Mysterious case of bird flu in Missouri shows complications of outbreak response

The first case of a person contracting bird flu without animal contact raises questions about the possibility of human-to-human transmission and highlights the complicated relationship between states and federal agencies in responding to an outbreak.

An extensive investigation into the case of a Missouri patient hospitalized on Aug. 22 has found no animal connection, officials with the U.S. Centers for Disease Control and Prevention (CDC) told reporters on Thursday.

Two of the patient’s contacts also became ill around the same time, the agency announced in a report the next day.

A health care worker developed mild symptoms but tested negative for influenza. Even more troubling, a family member also developed symptoms on the same day as the patient, but that person was not tested for influenza.

There is a blood test to check for antibodies to H5N1, a highly pathogenic bird flu, which can be performed as soon as possible. 10 days after infection, but antibody testing has not yet begun, CDC officials said Thursday.

The patient with confirmed infection answered lengthy questionnaires about recent activities, such as gardening, feeding birds, keeping pets at home, visiting agricultural shows or petting zoos, eating undercooked meat, or drinking raw milk. Nothing raised alarm bells with authorities about possible exposures.

“So far, epidemiologists have not identified a clear source of exposure,” said Nirav Shah, deputy director of the CDC.

“Missouri is working hard to dig deeper into the (epidemiology) to see if there are any undetected exposures,” said Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases.

Genomic sequencing has shown that the strain is related to the virus that likely began circulating among dairy cows late last year. The bovine-adapted variant has now spread to other wild and domesticated animals, including birds and mammals.

The patient’s sample showed at least two mutations, including one that could potentially make current vaccine candidates for H5N1 suitable less effective.

Shah told reporters last Thursday that there was no evidence of person-to-person transmission, calling the incident likely a “one-off” case.

“None of the individuals this individual came into contact with have developed any signs or symptoms,” Shah said Thursday.

Still, the CDC reported the other two symptomatic contacts last Friday, one of which was “later” shared by Missouri.

Missouri has not invited the CDC to assist with the investigation, though federal officials have been in contact with the state by phone and video calls, Shah said.

The CDC has limited enforcement powers at the state and local level, and recent Supreme Court rulings could have a chilling effect on regulatory action.

Missouri has been in “very close consultation” with the CDC since the case was discovered, “but we have not needed any more extensive on-site assistance at this time because we are still limited to one case with 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 during the 2024 outbreak, according to the CDC. Normally, testing is only done when someone who has been in close contact with infected animals develops symptoms.

That didn’t happen in Missouri. Health care workers suspected a respiratory infection like seasonal flu, but the swab wasn’t quickly evaluated as a possible H5N1 infection, Shah said. Instead, it was sent to the state lab along with other influenza A samples to be double-checked as part of routine surveillance for unusual flu strains.

Such monitoring normally occurs during the winter flu season, but in May the CDC advised that states should continue monitoring influenza A into the summer because of ongoing outbreaks on poultry and dairy farms.

This is the first time an H5N1 case has been identified using the national influenza surveillance system.

“Our influenza surveillance system is designed to find needles in haystacks, and as this case and others show, it works,” Shah said. “We found a needle, but we don’t know how it got there.”

Missouri has not reported any cases of H5N1 in cows, but the state had only tested 17 of about 60,000 dairy cows in July. The most recent outbreak of bird flu in poultry in Missouri was reported in February.

There are no unusual levels of flu activity or emergency room visits in this area compared to last year’s levels, Shah said. “But our investigation is not over.”

The Missouri patient, who has “serious” 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 primary reason for hospitalization at the time of admission,” Cox said. The patient was not seriously ill, was hospitalized for three days and has recovered.

It’s possible that the person’s medical conditions made him or her 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 likelihood of human-animal interactions increases, and therefore the likelihood of human cases increases.”