#18,788
Although Texas was among the first states to report HPAI H5N1 in dairy cattle herds (n=29) in the spring of 2024, it now ranks well behind California (n=767), Idaho (n=107), and Colorado (n=64) in the number of infected herds.
Doubts have been raised over the accuracy of these numbers, however, including a recent study (see Nature: A Mathematical Model of H5N1 Influenza Transmission in US Dairy Cattle),which strongly suggests that the H5N1 virus is far more widespread in U.S. dairy cows than has been reported.Nationally, Texas ranks 5th, just behind Michigan (n=31).
As the list below illustrates, 90% of Texas's detections occurred in the spring and summer of 2024, followed by a 3 month gap with no reports until December, followed by another 5 month gap lasting until May of 2025.
We've seen similar extended gaps in detections from other states, including Idaho and Michigan (see chat below). (Note: Confirmation dates may differ from collection dates).
All of which begs the question; how do we account for these gaps?
Possibilities include infected herds being missed, the reintroduction of the virus via cattle from another state, a new spillover from birds, or perhaps the virus was spreading unseen and reintroduced by some other animal host.
Collectively, our analysis indicates that genotype B3.13 viruses continuously circulated in Texas, even though no outbreaks were reported between July and December 2024.
It is unclear if the viruses were maintained in dairy cattle (apparently healthy or undiagnosed), or if they spilled over into other species and were reintroduced into dairy cows.
A link and some excerpts, then I'll have a bit more after the break.
Phylogenetic analysis of H5N1 influenza viruses isolated from dairy cattle in Texas in December 2024
Sangam Kandel1, Lavanya Babujee1, Lizheng Guan1, Randall Dahn1, David Pattinson, Peter J. Halfmann1, Amie J. Eisfeld, Gabriele Neumann, Alexis C. Thompson2, Ellen Ruth Alexander Morris3, Amy K. Swinford3, Keith Poulsen4, Kiril M. Dimitrov3, Yoshihiro Kawaoka
https://doi.org/10.1128/jvi.00580-25
Highly pathogenic avian influenza (HPAI) viruses of genotype B3.13 were first reported in dairy cattle in March 2024 and, by 4 March 2025, had infected at least 989 cattle herds in 17 states in the US (https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/hpai-confirmed-cases-livestock; accessed on March 24, 2025).
Importantly, spillover infections of poultry and mammalian species, including cats and humans, have occurred (https://www.cdc.gov/bird-flu/situation-summary/index.html; accessed on 4 March 2025). Texas reported several infected herds in the spring (1) and early summer of 2024 but saw fewer outbreaks during the second half of 2024, with reported outbreaks in July and December (2). Since September 2024, almost all reported outbreaks of genotype B3.13 viruses in dairy cattle have occurred in California.
Here, we characterized self-submitted samples from a farm in Texas that experienced an outbreak in December 2024. This farm had purchased dairy cattle from several states in the US.
We, therefore, asked whether the December 2024 outbreak in Texas was caused by B3.13 viruses that circulated undetected in dairy cattle or other species in Texas or resulted from the introduction of B3.13 viruses from California (having the highest number of affected farms at that time) or other states.
(SNIP)
The isolated viruses are closely related to B3.13 viruses isolated in Texas in the summer of 2024, supported by bootstrap values of >0.7 (Fig. 1). Phylogenetic analysis of individual viral gene segments demonstrated a close relationship between B3.13 viruses isolated in July and those isolated in December (Fig. S1).
This analysis also reports on a small number of recently acquired amino acid changes in various gene segments of the virus, which may - or may not - affect its behavior.
Testing of cattle remains largely voluntary, with many dairy farmers - fearing loss of income or the stigma of infection - opting for a `Don't test, don't tell' strategy. The USDA's powers and jurisdiction - like the CDC's - are often limited in these cases.The key point remains: The HPAI virus continued to circulate - under the surveillance radar - likely either in asymptomatic cattle, or some other animal hosts (e.g. mice, rats, voles, cats, etc.).
Additionally, beef cattle are almost never tested for HPAI, despite evidence that steers and bulls are susceptible to other influenza A viruses (see Virology: Detection of Antibodies Against Influenza A Viruses in Cattle).
And despite numerous reports of infected cats, mice, raccoons and rats in and around infected dairy herds, the extent and importance of these spillovers has yet to be clearly elucidated.
All of which suggests that reassuring reports - such as the USDA's most recent update (below) showing just 1 dairy herd infected with H5N1 in the past 30 days - may not be entirely accurate.
. . . we were unable to recapitulate transmission to sentinel cows via contaminated milking equipment and close contact with infected animals under experimental conditions.
They then went on to state:
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.While we've been lucky so far - and H5N1 hasn't sparked a large-scale human outbreak - these viruses continue to evolve along numerous pathways, and what emerges tomorrow, next month, or next year may not be as tame.
