An unusual bird flu outbreak in dairy cows has now affected at least 13 herds in six states, according to federal and state officials. These infections represent the first time that a highly pathogenic bird flu virus, which is often fatal in birds, has been detected in U.S. cattle.
At least one person in Texas has been diagnosed with bird flu after having contact with dairy cows presumed to be infected, state officials said on Monday. The patient’s primary symptom was conjunctivitis; the individual is being treated with an antiviral drug and is recovering, according to the Centers for Disease Control and Prevention.
The virus has been identified as the same version of H5N1, an influenza subtype, that is circulating in North American birds. Scientists have not found evidence that the virus has acquired the mutations it would need to spread easily between humans, officials have said.
The risk to the general public remains low, experts said. But these developments add a worrying dimension to an avian influenza outbreak that has already affected millions of birds and sea mammals worldwide.
Here’s what to know:
What is bird flu?
Bird flu, or avian influenza, is a group of flu viruses that are primarily adapted 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 quickly around the world. In the United States, it has affected more than 82 million farmed birds, the worst bird flu outbreak in U.S. history.
Since the virus was first identified, sporadic cases have been found in people in other countries. But a vast majority resulted from prolonged, direct contact with birds.
H5N1 does not yet seem to have adapted to spread efficiently among people, experts say.
How did cows get bird flu?
Cows were not thought to be a species at high risk.
“The fact that they are susceptible — the virus can replicate, can make them sick — that 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 testing 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 pick up when they hunt or eat infected birds.)
It is not entirely clear how the virus found its way into 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 cows been affected?
Although the virus is often fatal in birds, it appears to be causing relatively mild illness in cows.
“It’s not killing animals, and they seem to be recovering,” said Dr. Joe Armstrong, a veterinarian and cattle production expert at the University of Minnesota Extension. Last week, the U.S.D.A. said that there were no plans to “depopulate,” or kill, affected herds, which is the standard procedure when poultry flocks are infected with the virus.
The disease is primarily affecting older cows, which have developed symptoms that include a loss of appetite, a low-grade fever and a significant drop in milk production. The milk that the cows do produce is often “thick and discolored,” according to Texas officials. The virus has also been found in unpasteurized milk samples collected from sick cows.
It is not yet clear whether the bird flu virus is the sole cause of all the symptoms and illnesses that have been reported, experts cautioned.
How widespread is the problem?
It’s unclear. On Tuesday, the U.S.D.A. said that 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, Ohio’s Department of Agriculture announced that it had also received confirmation of an affected herd from the national laboratories.
So far, the virus has been found only in dairy cows and not in beef 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 cattle between states could transport the virus to new locations. The affected dairy in Michigan had recently imported cows from an infected herd in Texas. When the cows were transported, the animals were not displaying any symptoms. The farm in Idaho had also recently imported cows from an affected state, Idaho officials said.
How is it spreading?
That is a key, and still unanswered, question. It is possible that the infected cows are picking up the virus independently, especially if shared food or water sources have been contaminated.
A more worrisome possibility, however, is that the virus is spreading from cow to cow. On Friday, the U.S.D.A. noted that “transmission between cattle cannot be ruled out.”
Several scientists said that they would be surprised if there were not some degree of cow-to-cow transmission. “How else could it move so rapidly?” 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 acquires 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, a spokeswoman for the Texas Department of State Health Services. “We have tested around a dozen symptomatic people who work at dairies, and only the one person has tested positive” for the virus, she said in an email on Monday.
A sample of virus from the patient displayed one mutation that allows it infect people more easily. But to be capable of spreading from one person to another, the virus must acquire several additional mutations — not an easy evolutionary feat.
The C.D.C. is working with state health departments to monitor other people who may have been in contact with infected birds and animals, the agency said on Monday. It also urged people to avoid exposure to sick or dead birds and animals, and to raw milk, feces or other potentially contaminated materials.
How will officials know if bird flu adapts to spread between people?
Analyzing 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 tracking infections in birds and sea mammals and, now, cows. So far, the virus does not seem to have the ability to spread efficiently between people.
In 2012, scientists showed that H5N1 was able to spread through air between ferrets — a popular model for studying transmission of respiratory viruses among people — after acquiring five mutations.
A sample of bird flu isolated from a Chilean man last year had two mutations that indicate adaptation to infecting mammals. But those mutations have previously been seen without the virus evolving further to spread between people, experts said.
Is it safe to consume dairy products?
Federal officials have stressed that commercially processed milk remains safe to drink. Dairies are required to keep milk from sick animals out of the human food supply, and milk sold across state lines must be pasteurized, a process in which the milk is heated to kill potential pathogens. Pasteurization “has continually proven to inactivate bacteria and viruses, like influenza, in milk,” the Food and Drug Administration said in a new online guide to milk safety.
Dr. Gail Hansen, a veterinary public health expert and independent consultant, agreed that the risk was probably “very low.” She added, “I would not want people to stop drinking milk because of it.”
But the possibility could not be entirely ruled out, she said, expressing some concern that federal officials had been “overconfident in the face of so many unknowns.” If cows are shedding virus into their milk before they show signs of illness, that milk could potentially find its way into the commercial supply, she said. And different pathogens may require different pasteurization temperatures and durations; the specific conditions required to inactivate this particular virus remained unclear, Dr. Hansen said.
The risk of being infected by the virus by consuming unpasteurized, or raw, dairy products is unknown, the F.D.A. said. Raw milk is known to pose a variety of potential disease risks beyond avian influenza.
