The recent bird flu infection in a dairy worker in Texas has public health officials on high alert, though experts say the virus hasn’t become more contagious, either among cows or people.
Samples taken from the patient — whose only symptom was pinkeye — showed that the virus has not changed in ways that would make it easy for it to spread from human to human, and that currently available vaccines and medicines remain effective against it, according to the Centers for Disease Control and Prevention.
“Our assessment of the risk of avian flu to the general public right now remains low,” Dr. Nirav Shah, principal deputy director of the CDC, said in an interview Wednesday. “However, make no mistake, we are taking this very seriously.”
Upon learning from the U.S. Department of Agriculture last week that cows in a handful of dairy farms had tested positive for bird flu, the CDC contacted state health departments, asking them to work with farms to identify any person who may be showing symptoms.
“The case that we found in Texas, we found it because we went to look for it. We knew to look for it,” Shah said.
This current form of bird flu, a strain called H5N1, has been circulating in birds around the world since late 2021. It has infected and killed countless wild birds as well as led to the culling of tens of millions of birds on poultry farms across the United States.
The first human case in the U.S. was in 2022, in a prison inmate in Colorado who was working on a poultry farm. The case in Texas is the country’s second.
In a press briefing Wednesday, Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said the agency is in close contact with the CDC.
“Any case of H5N1 is concerning because it is highly dangerous to humans, although it has never been shown to be easily transmissible between people,” Tedros said.
While the two U.S. cases have both been mild, H5N1 infections outside the country have had a high mortality rate. Four cases in Cambodia were reported in February; one patient died. In 2023, there were six cases in Cambodia, four of which were fatal.
What are bird flu symptoms?
Bird flu is a respiratory virus. In severe cases in humans, it can cause pneumonia, according to the CDC. Symptoms can include fever, cough, sore throat, runny nose, body aches, headaches, fatigue and shortness of breath or difficulty breathing.
Cases can also be mild: In the 2022 case in Colorado, the man experienced a few days of fatigue.
The only symptom that the Texas patient developed was conjunctivitis, or pinkeye, and the person is recovering, according to the CDC. The agency is not aware that any of the person’s close contacts have developed flu-like symptoms, Shah said.
Still, what makes this particular case different is the link to cows; the fear is that the virus will mutate in a way that allows it to spread more easily among mammals and possibly humans.
“I don’t think that this news means that a flu pandemic is imminent,” said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan.
“I think that this means that we are getting yet another early warning signal that if H5N1 has enough opportunities to get into mammals, it’s going to do what viruses always do and it’s going to continue to adapt, to growing in them, because that is what viruses do.”
Is the bird flu virus changing?
Cows were likely exposed to the virus from some sort of interaction with a bird.
“It could have been with bird feces. It could have been with a dead bird,” Shah said.
Samples of the virus taken from the Texas patient as well as from a handful of infected cows have been sequenced.
Rasmussen said that the genetic sequencing of samples taken from cows showed that the virus was much more closely related to birds, suggesting it hadn’t been transmitting in cows for very long and hadn’t had the chance to mutate to spread more efficiently among them.
The virus sample taken from the patient had one additional mutation that’s linked to spread in mammals. Shah said that the change has been seen “in numerous other situations, going back 20 plus years” and is not associated with sustained spread between people.
The mutation is only one of a “constellation of these different mutations that have been associated with mammal to mammal transmission,” Rasmussen said. “So it really doesn’t look like this is thoroughly adapted to mammal to mammal transmission, so that’s good news.”
On the other hand, she said, “it’s still really early days. We’ve only just appreciated that the extent of infection in cattle might be really, really underestimated. So there’s a lot more research, I think, that we need to do to try to figure out how common this is in cattle and other people might potentially be at risk.”
One of the big questions that will be key to preventing further spread, whether among cows or from cows to people, is understanding how cows themselves transmit the virus.
In birds, Rasmussen said, the virus grows in the gastrointestinal tract. Other birds get sick if they come into contact with infected saliva, mucous or feces.
Humans can get sick if they touch something with the virus on it and then touch their mouth, eyes or nose, the CDC says. They can also breathe it in if they’re in an area with a lot of virus particles in dust or droplets. According to Rasmussen, the virus can bind to receptors deep in humans’ lungs, but not higher up in the respiratory tract, like the nose or mouth, which makes it hard to spread from human to human.
It can also bind to receptors in the eyes. That the Texas patient’s symptom was pinkeye “suggests that they didn’t get it by inhalation, they got it from direct contact with cattle and then maybe from rubbing their eyes or something like that,” she added.
It’s unclear how cows are infected and shed the virus, though. “It’s really, really hard to try to assess what the risks are to people around cows, if we don’t know where most of that virus is coming from on the cows,” Rasmussen said.
Pasteurized milk is safe to drink, according to the Food and Drug Administration, because the process kills the virus.
What’s more, any milk from an affected cow is thrown out before it can enter the milk supply, another step in keeping it safe, Shah said.
