Disease researchers have been unable to determine how a person in Missouri with no known exposure to animals or poultry became infected with an H5 bird flu virus, the chief deputy director of the Centers for Disease Control and Prevention said Thursday.
But Nirav Shah said the ongoing investigation has found no evidence of further spread of the virus, suggesting this case could turn out to be a one-off infection that defies explanation.
“Here’s the bottom line: Our flu surveillance system is designed to find needles in haystacks,” Shah said at a news briefing. “In this case we found such a needle, but we don’t know how it got there. Our investigation continues and we will keep everyone informed as we learn more.”
Shah also revealed that the CDC has so far failed to determine the full subtype of the H5 virus, explaining that the amount of RNA in the patient’s sample was low. The agency continues to try to identify the virus’s neuraminidase, or N, but Shah acknowledged that may not be possible in this case. Under those circumstances, the CDC would not be able to generate a complete genetic sequence of the virus from the Missouri patient.
Analysis the CDC was able to conduct shows that the hemagglutinin — the surface protein that gives the virus its H number — is closely related to the H5 viruses circulating in dairy cow herds in several states, Shah said.
Missouri is not one of the 14 states that have reported outbreaks in dairy cattle.
On Thursday, the U.S. Department of Agriculture confirmed 203 infected herds since the virus was first identified in late March as the cause of a decline in milk production at some dairy farms. The outbreak is believed to have begun in late 2023 or early 2024, likely in the Texas Panhandle, with the virus making its way from infected wild birds to livestock and then moving from cow to cow and farm to farm.
The Missouri Department of Health and Senior Services is leading the investigation at the scene. The person, who has recovered, is cooperating with state authorities, Shah said. He noted that there have been discussions with the person and his or her close contacts about providing blood samples to test for antibodies to the virus – work that could determine whether anyone close to the individual had an undetected infection that was passed on to this person. It is too early to take blood samples for such an effort, Shah said, adding that Missouri is willing to conduct this work if those close to the infected person agree to participate.
The unnamed person was taken to the hospital on August 22 with symptoms not normally associated with the flu: chest pain, nausea, vomiting, diarrhea and weakness. This person had multiple pre-existing health conditions, Shah noted. During the course of the investigation, a flu test was ordered, which led to the discovery of the mysterious H5 infection.
It is not unheard of for there to be cases in which researchers are unable to trace a human infection with new flu viruses back to a source of infection, Shah said, noting that of the more than 500 swine flu infections detected in the U.S. since 2010 about 8% of cases have been among humans without traceable contact with pigs or other infected people.
Shah also revealed that the CDC is working with commercial companies that make diagnostic tests to have them work on developing H5N1 tests, in case there is a need for them later. Early in the Covid-19 pandemic, the CDC developed a flawed test, and it and the Food and Drug Administration were slow to involve commercial testing companies in the test development process, a reality that hampered both the nation’s early response to Covid-19 was a huge criticism of the CDC.
Five companies – Aegis, ARUP, Ginkgo BioWorks, Labcorp and Quest – are involved in the effort. “We know that when the next response with laboratory needs arises – and they all do these days – we will have to contract with commercial laboratories. This initiative does that now, rather than in an emergency,” Shah said.
In addition to creating tests for the H5N1 virus, the companies are also commissioned to develop tests for the Oropouche virus. The agency expects to spend $5 million on this work this month, and could spend up to $118 million over the next five years, if necessary.
In related news, Eric Deeble, the USDA’s point person for the H5N1 outbreak response, revealed that California is using bulk tank testing to determine the extent of the H5N1 outbreak. The state, the nation’s largest dairy-producing state, announced in late August that it had detected the bird flu virus on three farms in the Central Valley. Since then, the company has found a further five infected herds through testing activities linked to the farms initially detected.
Correction: An earlier version of this article and a subheading incorrectly stated how many states have detected outbreaks of bird flu in dairy cattle.