At least one person in Texas has been diagnosed with bird flu after coming into contact with suspected infected dairy cows, state officials said Monday.
The announcement adds an alarming dimension to an outbreak that has affected millions of birds and marine mammals around the world and, more recently, cows in the United States.
So far, there is no evidence that the virus has evolved in ways that would help it spread more easily between people, federal officials said.
The patient’s main symptom was conjunctivitis. the person is treated with an antiviral drug and recovers, according to the Centers for Disease Control and Prevention.
The Department of Agriculture reported the first cases in dairy herds in Texas and Kansas last week, and a few days later, in an additional herd in Michigan. Preliminary tests indicate that cows in New Mexico and Idaho may also be infected.
The virus has been identified as the same version of H5N1, a subtype of influenza, circulating in North American birds.
The CDC is working with state health agencies to track 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.
This is only the second human case of H5N1 bird flu in the United States. The first was in 2022. The risk to the general public remains low, experts said. But testing and analysis continues, and there are many unanswered questions.
“This is a rapidly evolving situation,” the USDA said in a statement last week.
Here’s what you need to know:
What is bird flu?
Avian influenza or bird flu is a group of influenza viruses that are mainly adapted to birds. The particular virus in these new cases, called H5N1, was first identified in 1996 in geese in China and in humans 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 farmed birds, the worst outbreak of bird flu in US history.
Since the virus was first identified, it has been sporadic cases have been found in humans in other countries. But the vast majority came from prolonged, direct contact with the birds.
H5N1 does not yet appear to have adapted to spread efficiently between humans, experts say.
How did cows get bird flu?
Cows were not considered a high risk species.
“The fact that they’re susceptible — the virus can multiply, it can make them sick — that’s 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 started popping up 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.
There are several ways the virus could have found its way into cattle. The likely pathway, several experts said, is that infected wild birds, which shed the virus in their feces, saliva and other secretions, contaminated the cows’ feed or water.
But other free-ranging animals known to be susceptible to the virus, such as cats and raccoons, could also have carried the virus to dairy farms.
How have cows been affected?
Although the virus is often fatal in birds, it appears to cause relatively mild disease in cows.
“It doesn’t kill animals and they seem to recover,” said Dr. Joe Armstrong, a veterinarian and cattle production specialist at the University of Minnesota Extension. Last week, the USDA said there were no plans to “culling” or culling the affected flocks, which is the standard procedure when poultry flocks are infected with the virus.
The disease mainly affects older cows, which have developed symptoms including loss of appetite, low-grade fever and a significant drop in milk production. The milk the cows produce is often “thick and discolored,” according to Texas officials. The virus has also been found in samples of unpasteurized milk collected from sick cows.
It is not yet clear whether the bird flu virus is the sole cause of all the reported symptoms and illnesses, experts warned.
How widespread is the problem?
It is unclear. As of last Friday, the USDA’s National Veterinary Services Laboratory had confirmed bird flu infections in two flocks in Texas, two flocks in Kansas and one flock in Michigan.
Initial tests have indicated that additional herds in Texas, New Mexico and Idaho may also have the virus, but those findings have yet to be confirmed by the national laboratory. So far, the virus has only been found in dairy cows, not cattle.
But because cows are not regularly tested for bird flu and the disease was relatively mild, there could be other infected herds that went undetected, experts said.
And moving cattle between states could carry 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 did not show any symptoms. The Idaho farm also had recently imported cows from an affected state, Idaho officials said.
How does it spread?
This is a basic and unanswered question. It is possible that infected cows can contract the virus independently, especially if common food or water sources are contaminated.
A more worrying possibility, however, is that the virus is spreading from cow to cow. On Friday, the USDA noted that “transmission between cattle 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 he 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, this 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 of acquiring mutations that make it more dangerous to humans.
How will officials know if bird flu is adapting to spread between humans?
Analysis of the genetic sequence of the virus from infected birds, cows and humans can reveal whether H5N1 has acquired mutations that help it spread among humans.
Scientists are 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 humans.
In 2012, scientists showed that H5N1 was able to spread through the air between ferrets – a popular model for studying the transmission of respiratory viruses between humans – after acquiring five mutations.
A bird flu sample isolated from a Chilean man last year had two mutations that indicate adaptation to infecting mammals. However, these mutations had previously been observed without the virus evolving further to spread between humans, experts said.
Is it safe to consume dairy products?
Federal officials stressed that commercially processed milk remains safe to drink. Dairies are required to keep milk from sick animals away from human consumption, and milk sold across state lines must be pasteurized, a process in which the milk is heated to kill potential pathogens. Pasteurization “has been consistently shown to inactivate bacteria and viruses, such as influenza, in milk,” the Food and Drug Administration said in a new online milk safety guide.
Dr Gale Hansen, a veterinary specialist and independent consultant, agreed that the risk of contamination from pasteurized milk was probably “very low”. He added: “I wouldn’t want people to stop drinking milk because of this.”
But the possibility could not be completely ruled out, he said, expressing some concern that federal officials were “overconfident in the face of so many unknowns.” If cows shed virus in their milk before showing signs of illness, that milk could potentially find its way into the commercial milk supply, he said. And different pathogens may require different pasteurization temperatures and durations. The specific conditions needed to inactivate this particular virus remained unclear, Dr. Hansen said.
The risk of contracting the virus from eating unpasteurized or raw dairy products remains unknown, the FDA said. Raw milk is known to carry several potential disease risks beyond bird flu.
