A strain of bird flu that is highly lethal in birds has been confirmed in US dairy cattle in Texas and Kansas, the Department of Agriculture announced Monday.
It is the first time that cows infected with the virus have been identified.
Cows appear to have been infected by wild birds and dead birds were reported on some farms, the agency said. The Texas Animal Health Commission confirmed that the subtype of the flu known as H5N1 has been identified and said the virus resembled the version that has spread to birds across the country.
The results were announced after several federal and state agencies began investigating reports of sick cows in Texas, Kansas and New Mexico. The disease has mainly affected older cows, causing symptoms including reduced appetite, fever and a sudden drop in milk production. So far, the USDA said, there have been few or no reports of deaths in the affected herds.
In several cases, the virus was detected in unpasteurized milk samples collected from sick cows. Pasteurization should inactivate the flu virus, experts said, and officials emphasized that the milk supply was safe.
“At this stage, there is no concern for the safety of the commercial milk supply or that this circumstance poses a risk to the health of consumers,” the agency said in a statement.
Outside experts agreed. “It’s only been found in milk which is very abnormal,” said Dr. Jim Lowe, a veterinarian and influenza researcher at the University of Illinois College of Veterinary Medicine in Urbana-Champaign.
In those cases, the milk was labeled thick and syrupy, he said, and thrown away. The agency said dairies are required to divert or destroy milk from sick animals.
The cattle infections come on the heels of the first detection of bird flu in goats, which Minnesota officials announced last week.
So far, flu samples from sick cows have not contained genetic mutations known to make the virus more likely to infect humans, the agriculture service said, adding that the risk to the general public remains low.
“There’s no reason to panic yet,” said Stacey L. Schultz-Cherry, a virologist and flu specialist at St. Jude Children’s Research Hospital. “It just seems like this is another event due to contact with sick wild birds.”
However, he noted, cows were not considered among the species particularly susceptible to bird flu, and the outbreaks were another worrying twist in a global bird flu epidemic that has devastated wild bird populations in recent years.
The outbreak was caused by a new type of bird flu virus known as H5N1 that emerged in Europe in 2020. Wild birds can spread the virus, through their droppings and oral secretions, to farmed poultry and other animals . Outbreaks often occur in the spring and summer when migratory birds are on the move.
Although avian flu viruses are adapted to spread primarily among birds, the new version of H5N1 has become so widespread in wild birds that it has also repeatedly spread to mammals, especially scavenger species such as foxes, which can they feed on infected birds.
Infected wild birds could transmit the virus to cows by contaminating their feed or water, said Dr. Joe Armstrong, a veterinarian and cattle production specialist at the University of Minnesota Extension. “You go into a farm, especially during the migration season, and you have geese and ducks foraging like everybody else,” he said. “For me that’s the most likely route.”
But it’s also possible that free-ranging cats, which are not uncommon on farms and are known to be susceptible to the virus, could be involved in spreading the pathogen, he said.
Dr. Armstrong also cautioned that it was too early to conclude that bird flu was the main cause of disease in all the sick cows that have been reported.
Mammalian infections, which give bird flu viruses new opportunities to evolve, are always a concern, said Andrew Bowman, a molecular epidemiologist and influenza expert at Ohio State University. Scientists have long worried that a bird flu virus that evolved to spread more efficiently among mammals, including humans, could cause the next pandemic.
At this point, Dr. Bowman said, it remains unclear whether all the infected cows got the virus directly from the birds or whether the virus is also being spread from cow to cow.
“This is a question that should be resolved quickly,” he said. “If we have cattle-to-cattle transmission, that’s a different story. That definitely makes me a little more nervous.”
A closer look at the virus’s genomes will help scientists learn more about how the virus spreads and whether it is evolving in ways that will increase the risk to public health, the scientists said.
These early cases also mean cattle influenza surveillance should be stepped up, said Richard Webby, a virologist and influenza specialist at St. Jude Children’s Research Hospital.
“It’s pretty clear that we now need to get a better sense of how widespread this is in cows,” he said. “Influenza probably wasn’t an important part of a diagnostic test for a sick cow before this, but it’s certainly going to move forward.”
