The Missouri Department of Health and Human Services and the Centers for Disease Control announced Friday that a Missouri resident who had no reported contact with animals tested positive for H5 avian influenza. It is not yet clear whether the person was infected with the same strain of the virus causing an ongoing outbreak among dairy cows.
The person, who was hospitalized on Aug. 22, had several underlying health conditions. He has since recovered and been released from the hospital, according to a statement from state officials.
The CDC said this is the first case of H5 avian influenza detected through the country’s national influenza surveillance system and the first known case of H5 in an individual with no occupational contact with infected cattle or poultry.
A CDC spokesperson told STAT that analysis of the virus is still ongoing, but so far agency scientists have found no evidence of changes that would suggest the virus has evolved to make it more infectious to or between people.
While news that people with no history of contact with infected animals have been infected with the H5 virus is unsettling, experts who spoke with STAT cautioned that it’s too early to draw conclusions.
The 13 other H5 cases reported this year were discovered through surveillance of farmworkers who had contact with infected animals, according to the CDC: “In this case, the patient’s specimen initially tested positive for influenza A but was negative for seasonal influenza A virus subtypes, which prompted additional testing.”
The CDC is working to generate and analyze the complete genetic sequence of the virus when possible. At this time, the neuraminidase (the N in the virus name) has not yet been identified.
California, the nation’s largest milk-producing state, has confirmed cases of avian influenza in three dairy herds.
Missouri has not reported any cases of H5N1 in dairy cattle. Fourteen states, including three that border Missouri, have reported a total of 197 cases of infected cattle since the virus was first detected in cattle in late March. Infections have also been reported in poultry in the state.
The CDC said in a statement that no unusual flu activity has been detected in Missouri, and wastewater surveillance has not found any evidence of the H5 virus.
It will be important to know if this is the same H5 virus causing infections in cattle, as there are multiple variants of the H5 virus in wild birds and the person may have been infected with a different strain, experts said.
Florian Kramer, an influenza virologist at the Icahn School of Medicine at Mount Sinai in New York, noted that in early May, a person in Mexico who had no history of contact with infected animals tested positive for another avian flu virus, H5N2. The person’s source of infection has never been found. Such one-off cases sometimes occur, Kramer said.
Another possibility is that the person ingested raw milk or raw dairy products that were infected with the virus. “That’s something we can’t rule out,” Cramer said.
While the risks of drinking raw milk contaminated with the H5N1 virus are unclear, tests have shown that the contaminated milk can cause severe illness in mice. Similarly, many farms where cows have been infected with H5N1 have also reported deaths of cats on the premises.
Tests conducted by the Food and Drug Administration and the United States Department of Agriculture have shown that pasteurization kills the H5N1 virus in milk.
The Missouri person may have come into contact with wild birds or their droppings while cleaning bird feeders or handling dead birds collected by cats, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Cramer said he wouldn’t be too worried if any of these possibilities turn out to be the likely route of infection for this person. “The question is if this is a sign of low-level circulation (of the virus) in the population,” he said. “And we don’t know that, and of course that’s a concern.”
He suggested it’s important to test those around this person and anyone in the community who has flu-like symptoms. “This needs to be taken seriously. I hope, and I think it’s likely, that this is a one-off, but we’re not sure. We don’t know.”
Osterholm said he was encouraged that the state and CDC press releases made no mention of the possibility of further spread of the virus, as neither have reported symptoms among contacts of infected people or health care workers who have cared for infected people.
“What we have is implicit, not stated, but it doesn’t say that anyone else is under investigation for suspected illness,” he said.
Given what is known so far, Osterholm said the new developments don’t increase concern about H5N1. “Our concerns about the existing risk remain unchanged,” he said.
This is the 15th confirmed case of H5 infection in the United States and the 14th so far this year. (In 2022, a Colorado man was found to have the virus after helping clean up a chicken farm where infected birds were culled.)
All cases so far this year have been directly or indirectly linked to outbreaks in cattle: Cases in Michigan, Colorado and Texas have been traced to dairy workers and people who became infected while working on infected poultry farms.
