The bird flu virus sweeping dairy farms in several states has acquired dozens of new mutations, including some that may make it more adept at spreading between species and less vulnerable to antiviral drugs, a new study suggests.
Neither mutation is cause for alarm in itself. But they highlight the possibility that as the outbreak continues, the virus may evolve in ways that would allow it to spread easily between people, experts said.
“The flu mutates all the time — that’s what the flu does, sort of,” said Richard Webby, an influenza specialist at St. Jude Children’s Research Hospital, who was not involved in the work.
“The real key would be if we start to see some of these mutations become more prevalent,” Dr Webby said. “That would raise the risk level.”
The virus, called H5N1, has infected cows in at least 36 herds in nine states, raising fears that the milk could be infectious — concerns now largely put to rest — and highlighting the risk that many viruses jump to species on crowded farms. .
The study was published online on Wednesday and has not been peer reviewed. He is among the first to provide details of an Agriculture Department investigation that has been largely opaque until now, frustrating experts outside the government.
The outbreak likely began about four months before it was confirmed in late March and spread unnoticed through cows that had no visible symptoms, researchers found. This timing is consistent with estimates from genetic analyzes by other scientists.
The virus has been detected in some dairy herds with no known connections to affected farms, the authors said, supporting the idea of transmission from asymptomatic cows and suggesting that there may be infected herds that have yet to be identified.
The widespread nature of the outbreak also suggests efficient spread among cows, according to the new paper. This can pose significant risks to humans who interact closely with these animals.
“The fact that this is being transmitted to cows for a while is certainly concerning,” said Louise Moncla, an evolutionary biologist who studies bird flu at the University of Pennsylvania and was not involved in the work.
“I am very concerned that we will find cases in humans,” he said.
In the new study, researchers collected samples containing the virus from 26 dairy farms in eight states. Cows are not usually 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.
The virus then appears to have spread to dairy farms from Texas to Kansas, Michigan and New Mexico. On at least a dozen occasions since then, H5N1 has also spilled from cows back into wild birds and into poultry, domestic cats and a raccoon.
The findings should lead to large-scale surveillance not only of affected farms but also of those where infections have not been reported, said Dr. Diego Diehl, a virologist at Cornell and an author of the study.
Many of the other species likely became 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 he had died.
It is not uncommon for dairies to dump the waste milk into manure pits or lagoons. This “could certainly serve as a source of contamination for other susceptible species,” he said.
Researchers are closely monitoring H5N1 genetic sequences from cows for mutations that would allow the virus to infect or spread among mammals, including humans, more easily.
The only person diagnosed with bird flu during the current outbreak carried a virus with a mutation that allowed it to infect humans more effectively. One cow in the study also carried H5N1 with this mutation. More than 200 others were infected with versions of the virus that carried a different mutation that offered the same advantage.
Veterinarians began noticing unexplained drops in milk production in the cows in late January and sent samples for testing. The Department of Agriculture did not confirm infections until March 25.
“The more widespread H5N1 becomes, the more likely it is to hit 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 different species and causing sporadic human infections for over two decades, and so far we haven’t had a pandemic,” he said. “It’s one of those situations where it could happen next week, but it also could never happen.”