Tracing the emergence and spread of H5N1 in U.S dairy cattle
Summary author: Walter Beckwith
The spread of highly pathogenic avian influenza (HPAI) in U.S. dairy cattle can be traced to a single spillover event from a wild bird, researchers report, raising concern over growing pandemic risks as the virus evolves and leaps between species. HPAI viruses pose serious threats to animal health, agriculture, and potentially human health due to their ability to cross species barriers. A specific strain, H5N1 clade 2.3.4.4b, has spread globally, infecting wild birds, poultry and, mammals – including a small number of humans – underscoring its pandemic potential. Notably, in 2024, this strain was detected in dairy cattle across multiple U.S. states, marking an unusual and concerning expansion into a previously uncommon host. Here, Thao-Quyen Nguyen and colleagues investigated how this H5N1 strain evolved and spread following its arrival in North America in late 2021. Nguyen et al. analyzed genetic data from over 100 virus variants that emerged through mixing with local, low-pathogenicity bird flu strains. By combining these data with newly sequenced genomes and outbreak information from infected U.S. dairy cattle, the authors discovered that the outbreak originated from a single spillover from an avian source – likely in mid-to-late 2023 in Texas – followed by several months of undetected cow-to-cow transmission. The movement of infected or presymptomatic dairy cattle facilitated the rapid spread of the virus from Texas to several other states, including North Carolina, Idaho, Michigan, Ohio, Kansas, and South Dakota. According to the findings, after the avian influenza virus was introduced into cattle, it not only persisted but also spread from cattle to other species, including poultry, raccoons, cats, and wild birds such as grackles, blackbirds, and pigeons. Moreover, genetic analysis revealed mutations associated with mammalian adaptation, some of which have already become fixed in the viral population. “Our study demonstrates that [influenza A virus] is a transboundary pathogen that requires coordination across regulatory agencies and between animal and public health organizations to improve the health of hosts and reduce pandemic risk,” Nguyen et al. write.
Journal
Science
Article Title
Emergence and interstate spread of highly pathogenic avian influenza A(H5N1) in dairy cattle in the United States
Article Publication Date
25-Apr-2025
CDC taps Texas A&M School of Public Health to assess avian flu among dairy farm workers
The United States has seen 70 human cases and one death from avian flu to date
In April 2024, a month after avian influenza (HPAI H5N1) was detected in dairy cows in several states, the first known human case was reported in an individual in Texas who had been exposed to dairy cows.
The Centers for Disease Control and Prevention (CDC) believes this was the first case of H5N1 being spread from birds to cattle to a human—and the risk to humans remains low. Since H5N1 was first found in dairy herds, there have been 70 human cases, with four of these cases linked to exposure to sick dairy cows.
To find out more, the CDC has awarded a more than $3 million, one-year grant to a multidisciplinary research team led by experts at the Texas A&M University School of Public Health to assess the presence of avian influenza among dairy farm workers in Texas.
Unique Expertise And Teamwork
“Our School of Public Health is part of the largest land-grant institution in the nation,” said David Douphrate, PhD, who leads the project. “We are uniquely positioned to address agricultural health and safety issues, especially given our long track record of working within the dairy industry.”
Douphrate has more than 20 years of experience conducting research that addresses worker health and safety issues in agriculture, and dairy farms specifically.
“I am privileged to work with an outstanding team of experts in the School of Public Health,” he said. “Rebecca Fischer, PhD, is an infectious disease epidemiologist; Loni Taylor, PhD, is also an infectious disease epidemiologist as well as large animal veterinarian; Anabel Rodriguez, PhD, is an occupational epidemiologist; Timothy Erickson, PhD, is a brilliant biostatistician, and Jason Moats, PhD, has a long track record in emergency preparedness in farming operations.”
Douphrate added that others on the team are Robert Hagevoort, PhD, professor and extension dairy specialist at New Mexico State University, Matthew Nonnenmann, PhD, an industrial hygienist at the University of Nebraska Medical Center, and numerous research support personnel.
“Shoe Leather Epidemiology” Tackles The Virus At Its Source
“There is very little information about how many people have been affected by H5N1 and who they are,” Moats said. “Our goals are to first identify the scope of exposure and how many people have been exposed and—more importantly—to understand the workplace factors involved so we can reduce transmission.”
Douphrate said the team first sought input on how to conduct this research from dairy owners and dairy associations. The key to success, he said, is “protecting our trusted relationships with dairy owners and workers and taking all measures to ensure that confidentiality is maintained.”
Douphrate first meets with dairy owners to discuss the project and obtain their approval to collect data on their farms. At the site, researchers meet with dairy workers, explain the project, answer questions and obtain their consent to participate. A bilingual research team member then administers a questionnaire to each worker, and blood samples and nasal and eye swabs are collected from each worker for subsequent lab analysis. The process takes one to three days, depending on the number of workers.
Working with those affected where they live and work is what Fischer called active surveillance, or “shoe leather epidemiology,” a critically important capability that many public health organizations cannot offer. Instead, those organizations rely only on data they get from doctors and medical labs on those who have tested positive for a disease.
“To encounter a disease and determine how to stop it, we have to load up all of our stuff and go to the people affected,” Fischer said. “We have longstanding relationships with dairy farmers. and they know they can trust us even in the most sensitive situations—for farmers and workers who do not want the publicity associated with avian flu.”
Fischer said the blood test reveals if a worker has antibodies to avian flu, which indicate previous exposure to the virus. Swabs, on the other hand, determine if the worker “has avian flu today,” in which case the team facilitates the worker on where to receive health care services for treatment.
Relationships Built On Trust
“We are working with people who are largely ignored but who help make day-to-day life possible in this country,” Fischer said. “Our research is completely anonymous—we don’t collect names or birthdates, for example. We don’t take photos or ever talk to anyone about any aspect of what we do. That takes the fear out of being tested and helps ensure that our data are accurate and complete.”
The researchers learn how workers may have been exposed to the virus by combining the questionnaire data with the lab analysis. The test results and questionnaire answers tell the team what strain of flu they are dealing with and what to tweak to improve worker health and safety. If H5N1 is identified, the researchers help the farmers prevent its spread and keep the workers and cattle herds safe. Their findings are reported in aggregate so that individual farms cannot be identified.
“A former U.S. president once stated that a nation which cannot provide for itself through agriculture production is a nation at risk due to a reliance on external food sources,” Douphrate said. “Our nation’s ability to produce healthy foods is dependent on a healthy agricultural workforce. Researchers at the Texas A&M University School of Public Health are uniquely positioned to make a significant contribution to public health by responding to the recent H5N1 challenge.”
By Ann Kellett, Texas A&M University School of Public Health
