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A uthorities in Colorado are monitoring the health of about 10 people who were in contact with a man who tested positive for H5 bird flu when he was involved in culling poultry infected with the virus.

The man, who is incarcerated at a state correctional facility and was taking part in a pre-release work program, is being considered the country’s first case of H5 bird flu. The people being monitored worked with him and shared transportation to the poultry operation. 

State Epidemiologist Rachel Herlihy said Friday that all the individuals have tested twice for the virus and all tests have been negative so far. They were all offered the flu antiviral Tamiflu, which can be used both to treat and prevent flu infections, though Herlihy wasn’t certain all had agreed to take it. They will be monitored for 10 days after their last exposure to the unidentified man, who she said had recovered from a very mild illness.

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

In reality, it’s not clear the man was infected. Workers who were culling the birds underwent daily symptom checks; on April 20, they were tested using nose swabs. On April 26, the state’s public health laboratory concluded the man had been infected with an influenza A virus, but it was unable to subtype the virus.

The sample was then sent to the Centers for Disease Control and Prevention, which confirmed on April 27 that the swab contained H5 virus. The subtyping of the neuraminidase — the N in the virus’ name — is still underway, but the presumption is that the birds and the man were infected with H5N1 viruses.

The man was placed in isolation on April 26, after the state laboratory could not subtype the virus, Lisa Wiley, a public affairs officer for the Colorado Department of Corrections, said in an email. The move was taken out of an abundance of caution, she said.

In interviews with public health officials after the fact, the man acknowledged feeling fatigued for a few days around the time the test was taken. But he reported none of the respiratory symptoms normally associated with flu.

In a statement issued by the CDC on Thursday evening, the agency raised the possibility that the man may not have been infected — the swab may have just picked up the virus in his nostrils. But the positive test means he meets the definition of a case, the CDC said.

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

For years countries have been on high alert for H5N1 infections in people. In the mid-aughts, these viruses — labeled as highly pathogenic, because of their lethality to poultry — spread from Southeast Asia to South Asia and North Africa, leaving devastation in their wake. Millions of birds died from the virus or from the culling needed to stop the virus’ spread.

And occasionally the virus jumped from poultry to people. In the past 20 years or so, roughly 880 people in 19 countries have been infected with H5N1. Just over half of them died. Fortunately, though, the virus to date has not acquired the capacity to spread easily from person to person and the fears that an H5N1 pandemic might be looming have faded.

This case is unlikely to reignite them.

The virus has changed over time, Todd Davis, who leads the CDC influenza division’s zoonotic virus team, said in an interview. The H5N1 viruses from 15 years ago or so had some different genes that appeared to allow them to trigger more severe illness when they infected people, he said. Those changes, particularly in the polymerase gene, are not present in most current day H5N1 viruses.

And while the virus is still called H5N1, at a point a few years back it swapped genes with another flu virus, picking up a different neuraminidase. The new N1 is highly adapted to infect wild birds — speeding its recent spread through Europe, Africa and North America — but it seems less able now to infect people, Davis said.

Since that reassortment event, the newly configured H5N1 has only been seen to have infected two people — a man in the United Kingdom last December and the man in Colorado. The U.K. man kept poultry and the virus found its way into his flock. Though he tested positive, he experienced no symptoms.

“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 this virus is just not as capable of infecting or causing disease in humans.”

Still, flu viruses change constantly, and public health officials do not take lightly human infections with non-human flu viruses.

“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,” Davis said. “We’re still very much on high alert.”

Helen covers issues broadly related to infectious diseases, including outbreaks, preparedness, research, and vaccine development.

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