An unusual outbreak of bird flu in dairy cows has affected at least 13 herds in six states, according to federal and state officials. These infections represent the first time that a highly pathogenic avian influenza virus, which is often fatal in birds, has been detected in U.S. livestock.
At least one person in Texas has been diagnosed with bird flu after coming into contact with dairy cows that were presumed infected, state officials said Monday. The patient’s main symptom was conjunctivitis; The individual is being treated with an antiviral medication and is recovering, according to the Centers for Disease Control and Prevention.
The virus has been identified as the same version of H5N1, a subtype of influenza, that circulates among birds in North America. Scientists have found no evidence that the virus has acquired the mutations it would need to spread easily among humans, officials said.
The risk to the general public remains low, experts said. But these developments add a worrying dimension to an outbreak of bird flu that has already affected millions of birds and marine mammals around the world.
Here’s what you should know:
What is bird flu?
Avian influenza, or avian influenza, is a group of influenza viruses that primarily adapt to birds. The particular virus in these new cases, called H5N1, was first identified in 1996 in geese in China and in people in Hong Kong in 1997.
In 2020, a new, highly pathogenic form of H5N1 emerged in Europe and spread rapidly around the world. In the United States, it has affected more than 82 million farm birds, the worst bird flu outbreak in U.S. history.
Since the virus was first identified, sporadically Cases have been found in people from other countries. But the vast majority were the result of direct and prolonged contact with birds.
Experts say H5N1 does not yet appear to have adapted to spread efficiently between people.
How did cows get bird flu?
Cows were not thought to be a high-risk species.
“The fact that they are susceptible — the virus can replicate and make them sick — is something I wouldn’t have predicted,” said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital.
But this year, reports of sick cows began to emerge in Texas and New Mexico. Dead birds were also found on some of these farms and laboratory tests confirmed that some cows were infected with bird flu. (In Texas, three symptomatic cats also tested positive for the virus, the Texas Animal Health Commission said in an email. Feline infections are not new; cats are known to be susceptible to the virus, which they can contract when hunting or eating .infected birds.)
It is not entirely clear how the virus reached cattle. But one likely route, several experts said, is that infected wild birds, which shed the virus in their feces, saliva and other secretions, contaminated the cows’ food or water.
How have the cows been affected?
Although the virus is usually fatal in birds, it appears to cause a relatively mild illness in cows.
“It’s not killing the animals and they seem to be recovering,” said Dr. Joe Armstrong, a veterinarian and livestock production expert with the University of Minnesota Extension. Last week, the USDA said there were no plans to “depopulate” or kill affected flocks, which is standard procedure when poultry flocks are infected with the virus.
The disease mainly affects older cows, which have developed symptoms including loss of appetite, mild fever and a significant drop in milk production. The milk cows produce is often “thick and discolored,” according to Texas officials. The virus has also been found in unpasteurized milk samples collected from sick cows.
Experts warned that it is still unclear whether the bird flu virus is the sole cause of all the symptoms and illnesses that have been reported.
How widespread is the problem?
It is not clear. On Tuesday, the USDA said its National Veterinary Services Laboratories had confirmed bird flu infections in 12 herds to date: seven in Texas, two in Kansas and one each in Michigan, New Mexico and Idaho. On Wednesday, the Ohio Department of Agriculture announced it had also received confirmation of an affected herd from national laboratories.
So far, the virus has been found only in dairy cows and not in cattle.
But because cows are not routinely tested for bird flu and the illness has been relatively mild, there could be other infected herds that have escaped detection, experts said.
And the movement of livestock between states could transport the virus to new places. The affected dairy in Michigan had recently imported cows from an infected herd in Texas. When the cows were transported, the animals did not show any symptoms. The farm in Idaho had also recently imported cows from an affected state, Idaho officials said.
How is it spreading?
This is a key question and still unanswered. It is possible for infected cows to contract the virus independently, especially if shared food or water sources have been contaminated.
However, a more worrying possibility is that the virus is spreading from one cow to another. On Friday, the USDA noted that “transmission between livestock cannot be ruled out.”
Several scientists said they would be surprised if there wasn’t some degree of cow-to-cow transmission. “How else could it move so fast?” said Dr. Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.
If the virus can spread easily between cows, that could lead to larger, more sustained outbreaks. It would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk that it will acquire mutations that make it more dangerous to people.
What do we know about the human case?
This is only the second case of H5N1 bird flu in people in the United States; the first was in 2022.
The patient worked directly with sick dairy cows, said Lara M. Anton, spokeswoman for the Texas Department of State Health Services. “We have tested a dozen symptomatic people who work at dairies, and only one person tested positive” for the virus, she said in an email Monday.
A virus sample from the patient showed a mutation that allows it to infect people more easily. But in order to spread from one person to another, the virus must acquire several additional mutations, which is no easy evolutionary feat.
The CDC is working with state health departments to monitor other people who may have come into contact with infected birds and animals, the agency said Monday. It also urged people to avoid exposure to sick or dead birds and animals, as well as raw milk, feces or other potentially contaminated materials.
How will officials know if bird flu is adapted to spread between people?
Analysis of the genetic sequence of the virus from infected birds, cows and people can reveal whether H5N1 has acquired mutations that help it spread among people.
Scientists have been closely monitoring infections in birds and marine mammals and, now, cows. So far, the virus does not appear to have the ability to spread efficiently between people.
In 2012, scientists showed that H5N1 could spread through the air between ferrets (a popular model for studying the transmission of respiratory viruses between people) after acquiring five mutations.
An avian flu sample isolated from a Chilean man last year had two mutations that indicate adaptation to infect mammals. But such mutations have been observed before without the virus evolving further and spreading between people, experts said.
Is it safe to consume dairy products?
Federal officials have emphasized that commercially processed milk remains safe to drink. Dairies must keep milk from sick animals out of the human food supply, and milk sold across state lines must be pasteurized, a process in which milk is heated to kill potential pathogens. Pasteurization “has continually been shown to inactivate bacteria and viruses, such as influenza, in milk,” the Food and Drug Administration said in a new online guide on milk safety.
Dr. Gail Hansen, a veterinary public health expert and independent consultant, agreed that the risk was probably “very low.” She added: “I wouldn’t want people to stop drinking milk because of that.”
But the possibility cannot be completely ruled out, he said, expressing some concern that federal officials had been “overconfident in the face of so many unknowns.” If cows shed viruses into milk before showing signs of illness, that milk could potentially find its way into the commercial supply, he said. And different pathogens may require different pasteurization temperatures and durations; The specific conditions required to inactivate this particular virus are still unclear, Dr. Hansen said.
The risk of becoming infected with the virus from consuming raw or unpasteurized dairy products is unknown, the FDA said. Raw milk is known to pose a variety of potential disease risks beyond avian influenza.