An inmate in Colorado has become the first human in the US to test positive for the H5N1 strain of bird flu, amid the worst outbreak of the virus in seven years and a cull of millions of poultry in dozens of states.

The unnamed prisoner contracted the infection during a work release assignment at a farm in Montrose county where workers were euthanizing an infected flock, the Colorado department of public health and environment said.

The federal Centers for Disease Control and Prevention (CDC) said the health risk to the general public remained low.

“The patient reported fatigue for a few days as their only symptom and has since recovered,” the agency said, adding that the man, who was younger than 40, was treated with the antiviral drug oseltamivir.

The Colorado case “does not change the human health risk assessment”, the CDC said.

“[The agency] has tracked the health of more than 2,500 people with exposures to H5N1 virus-infected birds and this is the only case that has been found to date. Other people involved in the culling operation in Colorado have tested negative for H5 virus infection, but they are being retested out of an abundance of caution.”

The only other known human case of the strain of bird flu currently predominant was in the UK in December last year, the CDC noted, although almost 900 cases have been reported worldwide from earlier H5N1 viruses since 2003.

The US agriculture department has been tracking the spread of avian influenza across 30 states and affecting more than 35 million birds, according to its latest figures.

The Guardian reported on Thursday that more than 22 million birds have been killed in an attempt to contain the outbreak, the majority in Iowa, the biggest US producer of eggs. That figure didn’t include a commercial egg farm in Knox, Nebraska, where 2.1 million poultry were reported affected on Wednesday.

At one egg factory in Rembrandt, Iowa, about 250 employees were fired after roasting alive 5.3 million chickens, the largest cull at any factory farm in the country, in circumstances some fired workers and animal rights groups say were inhumane.

The Storm Lake Times revealed that birds at Rembrandt were culled using a system known as ventilation shutdown plus (VSD+) in which air is closed off to the barns and heat pumped in until the temperature rises above 104F (40C).

“They cooked those birds alive,” said one of the workers involved.

Earlier in the week, an H3N8 strain of bird flu was detected in humans for the first time in Henan province in China.

The four-year-old boy infected had been in contact with chickens and crows raised at his home, according to reports from China’s National Health Commission.

Avian influenza, or bird flu, is a highly contagious viral disease. The first reports of human cases came in the 1990s.

Some strains have been passed to humans but it is currently very rare and usually occurs only after very close contact with infected birds or animals.