At least one person in Texas has been diagnosed with bird flu after coming into contact with presumably infected dairy cows, state officials said Monday.
The announcement adds a worrying dimension to an epidemic that has affected millions of birds and marine mammals around the world and, more recently, cows in the United States.
So far, there are no signs that the virus has evolved in ways that would help it spread more easily among people, federal officials said.
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 about a dozen symptomatic people who work on dairies and only one person tested positive” for the virus, she said in an email.
The patient's main 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 Department of Agriculture announced the first cases last week on dairy farms in Texas and Kansas and, a few days later, in another herd in Michigan. Preliminary testing suggests that cows in New Mexico and Idaho may also be infected.
The virus was identified as the same version of H5N1, a subtype of influenza, which circulates among birds in North America.
The CDC is working with state health departments to monitor other people who may have been in contact with infected birds and animals, the agency said Monday. He also urged people to avoid exposure to sick or dead birds and animals and to raw milk, feces or other potentially contaminated materials.
This is only the second case of H5N1 avian influenza in the United States; the first occurred in 2022. The risk to the general public remains low, experts say. But testing and analysis are ongoing, and there are many unanswered questions.
“This is a rapidly evolving situation,” the USDA said in its announcement last week.
Here's what to know:
What is bird flu?
Avian influenza, or avian influenza, is a group of influenza viruses adapted primarily 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 the H5N1 virus emerged in Europe and rapidly spread throughout the world. In the United States, it affected more than 82 million farmed birds, marking the worst avian influenza epidemic in U.S. history.
Since the virus was first identified, sporadic cases have been found in people from other countries. But the vast majority are the result of direct and prolonged contact with birds.
H5N1 does not yet appear to have adapted to spread effectively among people, experts say.
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, it can 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 avian influenza.
There are several ways the virus could have spread in livestock. The most likely cause, according to several experts, is that infected wild birds, which shed the virus in feces, saliva and other secretions, have contaminated the cows' food or water.
But other free-range animals known to be susceptible to the virus, such as cats and raccoons, may also have brought the virus to dairy farms.
How were the cows affected?
Although the virus is often fatal in birds, it appears to cause relatively mild illness in cows.
“It's not killing the animals and they appear to be recovering,” said Dr. Joe Armstrong, a veterinarian and cattle production expert at 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 primarily affects older cows, which have developed symptoms including loss of appetite, mild fever and a significant drop in milk production. According to Texas officials, the milk produced by cows is often “thick and discolored.” The virus has also been found in samples of unpasteurized milk collected from sick cows.
It is not yet clear whether the avian influenza virus is the sole cause of all reported symptoms and illnesses, experts warn.
How widespread is the problem?
It is not clear. Last Friday, the USDA National Laboratory of Veterinary Services confirmed avian influenza infections at two farms in Texas, two farms in Kansas and one farm in Michigan.
Initial testing has suggested that other farms in Texas, New Mexico and Idaho may also have the virus, but those results have not yet been confirmed by the national laboratory. So far the virus has only been found in dairy cows and not in beef cattle.
But because cows are not regularly tested for bird flu and the disease has been relatively mild, there may be other infected herds that have escaped detection, experts said.
And the movement of livestock between states could carry the virus to new locations. The affected farm in Michigan had recently imported cows from an infected herd in Texas. When the cows were transported, the animals showed no symptoms. The Idaho farm recently imported cows from an affected state, Idaho officials said.
How is it spreading?
This is a fundamental and still unanswered question. It is possible for infected cows to contract the virus independently, especially if shared sources of food or water have been contaminated.
A more worrying possibility, however, is that the virus spreads from cow to cow. The USDA noted Friday that “cattle-to-cattle transmission 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 quickly?” said Dr. Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.
If the virus could spread easily among cows, this could lead to larger and more prolonged epidemics. It would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk of acquiring mutations that make it more dangerous to humans.
How will officials know whether bird flu is adapted to spread among people?
Analysis of the virus's genetic sequence from infected birds, cows and people can reveal whether H5N1 has acquired mutations that make it easier to spread between people.
Scientists have closely monitored 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 was able to spread through the air among 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 mammalian infection. But such mutations have been observed before 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 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, agrees that the risk of contracting the infection from pasteurized milk is probably “very low”. She added: “I wouldn't want people to stop drinking milk because of this.”
But the possibility couldn't be entirely ruled out, he said, expressing some concern that federal officials had been “overconfident in the face of so many unknowns.” If cows shed the virus in their milk before showing signs of illness, that milk could potentially end up in the commercial supply, she said. And different pathogens may require different pasteurization temperatures and durations; the specific conditions required to inactivate this particular virus remain unclear, Dr. Hansen said.
The risk of contracting the virus by consuming unpasteurized or raw dairy products remains unknown, the FDA said. Raw milk is known to carry a number of potential disease risks beyond avian influenza.