CNN  — 

The bird flu spreading through cattle in the United States is an “enormous concern” the chief scientist of the World Health Organization said Thursday as he called for more tracking and preparation for the virus.

So far, there is no evidence that the highly pathogenic H5N1 flu virus can spread from person to person. This flu strain was first detected in birds in 1996 and has primarily been a threat to farmed and wild fowl, but in the past two years, an increasing number of mammals have tested positive with the virus, indicating that the virus is looking for new hosts and moving closer to people.

“The great concern, of course, is that in doing so and infecting ducks and chickens — but now increasingly mammals — that that virus now evolves and develops the ability to infect humans. And then critically, the ability to go from human-to-human transmission,” Dr. Jeremy Farrar, a British medical researcher and chief scientist at WHO since 2023, told reporters on Thursday in Geneva.

“We have to watch, more than watch, we have to make sure that if H5N1 did come across to humans with human-to-human transmission that we were in a position to immediately respond with access equitably to vaccines, therapeutics and diagnostics.”

Farrar’s comments came in a response to a question during a news conference about WHO’s new definition for airborne pathogens.

Though H5N1 doesn’t spread from person to person, humans can catch it when they’re exposed to infected animals. In the US, one person in Texas has tested positive for H5N1 this year. That person was working with cows when they tested positive, and it is believed they caught it from an infected cow.

They are only the second documented case of human H5N1 in the United States. The first was in a poultry worker in Colorado in 2022.

Though it doesn’t spread easily to humans, researchers are worried about it because of how deadly these infections can be. Since 2003, 889 human cases and 463 deaths have been reported from 23 countries, giving this virus a case fatality rate of 52%, according to the WHO.

“This virus is a really scary virus. It’s something I would hate to see in humans,” Dr. Richard Webby, director of the WHO’s coordinating center for studies on the ecology of influenza and a scientist at St. Jude’s Children’s Research Hospital, told CNN.

Webby, who has been studying H5N1 for 20 years, said even though this virus has never figured out how to efficiently make people sick, “what’s happening now there are lots more small mammals being infected with this virus and we’ve ever seen in the past 20, close to 25 years of monitoring, so that is absolutely concerning,” he said.

Webby said H5N1 is more active now than it ever has been, but it still hasn’t developed key mutations that scientists are worried about that could help it infect people.

“Because we haven’t seen those changes does suggest to me that this virus has got quite a hurdle to overcome to become a real sort of human pathogen,” Webby said.

So far in the United States, the US Department of Agriculture’s Animal and Plant Health Inspection Services has reported 29 infected herds in eight states — Texas, New Mexico, Kansas, South Dakota, Idaho, Michigan, Ohio and North Carolina.

The US Centers for Disease Control and Prevention says that while the current risk to public health from H5N1 is low, it is monitoring the situation carefully. In an update posted to its situation summary on Thursday, the CDC said is studying the virus isolated from the recent human case in the US and found it to be susceptible to antiviral medications.

The agency also said it has created a candidate vaccine virus that can be used as a template to make a vaccine if needed.