The avian influenza virus spreading across dairy farms in several states has acquired dozens of new mutations, including some that could make it better at spreading between species and less susceptible to antiviral drugs, according to a new study.
None of the mutations in themselves are cause for alarm. But they highlight the possibility that, as the epidemic continues, the virus could evolve in ways that would allow it to spread easily among people, experts said.
“The flu mutates all the time — that's what, in a sense, the flu does,” said Richard Webby, a flu expert at St. Jude Children's Research Hospital, who was not involved in the work.
“The real key would be if we started to see some of these mutations become more widespread,” Dr. Webby said. “This would increase the level of risk.”
The virus, called H5N1, has infected cows on at least 36 farms in nine states, raising fears that milk could be contagious – concerns now largely put to rest – and highlighting the risk that many viruses could jump from one species to another. another in crowded herds.
The study was published online Wednesday and has not been peer-reviewed. He is among the first to provide details of an Agriculture Department investigation that has so far been mostly opaque, frustrating experts outside the government.
The researchers found that the outbreak most likely began about four months before it was confirmed in late March and spread unnoticed through cows that had no visible symptoms. This timing is consistent with estimates from other scientists' genetic analyses.
The virus has been detected in some dairy herds with no known links to affected herds, the authors said, supporting the idea of transmission from cows without symptoms and suggesting there may be infected herds that have not yet been identified.
According to the new study, the widespread nature of the epidemic also suggests effective spread among cows. This could pose significant risks to people who interact closely with those animals.
“The fact that this virus has been transmitting to cows for some time is definitely concerning,” said Louise Moncla, an evolutionary biologist who studies avian influenza at the University of Pennsylvania and was not involved in the work.
“I'm very worried about being able to find cases in people,” she said.
In the new study, researchers collected virus-containing samples from 26 dairy farms in eight states. Cows are not generally susceptible to this type of flu, but H5N1 appears to have acquired mutations in late 2023 that allowed it to jump from wild birds to cattle in the Texas Panhandle, researchers said.
It therefore appears that the virus has spread to dairy farms from Texas to Kansas, Michigan and New Mexico. Since then, in at least a dozen cases, the H5N1 virus has also spread from cows to wild birds, poultry, domestic cats and a raccoon.
The findings should prompt large-scale surveillance not only of affected herds but also of those without reported infections, said Dr. Diego Diel, a Cornell virologist and author of the study.
Many of the other species were likely infected after coming into contact with contaminated milk, which can contain very high levels of the virus, Dr. Diel said. A separate study published earlier this week reported that about a dozen cats fed raw milk had died.
It is not uncommon for dairies to dump discarded milk into manure pits or lagoons. This “could certainly serve as a source of infection for other susceptible species,” she said.
Researchers are carefully monitoring the genetic sequences of H5N1 from cows for mutations that would allow the virus to infect or spread more easily among mammals, including humans.
The only person diagnosed with bird flu during the current outbreak carried a virus with a mutation that allowed it to infect people more efficiently. One cow in the study also carried the H5N1 virus with that mutation. More than 200 others have been infected with versions of the virus carrying a different mutation that offers the same benefit.
Veterinarians began observing unexplained drops in milk production in the cows in late January and sent samples for testing. The Department of Agriculture did not confirm the infections as of March 25.
“The more H5N1 spreads, the greater the chance that it could encounter a combination of mutations that could increase the risk to humans,” said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle.
“On the other hand, H5N1 has been circulating in various species and causing sporadic human infections for over two decades, and so far we have not had a pandemic,” he said. “It's one of those situations where it could happen next week, but it might never happen.”