Health authorities in Colorado on Friday announced that a man has been infected with H5 bird flu, marking the first case of the disease in the United States, according to CDC. But experts say there’s no reason to be concerned about the disease spreading widely throughout the country.

Bird flu reported in Colorado

According to authorities in Colorado, the man infected with the virus was part of a pre-release work program at a state correctional facility culling poultry.

All workers who were culling the poultry had symptom checks each day, and on April 20, the workers were tested with nose swabs. On April 26, Colorado’s public health laboratory determined the man was infected with an influenza A virus, and further analysis from CDC discovered on April 27 the virus was an H5 virus.

CDC is still determining the neuraminidase of the virus—the N part of the virus’s name—but the agency is assuming the man was infected with an H5N1 virus, also known as bird flu, STAT News reports.

The man told public health officials after he tested positive that he had felt fatigued for a few days, but did not report the respiratory symptoms typically associated with influenza.

CDC on Thursday issued a statement saying it’s possible the man wasn’t infected with H5N1 and that the nasal swabs simply picked up the virus in his nostrils. However, the agency said the man still meets the definition of a case.

“There’s some additional work that CDC can do, but we might not ever know if this was truly an infection or not,” said Rachel Herlihy, Colorado’s state epidemiologist.

Around 10 people in contact with the man are being monitored for symptoms, STAT News reports. Herlihy said all the individuals have been tested twice for the virus and have all tested negative. The individuals were also offered Tamiflu, an antiviral that can be used to prevent flu infections.

“We’re being cautious,” Herlihy said. “We’re using isolation, we’re using treatment, we’re monitoring contacts—doing all of those things. But we continue to believe that the overall risk is low.”

What experts are saying

In the past two decades, around 880 people in 19 countries have been infected with H5N1, and just over half have died, STAT News reports.

According to Todd Davis, leader of CDC’s influenza division’s zoonotic virus team, the H5N1 virus has changed over time. The version of the virus from 15 years ago seems to have had different genes that allowed it to cause more severe illness in people, but those changes are not present in the version of the virus currently circulating.

Instead, the new version of the virus is extremely adept at infecting wild birds, but seems less able to infect people, Davis said.

“There have only been two detected human cases since the virus was first detected in 2018,” Davis said. “That’s also pretty good evidence that the virus is just not as capable of infecting or causing disease in humans.”

Still, it’s important to pay close attention and be cautious, Davis added.

“Our challenge at the moment is trying to keep up with those hundreds of people who are involved in culling operations or disinfection of infected farms, and making sure that we don’t see any changes in the virus that might increase the propensity for severe illness or transmissibility,” he said. “We’re still very much on high alert.”

Michael Osterholm, director of the University of Minnesota‘s Center for Infectious Diseases Research and Policy, agreed that increased surveillance of H5N1’s spread among humans is important.

“Any time you’re dealing with H5N1, you sleep with one eye open,” he said. (Branswell, STAT News, 4/29; Branswell, STAT News, 4/28; Torchinsky, NPR, 4/29; Kamp, Wall Street Journal, 4/28)