The case has alarmed disease trackers monitoring for the worst-case scenario: human-to-human transmission of the pathogen, which has happened infrequently worldwide and typically among family members engaged in work with animals. And it raises questions about whether this pathogen is now more easily transmitted among mammals.
But federal officials said the infection does not change the health risk assessment for bird flu among the U.S. general public, a risk that the CDC considers to be low. However, people with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other animals, are at greater risk of infection.
In 2022, a person in Colorado tested positive for the same strain of avian flu. The person had direct exposure to poultry and was involved in culling poultry that had presumptive H5N1 bird flu. The person reported fatigue for a few days as their only symptom and recovered, according to the CDC.
Still, any time the virus changes — its recent emergence in cattle and the likelihood of cow-to-cow transmission represents a change — “that makes me sit up and take notice,” said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security.
There are several ways the virus could evolve, disease experts have said: It could remain primarily a threat to animal health and then recede, as it has in the past. It may continue to circulate among animals, but not routinely infect humans. Or, in the worst case, it evolves to spread easily between people and becomes the next pandemic, Rivers said.
The virus was detected in dairy herds in Texas and Kansas last week and has since spread to additional herds in at least five states, adding evidence that the virus may be spreading cow-to-cow. The strain has been confirmed in Michigan, and presumptive positive tests have also been reported from Idaho and New Mexico, federal officials said Friday.
Epidemiologists have been worried about the growing number of mammals infected by highly pathogenic avian influenza — commonly known as HPAI — around the world. Avian influenza has been spreading around the world since 2020 and has been documented to infect dozens of other mammalian species, but it rarely spreads between them. Last month, HPAI was found in a baby goat in Minnesota, the first case in U.S. livestock.
Scientists are concerned that the virus may have mutated in ways that could allow it to better infect people. In a statement Friday, the U.S. Department of Agriculture said it has not identified changes to the virus that would make it more transmissible to humans.
“While cases among humans in direct contact with infected animals are possible, this indicates that the current risk to the public remains low,” the agency statement said.
Texas officials are providing guidance to affected dairies about how to minimize workers’ exposure and how people who work with affected cattle can monitor for flu-like symptoms and get tested. Illnesses in people with H5N1 flu infections have ranged from mild, such as eye infection and upper respiratory symptoms, to severe, such as pneumonia and death.
Texas has issued a health alert asking health-care providers around affected dairies to be vigilant for possible human cases and is providing testing and treatment recommendations.
This is a developing story and will be updated.
