Jason Lombard, Seema S Lakdawala
doi: https://doi.org/10.1101/2025.07.31.666798
Abstract
Transmission routes of highly pathogenic H5N1 between cows or to humans remain unclear due to limited data from affected dairy farms. We performed extensive air, farm wastewater, and milk sampling on 14 H5N1 positive dairy farms across two different California regions.
Virus was detected in the air in milking parlors and from exhaled breath of cows. Infectious H5N1 virus was detected in the air and water streams; sequence analysis revealed viral variants on a farm in these locations. Longitudinal analysis of milk from the individual quarters of cows revealed a high prevalence of subclinical H5N1 positive cows and a heterogeneous distribution of infected quarters that maintained a consistent pattern over time. Our data highlight potential modes of H5N1 transmission on dairy farms.
(SNIP)
DiscussionElucidating the routes of transmission of H5N1 between cows is critical to defining successful mitigation strategies. In this study we successfully detected H5N1 in the air and in reclaimed farm wastewater on separate dairy farms on multiple days. This included infectious air samples from three different milking parlors and viral RNA from the exhaled breath of rows of cows on two distinct farms.
Additionally, we detected viral RNA in farm wastewater at multiple sites on various farms and infectious virus at two different sites on the same farm. Together, these results highlight the expansive environmental contamination of H5N1 on affected dairy farms and identify additional sources of viral spread between cows and to humans.
(SNIP)
Taken together, our data confirm the presence of infectious H5N1 virus in the air and reclaimed farm wastewater sites. In addition, we observed high viral loads in the milk of cows, including those without clinical signs, and heterogenous patterns of H5N1 positivity by quarter, suggesting that multiple modes of H5N1 transmission likely exist on farms.
These transmission routes could include contaminated milking equipment from an infected cow, aerosols generated within the milking parlor, and/or contact of teats with contaminated water used to clean housing pens.
Multiple mitigation strategies should therefore be implemented to reduce the risk of H5N1 spread within a herd and to humans. Respiratory and ocular personal protective equipment (PPE) for farm workers to prevent deposition of virus-laden aerosols on these sites, especially in the milking parlor.
Disinfection of milking equipment between each milking of each cow, such as with consistent use of backflush system, could also reduce spread of H5N1 between cows. Treatment of milk from sick cows to inactivate H5N1 prior to disposal as well as treatment of waste streams prior to their use on fields or on farms should also be considered.
Finally, identification of infected cows, regardless of clinical signs, for isolation will help reduce the transmission of H5N1 on farms.
The fact that we are nearly 17 months into this bovine outbreak, and we've yet to get a good handle on how cow-to-cow and cow-to-human transmission takes place, should give everyone pause.
Many dairy farmers - fearing loss of income or the stigma of infection - have been reluctant to allow their livestock to be tested (see EID Journal: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA), and many farm workers - worried about their jobs or immigration - are unwilling to be tested.While we get lucky - and it turns out there is some species barrier that prevents HPAI H5 from becoming a pandemic - `Don't test, don't tell' is a very risky strategy.
Ignoring the public health risks - or implemented half-hearted or ineffective measures - only provides the virus with more opportunities to evolve and adapt, and increases the chances that H5N1 - or one of its descendants - will get lucky.