Bird flu is spreading into unprecedented territory in the United States, infecting scores of cows at dairy farms in nine states and jumping to one farm worker — only the second confirmed case ever of a human getting infected by the H5N1 virus within the United States. Federal and local agencies are working to monitor and curtail the spread of H5N1. We asked scientists who study bird flu what we know about the virus and how it might be kept at bay or could expand into the next pandemic.
What is the reach of avian flu?
Avian flu is, as its name suggests, most commonly found in wild and farm birds worldwide, with periodic outbreaks occurring among mammals. H5N1, a particular strain of bird flu, periodically flares up more expansively; it was cited for causing more than 100 million bird deaths globally in 2022 and has been detected in dozens of species of mammals. In the United States, the virus has been detected in more than 200 different mammals, including cats, goats, and raccoons.
Most often, the virus gets transmitted when one animal eats an infected animal or comes in contact with bodily materials like feces and saliva from an infected animal, as explained by the Centers for Disease Control and Prevention (CDC).
What risk does the virus pose to humans?
Avian flu viruses rarely infect humans. The World Health Organization (WHO) reports that from 2003 to April 1, 2024, 23 countries reported a total of 889 human cases of H5N1. The most common symptoms in people, according to the CDC, include eye redness (conjunctivitis), respiratory difficulties, fever, cough, sore throat, and pneumonia.
However, more than half of the worldwide human cases (463) resulted in death. “It is a highly pathogenic strain,” explains Erin M. Sorrell, PhD, MSc, senior scholar at the Johns Hopkins Center for Health Security and associate professor at Johns Hopkins Bloomberg School of Public Health in Maryland. “It has the ability to replicate outside the traditional locations where low-pathogenic influenza does: the intestinal tract for poultry; the upper and lower respiratory tracts in humans. The virus becomes systemic in its infection.”
In the United States, the first known case of transmission to a human occurred in 2022. The CDC says that person, who worked directly with infected poultry, reported mild fatigue and recovered.
The second known case was reported in March 2024, in a dairy farm worker in Texas. The only symptom was conjunctivitis in both eyes, which receded, according to a case assessment by the CDC and Texas health agencies.
“As of now, this is a low-risk situation for humans,” says Michael Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. But that could change as the virus spreads among more animals, he warns.
Is there concern that the virus might spread to more people?
There are varying levels of concern among scientists who are watching the outbreak. One reason for that concern is that it has begun spreading to dairy cows — animals that have a lot of close contact with humans.
“Dairy cows do not normally get infected with Influenza A viruses,” which is what bird flu is, says Jenna Guthmiller, PhD, assistant professor in the Department of Immunology and Microbiology at the University of Colorado Anschutz School of Medicine.
The assessment of the Texas case notes that the worker “reported no contact with sick or dead wild birds,” but “direct and close exposure to dairy cows.” Some of those cows showed signs of illness that had been appearing at other dairy farms in the area, including decreased milk production, reduced appetite, lethargy, fever, and dehydration. A map from the U.S. Department of Agriculture (USDA) shows the states where the virus recently has been found.
Tests of cattle in the area where the worker got infected determined that the bovine outbreak started when a wild bird infected a cow at a Texas farm in December, perhaps through bodily secretions into the cattle’s food or water supply.
The concern is that cows might become hosts that spread the virus to humans, Guthmiller says. That spread can occur a number of ways, including through milk from the udders that farm workers could absorb through their eyes, nose, or mouth.
So far, the risk of humans getting infected is “low,” the CDC states in summaries about H5N1. But Rick Bright, PhD, former deputy assistant secretary for preparedness and response at the Department of Health and Human Services (HHS), says that because the virus is spreading among both birds and cows on farms where people work, “the opportunity for infecting a human is increasing.”
Why might cows pose a particular threat of spreading H5N1?
Close-up contact with people
In the milking parlor of a farm, people work extremely close to the cows’ udders, even if they are primarily guiding the milking machines.
“Your face, your shoulders, your head are at the level of the cow’s udders,” says Guthmiller, who milked cows on the dairy farm where she grew up. “We are literally putting our faces right where the flu viruses are coming out.”
The machines
If a milking machine gets infected with the virus from one cow, the infection can be spread to other cows and to workers who use the same machine.
“The main transmission that people believe is occurring is through these milkers becoming contaminated, then moving on to the next cow,” Guthmiller says. She says it would add significant time and cost for a farm to try cleaning the milkers after each cow.
Delayed diagnosis
“It seems that this virus is transmitting [from cow to cow] before cows even show any symptoms, or their symptoms are mild,” Guthmiller says. “Cows can’t tell us that they’re not feeling well.”
Data that signal warning signs of an illness, like decreased production from a few cows, take a long time to come to the notice of managers who are monitoring hundreds or thousands of cows. Meanwhile, humans and animals continue to have close contact with infected cattle.
Cow trades
Cows are routinely traded between farms. “They’re going from one farm to another, which could lead to outbreaks on more farms,” Guthmiller says.
