H5N1 RNA detected in semen of California dairy bull, study finds

U.S. scientists have detected genetic traces of highly pathogenic avian influenza, or H5N1, in semen from a dairy bull in California, according to a new report in a federal disease journal. The finding is a first in cattle and raises fresh questions about how the virus might move through herds and across farms.

But the authors of the research letter, published Wednesday in the Centers for Disease Control and Prevention’s journal Emerging Infectious Diseases, emphasize that they did not find live virus and do not know whether semen can transmit H5N1.

“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,” wrote lead author A. Lim and colleagues.

The team was investigating a 2024 outbreak of H5N1 in a 4,500‑head Holstein dairy in California. The virus belongs to clade 2.3.4.4b, genotype B3.13, the strain that has spread among U.S. dairy cattle since it was first identified in March 2024.

The outbreak on the California dairy began in August 2024, the paper says, and was recognized in October when milk from a bulk tank tested positive for H5N1 by a laboratory method called reverse transcription PCR, or RT‑PCR, which detects viral RNA. Over the next three weeks, about 60% of the lactating cows became sick, with sharply lower milk production, mastitis (udder infection), lethargy, dehydration, loss of appetite and fevers above 104 degrees.

Roughly a month after infection was first confirmed in milk, veterinarians collected samples from three 3‑year‑old Holstein bulls on the farm. One of them, labeled “bull 1,” had influenza A viral RNA detected “at low levels” in a semen sample. The bull had no symptoms of illness at the time.

Additional samples from that same animal — including deep nasal swabs, scrapings from the prepuce (the sheath around the penis), pre‑ejaculate seminal fluid, and blood serum — all tested negative for influenza A. All specimens from the other two bulls on the farm, referred to as “bulls 2 and 3,” were also negative across the board.

Further testing identified the semen‑associated virus as H5N1, clade 2.3.4.4b, genotype B3.13, matching the strain in U.S. dairy cattle. The researchers were able to sequence part of the viral genome from the semen and deposit it in the global GISAID database under accession number EPI_ISL_20206713. In their analysis, that partial sequence clustered with other bovine B3.13 viruses and was “most closely related to a sample collected from a dairy farm worker in California during the same period.”

To determine whether infectious virus was present, the team tried to grow H5N1 from the semen in 10‑day‑old specific‑pathogen‑free chicken embryos and in Madin‑Darby canine kidney (MDCK) cells, a standard lab cell line for influenza. They report that no embryos died, the cell cultures showed no visible damage, and follow‑up hemagglutination tests were negative. In practical terms, they could not recover live virus.

Serologic testing of blood from all three bulls using an influenza A antigen ELISA, which looks for evidence of an immune response, came back negative. Bull 1’s value was close to the assay cutoff, with what the authors describe as “potentially indicating seroconversion,” meaning the potential start of an antibody response, but still in the negative range. The team requested follow‑up samples months later, but bull 1 had been culled and no additional testing was possible.

The authors list several major limitations. The semen sample contained only a low amount of viral RNA and there was limited volume available for repeat and confirmatory work. They were unable to assemble a full genome from the semen sequence. And they note the possibility that the ejaculate was contaminated during collection, which could have introduced virus from outside the reproductive tract.

For that reason, they caution against assuming that H5N1 can be spread through semen based on this single detection. The finding shows viral RNA in one sample from one bull, not a proven route of infection.

Still, the report lands in the middle of a broader H5N1 crisis in U.S. dairy cattle and adds a new data point to how the virus may behave in an unexpected host. The CDC previously documented human spillover from the California dairy outbreak: “During September 30–December 24, 2024, a total of 38 persons received positive test results for HPAI A(H5N1) viruses in California; 37 were dairy farm workers with occupational exposure to sick cows, and one was a child aged <18 years with an undetermined exposure,” the agency wrote in its Morbidity and Mortality Weekly Report.

Federal officials have focused on two main fronts: protecting the food supply and tightening farm biosecurity. The U.S. Department of Agriculture, which oversees animal health and meat and dairy safety, has required testing of dairy cattle before they cross state lines and rolled out a National Milk Testing Strategy to screen bulk tanks and silos for H5N1.

USDA has repeatedly said that commercial milk and meat remain safe, citing pasteurization and proper cooking as effective protections, and has assessed the risk to consumers as low. The agency has also warned that the larger danger lies inside the farm gate, where the virus can move between animals and into workers.

“USDA continues to emphasize to farmers nationwide that biosecurity is the best weapon against the spread of H5N1, and farms should practice good biosecurity even if the virus has not been detected in their state or vicinity,” the department said in an Oct. 30, 2024, statement.

Semen is already a known pathway for spreading several cattle diseases, including bovine viral diarrhea virus and bovine herpesvirus 1, and avian influenza viruses have been linked to risks through poultry semen. Until now, H5N1 in bovine semen was treated as a theoretical concern.

Lim and colleagues argue that even an isolated RNA detection deserves follow‑up in that context. If future research shows that some infected bulls can shed H5N1 in semen, it could complicate disease control by enabling quiet spread within herds or through artificial insemination and semen trade. For now, the new paper stops short of that conclusion and instead calls for more studies and formal risk assessments.

The finding illustrates how a virus that started in wild birds continues to test surveillance systems in livestock and human health. It also underscores how much remains unknown about H5N1’s behavior in cattle, even as the virus keeps resurfacing on U.S. farms.