EID Journal: Highly Pathogenic Avian Influenza A(H5N1) Virus RNA in Bovine Semen, California, USA, 2024

 

#19,121

A little over two years ago HPAI H5N1 was detected in dairy cattle in Texas and Kansas, which prompted me to write A Brief History Of Influenza A In Cattle/Ruminants, where we looked at a number of past papers on both influenza and influenza-like illnesses in cattle and goats.

While relatively rare, both influenza and influenza-like illnesses in cattle and goats had been observed prior to 2024. In some cases, viruses were identified, while in other cases they were not.

Over the next twelve months - despite only limited testing - we saw hundreds of dairy herds across more than a dozen states infected with HPAI, along with sporadic spillovers into goats, alpacas, pigs, and even a sheep in the UK.

From the beginning, testing was restricted almost exclusively to lactating dairy cows - and for practical/logistical reasons that testing has often been indirect - via bulk tank testing of milk. 

• The USDA's Dairy Herd Status Program website hasn't been updated since Jan 8th, 2006 , but in its first year showed just 130 herds (out of an est. 36,000) from 21 states enrolled in the voluntary herd monitoring program.  

• Far more testing is done, however, via the National Milk Testing Strategy (NMTS) and individual state programs, which test pooled milk from many farms and is primarily used to identify which regions/states have the virus. 

Since the prevailing theory is that HPAI mainly affects lactating dairy cows - due to the virus's affinity to bovine mammary cells - very little testing of dry cows, heifers, calves, or beef cattle (bulls & steers) has been done. 

This, despite a March 2025 study (see Virology: Detection of Antibodies Against Influenza A Viruses in Cattle)  which reported that bulls and steers were just as likely to carry antibodies to (non-HPAI H5) IAV as cows and heifers (also see a more recent report on H1N1 in a European (male) Bison).

Admittedly, the number of non-lactating cattle far exceeds the number of dairy cattle in the U.S., and the logistics of testing and tracking individual (often asymptomatic) beeves would be daunting.  

But this represents a massive, largely untested, potential reservoir for the virus. 

All of which brings us to a new report in the EID Journal which finds (limited) evidence of H5N1 RNA in bovine semen, which could present an inadvertent (and largely hidden) way to spread the virus. 

In short, researchers tested 3 bulls on a California dairy farm during an outbreak of H5N1 (B3.13) in October of 2024. They found low-levels of H5N1 RNA in a semen sample from one bull (confirmed by PCR).  No infectious virus was isolated, possibly due to the weak sample.

While something less than a smoking gun, this is enough of a signal to warrant additional research. I've posted the link, and some highlights, from the report.  I'll have a bit more after the break.

Research Letter

Highly Pathogenic Avian Influenza A(H5N1) Virus RNA in Bovine Semen, California, USA, 2024

 

Ailam Lim1, Keith Poulsen1, Leonardo C. Caserta, Lizheng Guan, Eryn Opgenorth, Maxwell P. Beal, Amie J. Eisfeld, Yoshihiro Kawaoka, and Diego G. Diel

Abstract

Since March 2024, highly pathogenic avian influenza (HPAI) A(H5N1) virus has infected dairy cattle in the United States, prompting concern about novel transmission routes. During an outbreak in California, HPAI H5N1 RNA was detected in an asymptomatic bull’s semen. Although infectious virus was not isolated, semen-associated transmission risks and biosecurity practices remain a concern.

Since March 2024, detection of clade 2.3.4.4b highly pathogenic avian influenza (HPAI) A(H5N1) in US dairy cattle has raised concerns about the virus’s ability for cross-species transmission, adaptation to mammals, and novel transmission routes, including milk (1,2). Multiple pathogenic viruses are transmitted in bovine semen, and detection of HPAI in turkey semen has prompted questions about the potential role of HPAI transmission in bovine semen (3,4).

 Shedding of HPAI H5N1 in bovine semen could result in silent viral spread within herds and across geographic regions through artificial insemination. Although clinical HPAI disease has been reported in female calves and pregnant animals, reports of diseased bulls in dairy farms or beef cattle are lacking.

Many questions about the pathophysiology of HPAI H5N1 in US dairy herds remain unanswered, but movement of lactating cows is a recognized risk factor for interstate disease spread. In this diagnostic study, we sought evidence of HPAI H5N1 shed through semen in natural breeding bulls on an HPAI H5N1–affected dairy farm in California.

(SNIP)

Because of the limited semen volume available for analysis, we did not perform further confirmation testing at the national reference laboratory. We requested additional samples several months later for convalescent testing, but bull 1 had been culled from the herd. The significance of identifying HPAI H5N1 in bovine semen remains uncertain. The virus could have been actively shed in semen, or the ejaculate could have been contaminated during collection.

Although detecting RNA does not confirm the presence of infectious virus, this finding warrants further investigation into whether HPAI H5N1 can be shed in semen and raises questions about farm biosecurity amid the ongoing outbreak. High viral load in the environment during a herd outbreak was well documented (S. Lakdawala et al., unpub. data, https://www.biorxiv.org/content/10.1101/2025.07.31.666798v3; C. Stenkamp-Strahm et al., unpub. data, https://www.medrxiv.org/content/10.1101/2025.09.03.25335023v1). Good biosecurity measures are essential to prevent infections and, if infection occurs, slow disease spread on the farm.

In conclusion, further research and risk assessments are needed to determine tissue tropism of HPAI H5N1 in reproductive organs and whether naturally infected bulls shed virus in semen, and, if so, evaluate the risk for disease spread on dairy farms and with artificial insemination programs. Repetition and confirmation of these findings would have implications for natural breeding and biosecurity for artificial insemination collection centers, suggesting the need for increased caution in preventing silent intraherd spread.

(Continue . . . )

More than 2 years after the first discovery of H5N1 in dairy cows in Texas there remain far too many unanswered questions regarding the prevalence, and spread, of HPAI in American (and global) livestock. 

Over that time we've seen a number of studies suggesting that Influenza A in general (including HPAI H5), can and does infect livestock, including bovines, pigs, and goats. A few of many studies include:

Netherlands: NOS.NL Reports 5 Dairy Cows Have Now Tested Positive for H5N1

Transboundary & Emerg Inf: Serological Evidence of HPAI (H5N1) in Invasive Wild Pigs in Western Canada,

Preprint: Highly Pathogenic Avian Influenza H5 Virus Exposure in Goats and Sheep (in Pakistan).

Ten days ago, in Preprint: Bovine H5N1 Influenza Viruses Have Adapted to More Efficiently Use Receptors Abundant in Cattle, we saw new evidence that  H5N1 was undergoing active and robust viral adaptation in cattle.

And yet there appears to be little sense of urgency among farmers, or regulatory agencies. Surveillance remains largely passive, and in some regions of the world, practically non-existent.

While H5 may find a dead-end in cattle, we are tempting fate by ignoring the current trajectory, which suggests HPAI is increasing in - and adapting to - farmed mammals.