In April, the U.S. Department of Agriculture (USDA) mandated that dairy cows receive a negative test for Influenza A before being transported to another state.
The lack of bird flu experience at dairy farms
“The poultry industry has dealt with avian influenza for years,” says Sorrell at Johns Hopkins. The common responses include culling entire flocks to stop the spread.
The dairy industry has no experience with testing and responding to cows with avian flu. It is much easier, faster, and cheaper for poultry farms to buy and raise new birds than for a dairy farm to do the same with cows, Sorrell explains.
What don’t we know?
Scientists are still learning about the extent of the disease and how it is being transmitted on farms.
“We don’t know how many cows are infected, we don’t know how many people have been exposed,” says Bright, now chief executive of Bright Global Health, which focuses on responses to public health emergencies.
Getting the answers requires surveillance of animal and human populations — no easy task. Disease surveillance in the United States is a fractured and uneven endeavor, carried out mostly on a voluntary basis by state and local governments with guidance and prodding from federal agencies.
Scientists who have been critical of the federal effort say surveillance should include more testing of farm workers and cattle. But for various reasons (including lack of trust in government regulators), workers and farm managers are wary of government officials conducting tests. The tests could also help discover viral mutations that might facilitate the spread of the infection among humans.
Bright worries that by the time the virus is confirmed in even a small number of people (say, 10), it will have spread so widely that an epidemic might be at hand. Some scientists speculate that more dairy farmers already have become sick, but haven’t been tested for H5N1.
“If we haven’t caught the virus before it mutates to efficiently transmit person-to-person, all bets are off in terms of being able to control it,” Bright says.
How can dairy farm workers protect themselves?
Farm workers could wear personal protective equipment (PPE) — such as gloves, goggles, and masks — but PPE is rare on dairy farms. (The infected worker in Texas reported wearing gloves.)
“Most diseases of cows don’t affect humans, so there’s very little biosecurity on dairy farms,” Guthmiller says. “We’re not concerned with getting something from the cows.”
Plus, PPE is especially uncomfortable and inconvenient in a milking facility. Masks impede breathing and get soaked from sprayed milk. Goggles get sprayed as well and they decrease visibility. “People aren’t wearing safety glasses,” Guthmiller says. “That would be an annoyance.”
The CDC has asked local jurisdictions to offer PPE to dairy farms. Reports are that few farms have taken up the offers.
Is it safe to drink milk and eat beef?
Pasteurized milk does not transmit H5N1 and is safe to drink, the FDA says. The conclusion is based on repeated qPCR (quantitative polymerase chain reaction) tests of pasteurized milk and other pasteurized dairy products (including cottage cheese and sour cream) that showed no live, infectious bird flu virus. The agency said that fragments of the flu were found in some samples, but that those fragments had been inactivated by the pasteurization process.
“Influenza viruses are very sensitive to heat” from the pasteurization process, which kills them, Sorrell says.
On the other hand, raw milk, which has not gone through that process, is not protected from the virus, Sorrell explains. A study of cats that got sick or died from H5N1 this year concluded that they had been infected by drinking unpasteurized milk from the cows on dairy farms.
The USDA conducted testsof commercially sold ground beef in states where H5N1 had been confirmed in cattle. No samples have shown traces of H5N1.
Are there vaccines to protect against bird flu?
The United States has two candidate vaccines available to manufacturers for the production of a vaccine against H5N1 if an outbreak occurs among humans, according to the CDC.
Some scientists are not convinced that vaccine production could ramp up quickly enough to meet the need. Bright notes that the vaccines are produced through the decades-old process of injecting a virus into eggs, then harvesting the fluid to create an inactivated virus. It’s a slow process.
Some scientists, including at the University of Pennsylvania Perelman School of Medicine, are exploring the use of mRNA technology to speed up production of a vaccine for bird flu, in much the same way as some COVID-19 vaccines were quickly developed.
Bright and Osterholm note that adjusting vaccines to address mutations of a virus is a perpetual challenge. “Anything we have in the stockpile right now may not even come close to matching up with the new virus,” Osterholm says.
What is being done to contain the virus?
In early April, the CDC sent a health advisory to clinicians and state public health departments to consider the possibility of H5N1 infection in patients who develop respiratory illness or conjunctivitis, and who had been exposed to potentially sick or dead birds or livestock.
The USDA and HHS have instituted several ways to financially support farms to help curtail the spread of H5N1, including reimbursing them for PPE, for costs of testing and treating their cattle, and for lost milk production in herds affected by the virus.
The CDC is working with state and local health departments to monitor people exposed to infected cattle, and is boosting its analysis of data about Influenza A among people, including in wastewater samples and emergency room visits.
For a person who does become infected by the virus, the CDC recommends antiviral drugs that are used for influenza.
Despite these and other steps, some scientists worry that government agencies are not doing enough to monitor the often-invisible spread of H5N1. “The thing that is a concern to me is the lack of [widespread] testing” of animals and milk on farms, Guthmiller says. “By not having a better grasp of this outbreak, it could get a lot worse for dairy cows, for our food chain, and could have the potential to jump into a pandemic.”
