Emerging Infectious Diseases • Vol. 32, Number 5 - May 2026
15 April 2026
A new diagnostic study has identified the presence of highly pathogenic avian influenza (HPAI) A(H5N1) virus RNA in the semen of an asymptomatic dairy bull, marking a potential shift in how researchers understand the virus's spread within the cattle industry.
Since March 2024, the clade 2.3.4.4b H5N1 virus has been circulating among dairy cattle in the United States. In October 2024, an outbreak occurred at a 4,500-head Holstein dairy farm in California, where infection was initially confirmed through bulk tank milk samples. While lactating cows on the farm exhibited symptoms such as decreased milk production, mastitis, and fever, the bulls remained asymptomatic. Approximately four weeks after the initial detection, researchers collected diagnostic samples from three 3-year-old Holstein bulls on the affected farm. These samples included deep nasal swabs, preputial scrapings, and semen. Testing conducted at the Wisconsin Veterinary Diagnostic Laboratory revealed the following: Low levels of influenza A virus (IAV) RNA were detected specifically in the semen of one bull. RT-PCR and targeted sequencing confirmed the strain as H5N1 lineage 2.3.4.4b. The viral RNA clustered within the B3.13 genotype, closely matching samples from a dairy farm worker in California from the same period. Although RNA was present, researchers were unable to isolate an infectious virus from the samples using embryonated chicken eggs or Madin-Darby canine kidney cells. Subjectively, the affected semen appeared to have low sperm concentration and volume.
The detection of H5N1 RNA in bovine semen raises critical questions regarding "silent" viral spread. While it remains uncertain if the virus was actively shed or resulted from environmental contamination during collection, the findings suggest a possible risk of transmission through natural breeding or artificial insemination programs.
