The H5N1 bird flu continues to spread among American dairy cows, with infections showing up in 93 herds in 12 states as of late Wednesday.
The CDC says there's no evidence humans can easily infect each other with bird flu, and the risk to the general public remains low right now.
The CDC has documented three human cases of H5N1 bird flu this year in the United States. All were dairy workers who had mild cases and were sickened after being exposed to sick dairy cows.
The first patient, in Texas, and the second patient, in Michigan, both suffered conjunctivitis, or mild eye infections. The third, also in Michigan but at a different farm, had a mild eye infection and a cough.
"Could we be seeing more conjunctivitis cases, eye infections out there? [We] absolutely could, that are not getting detected," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "We need to know more about the viruses themselves to understand, are they changing?"
Scientists continue to watch how H5N1 mutates. To date, it's been able to infect at least 48 species of mammals worldwide since 2020, including cats, dogs, alpacas and seals.
"It's clear that this virus is changing substantially in ways that are now infecting all these other mammal species. And not only is it infecting them, but it's causing multi-organ involvement, including severe brain damage. We hadn't seen that before, so surely to other mammals it's a real problem," Osterholm said.
The more mammals that can get sick, the higher the risk to humans.
"It's not as if it's one switch that the virus that flips and then all of a sudden it can spread. It's a series of different mutations," infectious disease physician and senior scholar at Johns Hopkins Center for Health Security Dr. Amesh Adalja told Scripps News.
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Health experts stress the need for more testing in case the virus changes such that it could spread easily from human to human, also known as sustainable human-to-human transmission.
"One of the misunderstandings that is prevalent right now in the public is that somehow public health isn't doing its job trying to get testing done, and in fact, that's just not the case," Osterholm said. "The challenge we have right now is that there are many people in the agricultural industry that don't want testing done, they don't want to disrupt their operations. These are not animals that are getting desperately sick, you know, they recover. [Dairy farmers] want to keep their operations really kind of, I'd say, below the radar screen. So, it's been a real challenge to get to these farms with permission from the farmers and agriculture in general to get the testing done."
As for the public's food supply, the USDA says cooked beef and pasteurized milk are safe to consume.
Other strains of the virus are also popping up around the globe. The World Health Organization says a 4-year-old in India was hospitalized for three months with the H9N2 strain, and H5N2 was found in a man from Mexico who had died. These are different strains than the H5N1 causing the dairy cow outbreak in the U.S.
"I think that's reassuring," Adalja says. "The thing to remember is, that bird flu viruses are always going to be spreading and most of them don't pose risks to humans. But we do know that when it comes to pandemics, avian influenzas are very high on the list and eventually one will have the ability to spread the way the 1918 H1N1 virus was able to. That's why there is such a concerted effort to run down these flu viruses, to make sure that they're contained, to understand their biology, so that we are able to act very proactively when something does pose a risk to human health."