Dairy farmer Paul Miller feeds his cows at Fairvue Farms in Woodstock, Connecticut, in July 2024. More than 1,000 cases of H5N1 flu had been confirmed in U.S. cattle as of May.
Photo by Suzanne Kreiter, courtesy The Boston Globe/Getty Images
As the bird flu outbreak that has killed millions of poultry and cattle across the U.S. continues to drag on, scientists are warning that the H5N1 virus is only a few mutations away from evolving to person-to-person transmission.
While the Centers for Disease Control and Prevention says risk to humans remains low for now, the fast spread of the virus through herds and flocks, the rising number of human cases and America’s questionable level of preparedness has some public health experts nervous.
“The alarm bells are sounding because H5N1 is creeping toward the moment when human-to-human transmission is possible,” Stephen Morrison, PhD, senior vice president of the Center for Strategic and International Studies and director of its Global Health Policy Center, told The Nation’s Health. “It could become very dangerous.”
H5N1 is a contagious avian influenza virus with near 100% fatality for wild and farmed bird populations. In April, the Global Virus Network, representing over 80 virologists in 40 countries, published an international call to action on the virus in The Lancet Regional Health-Americas.
The virus first emerged in China in 1996 as migratory bird flu. Since 2003, over 20 countries have reported nearly 1,000 human cases, with a death rate of about 50%, according to the Global Virus Network’s call to action. The current outbreak in the U.S. began in February 2022, with more than 171 million birds affected and animal cases in every state.
While H5N1 cases among poultry and cattle had been falling this spring, they were expected to rise again with bird migrations in the U.S. during May and June. Wild birds are the primary spreaders of H5N1.
Alarm in the U.S. grew when H5N1 jumped to dairy herds in March 2024. Cows and people have similar genetics, and dairy farmers interact closely with cows, increasing risk of virus spillover to humans.
“Now we have the virus in a mammalian host,” Richard Webby, PhD, an influenza expert at St. Jude Children’s Research Hospital, in Memphis, Tennessee, told The Nation’s Health. “The concern was that would drive the virus to be more infectious for mammals, including humans.”
As of May, 70 cases of the disease had been detected in humans, with one death. Though a Louisiana man died after contact with backyard birds, most human infections have occurred among poultry and dairy workers.
A pandemic of the disease among humans would cause substantial illnesses and deaths, scientists say.
“This has the most capacity to create severe disease,” Webby said. “My gut feeling is that it would be more impactful than the COVID pandemic.”
Protecting workers from infection
What keeps some scientists up at night is the possibility H5N1 could evolve to become more lethal to its human hosts. The 2025-2026 U.S. flu season will carry weight because some people may carry both influenza and H5N1 viruses, researchers predict. The two viruses could exchange genetic information to birth a robust H5N1 that is able to transmit between humans.
Mike Weber, co-owner of Sunrise Farms, holds a chicken at his farm in February in Petaluma, California. It took the farm a year to rebuild after losing half a million birds to avian flu.
Photo by Justin Sullivan, courtesy Getty Images
“We should be doing everything we can to reduce every single human infection, because we don’t want this virus to have the opportunity to gain all of the features needed to become successful at human-to-human transmission,” Seema Lakdawala, PhD, an associate professor of microbiology and immunology at Emory University, told The Nation’s Health.
Since H5N1’s detection in U.S. cattle herds, biosecurity work has helped contain spread to other farms. But under the Trump administration, federal and state biosecurity responses have been hobbled by firings and funding cuts, leading to layoffs that have compromised surveillance, detection and containment, some experts say.
A fall CDC report estimated that at least 7% of U.S. dairy farm workers have been infected by H5N1. Some workers surveyed reported having no symptoms. And while CDC immediately broadened testing efforts and encouraged use of personal protective equipment, the agency has not mandated H5N1 vaccines for front-line dairy workers, puzzling some experts.
“I think it would be a good move to vaccinate vulnerable populations,” Erin Sorrel, PhD, MSc, senior scholar at Johns Hopkins for Health Security and an associate professor at Johns Hopkins Bloomberg School of Public Health, told The Nation’s Health.
Morrison pointed to the complexity of a federal mandate for H5N1 vaccination. The dairy industry generally does not want government interference on dairy farms, which can slow production by taking herds and workers offline; the strain is not lethal for infected cow herds, which almost always recover and return to milk production. Also, given the Trump administration’s recent skepticism around vaccines, a federal mandate would be hard to muster, he said.
Meanwhile, some farm workers on the front lines balk at H5N1 testing because they want to keep working, Morrison said.
“The farmworkers themselves are often quite marginalized, not very powerful, dependent on their incomes,” he said. “They don’t want to be taken out of the work stream, and a very high portion are undocumented.”
A recent study found half of dairy workers in Wisconsin, second only to California in dairy herds, are residing in the U.S. without legal permission.
“Those workers don’t want to be drawing attention and increase their risk of deportation,” Morrison said, referring to the Trump administration’s recent aggressive immigrant deportation campaign.
“Dairy farm owners are just like every other private business owner,” said David Douphrate, PhD, MPT, MBA, associate professor of environmental and occupational health at Texas A&M “They do not let anyone or everyone on their place of business. There needs to be trust between the owner and the researcher and an understanding of how the research will be of benefit to the health and safety of workers.”
Douphrate, who has spent over two decades researching worker health and safety and training workers in the dairy industry, is the current research lead on a $3.8 million CDC grant to assess H5N1’s presence in Texas dairy workers. Data collection began in December.
The project builds on Texas A&M’s trusted relationship with dairy owners and includes workers in decisionmaking, he said. Confidentiality is granted to owners and workers, improving compliance.
The grant project, which ends July 18, did not include H5N1 vaccination. But if vaccinations enter the conversation, input would be sought from industry, dairy owners and workers on feasibility and best practices, Douphrate said.
“My relationships with the industry and owners would, at a minimum, enable a starter conversation as to how a vaccine delivery program might happen,” he told The Nation’s Health.
Texas A&M plans to share its assessment in reports and presentations later this year.
For more information on H5N1 in humans, visit www.cdc.gov/bird-flu. For more on cases in livestock, visit www.aphis.usda.gov/h5n1-hpai.
- Copyright The Nation’s Health, American Public Health Association
