The first case of a person to contract bird flu after no known contact with animals is raising questions about the possibility of human-to-human transmission and emphasizing the complicated relationship between states and federal agencies in outbreak response.
An extensive investigation into the case of a patient in Missouri who was hospitalized on August 22 revealed no links to animals, officials at the US Centers for Disease Control and Prevention (CDC) told reporters on Thursday.
Two contacts of the patient also became ill around the same time, the agency said in a report the next day.
A healthcare worker suffered mild symptoms but tested negative for influenza. More worryingly, a member of the household also developed symptoms on the same day as the patient – but that person was not tested for the flu.
There is a blood test to check for antibodies against H5N1, a highly pathogenic bird flu, which can be done as soon as possible 10 days after infection, but antibody testing has not begun, CDC officials said Thursday.
The patient with a confirmed infection responded to long questionnaires about recent activities, such as gardening, having a bird feeder, keeping any pets at home, visiting agricultural fairs or zoos, eating uncooked meat or mourning to drink milk. Nothing raised alarm bells with officials about possible exposure.
“So far, epidemiologists have not found a clear source of exposure,” said Nirav Shah, principal deputy director of the CDC.
“Missouri is working very hard to get deeper into the [epidemiology] to see if there is any undetected exposure that is possible,” said Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases.
Genomic sequencing revealed that the strain is related to the virus that probably began circulating among dairy cows late last year; the bovine-adapted variant has since spilled over into other wild and domesticated animals, including birds and mammals.
The patient’s sample showed at least two mutations, including one that could make current vaccine candidates for H5N1 less effective.
Shah told reporters last Thursday that there was no evidence of person-to-person transmission, calling the event a likely “one-off” case.
“None of the individuals that this individual came in contact with developed any signs and symptoms,” Shah said Thursday.
Still, the CDC reported the other two symptomatic contacts last Friday, one of which was “subsequently” shared by Missouri.
The CDC has not been invited by Missouri to help with the investigation, although federal officials are in contact with the state via phone and video calls, Shah said.
The CDC has limited authority for compliance at the state and local level, and recent Supreme Court decisions can have a chilling effect on regulatory action.
Missouri has been in “very close consultation” with the CDC since the case was detected, “but we have not seen a need for more extensive on-site assistance as we are still limited to one case with a low risk of sustained transmission.” , Lisa Cox, communications director for the Missouri Department of Health and Senior Services, told the Guardian.
Only 240 people or so were tested for H5N1 in the 2024 outbreak, according to the CDC. Typically, tests are only done when someone in close contact with infected animals develops symptoms.
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In Missouri, that’s not what happened. Health workers suspected a respiratory infection like the seasonal flu, but the swab was not quickly detected as a potential H5N1 infection, Shah said. Instead, it was sent along with other influenza A samples to the state lab to be double-checked as part of routine surveillance looking for any type of unusual flu strains.
This type of monitoring usually occurs during the winter flu season, but in May, the CDC recommended that states continue to monitor for influenza A throughout the summer due to ongoing outbreaks on poultry and dairy farms.
This is the first time an H5N1 case has been detected with the national influenza surveillance system.
“Our flu surveillance system is designed to find needles in haystacks, and as this case and others show, it works,” Shah said. “We found such a needle, but we don’t know how it got there.”
Missouri has not reported any cases of H5N1 in cows, but the state has only 17 tested of around 60,000 dairy cows from July. The most recent outbreak of bird flu among Missouri poultry was reported in February.
There were no unusual levels of flu activity or emergency department visits in this area compared to last year’s levels, Shah said. “But our investigation is not over.”
The Missouri patient, who has “significant” underlying conditions, experienced “acute” symptoms of chest pain, nausea, vomiting, diarrhea and weakness, Shah said.
“The patient has underlying chronic medical conditions, and respiratory symptoms were not the main cause for hospitalization on admission,” Cox said. The patient was not seriously ill, was hospitalized for three days and recovered.
It’s possible that the individual’s existing medical conditions made them more vulnerable to disease, Shah said.
As the virus continues to spread among animals, officials remain vigilant for more cases among humans, Shah said. “As more animal species exhibit and harbor H5, the possibility of animal-human interaction increases, and thus the possibility of human cases increases.”
