Concern is growing among health officials over the latest bird flu outbreak after a person in Texas caught the virus from an infected dairy cow in the first known instance of a mammal passing the disease to a human.
Even scientists who have tracked avian influenza for years are confused by its recent spread in dairy cattle populations.
"We are in fairly unprecedented, uncharted territory, globally in relationship to avian influenza," Dr. Peter Rabinowitz, director of the UW Center for One Health Research, told NPR.
While the the highly pathogenic avian influenza A (H5N1) virus has been detected in dozens of mammal species over the past two decades, it was recently detected for the first time in goats and cows.
On March 20, the Minnesota Board of Animal Health reported that goats on a farm where a backyard poultry flock was previously infected tested positive for the virus. Five days later, the U.S. Department of Agriculture reported the H5N1 virus was detected in dairy cows in Texas and Kansas. It was also later confirmed in cows in Idaho, Michigan and New Mexico.
Unpasteurized milk from sick cattle collected from two dairy farms in Kansas and one in Texas, as well as a throat swab from a cow in another dairy in Texas, tested positive for the same strain that is widespread among birds globally, according to the Centers for Disease Control and Prevention.
On April 1, the CDC reported a dairy worker in Texas tested positive for the virus, saying the person was likely infected by sick cattle. The patient reported eye redness (consistent with pink eye) as their only symptom, and is recovering, per the CDC.
It's only the second person reported to have tested positive for bird flu in the U.S. -- a previous human case occurred in 2022 in Colorado -- and the first known case of the virus spreading from a mammal to a human.
"It is important whenever there's a change – and this is a change that cattle are infected for the first time that we know about, and this is only the second human case in the U.S. that's been detected – every time there's such a change, we need to dig into that and make sure there's nothing more than just a single one-off case from industrial exposure," Dr. Thomas Inglesby, director of the Johns Hopkins Center for Health Security, told CNN.
Officials stress that the risk to the public still remains low, noting that human cases detected thus far are mostly linked to close contact with infected animals and contaminated environments.
"The fact that it spread to one human case is certainly making us want to take this very seriously. But folks should know that right now, the risk to them is very low," CDC Director Dr. Mandy Cohen told ABC News.
In any event, concern is growing.
"The increasing number of H5N1 avian influenza detections among mammals — which are biologically closer to humans than birds are — raises concern that the virus might adapt to infect humans more easily," the World Health Organization said. "In addition, some mammals may act as mixing vessels for influenza viruses, leading to the emergence of new viruses that could be more harmful to animals and humans."
Human illnesses with H5N1 bird flu have ranged from mild (eye infection, upper respiratory symptoms) to severe illness (such as pneumonia) that have resulted in death in other countries, according to the CDC.
The symptoms of H5N1 infection in humans includes fever, cough, sore throat and muscle aches. Other early symptoms may include abdominal pain, chest pain and diarrhea. The infection may progress quickly to severe respiratory illness (difficulty breathing or shortness of breath, pneumonia) and neurologic changes (altered mental status, seizures).
There's currently no evidence that the virus can spread among people, but researchers are staying on high alert.
The CDC said it is working closely with state and federal agencies, including USDA and FDA, and local health authorities to further investigate and closely monitor the situation.
According to FDA and USDA, there are no concerns with the safety of the commercial milk supply at this time because products are pasteurized before entering the market. Dairies are required to send only milk from healthy animals into processing, while milk from impacted animals is destroyed so that it does not enter the food supply.