THURSDAY, July 25, 2024 (HealthDay News) -- The bird flu is now jumping between species of mammals, a step that draws the virus closer to hopping into human beings, a new study warns.

Researchers have tracked transmission of avian influenza between dairy cows in herds, as well as from cows to cats and a raccoon.

“This is one of the first times that we are seeing evidence of efficient and sustained mammalian-to-mammalian transmission of highly pathogenic avian influenza H5N1,” said senior researcher Dr. Diego Diel, director of the Virology Laboratory at the Animal Health Diagnostic Center in the Cornell University College of Veterinary Medicine.

Still, genetic analysis of the virus did not reveal any mutations that would lead to enhanced transmissibility of H5N1 in humans, Diel said. The findings were published July 25 in the journal Nature.

However, mammal-to-mammal transmission does raise concerns that the virus might eventually adapt to spreading in humans, Diel said.

So far, 11 human cases have been reported in the United States, with the first dating back to April 2022, researchers said.

Four human cases are linked to cattle farms and seven to poultry farms, including an outbreak of six cases over the last few weeks in Colorado, researchers said.

These recent human cases contracted the same bird flu strain identified in the study as the one circulating in dairy cows, researchers said.

Luckily, all human cases to date have had mild symptoms, and the virus has not developed an ability to pass easily between humans.

“The concern is that potential mutations could arise that could lead adaptation to mammals, spillover into humans and potential efficient transmission in humans in the future,” Diel said in a Cornell news release.

For the study, researchers used genetic sequencing to track the viral strains that transmitted between cows when infected animals from Texas were moved to a farm with healthy cows in Ohio.

They also found that the virus was transmitted to cats, a raccoon and wild birds found dead on affected farms.

The cats and raccoon likely became infected from drinking raw milk from infected cows, researchers said.

More than 100 million cases of avian flu in poultry have been reported in the United States, and 168 dairy herds have been affected, the U.S. Centers for Disease Control and Prevention says.

Outbreaks in poultry have been reported in 48 states, and 13 states have had outbreaks in dairy cows.

More information

The U.S. Centers for Disease Control and Prevention has more about the bird flu.

SOURCE: Cornell University, news release, July 24, 2024