by Natalie Wallington, The Kansas City Star

Colorized transmission electron micrograph of Avian influenza A H5N1 viruses. Credit: Public Domain

A mysterious human case of the H5N1 bird flu was recently discovered in Missouri—and experts are still working to determine how the patient was infected.

The avian flu has been circulating among poultry and wild birds since 2022, but made the interspecies jump to dairy cows earlier in 2024. Just over a dozen humans have been infected in the U.S. so far that health officials know about, most of them livestock workers who have made direct contact with sick animals.

But that pattern was broken when the Missouri Department of Health and Senior Services announced it found a human case in late August among the general public. The patient reported no contact with animals, leaving experts wondering how they contracted the flu—and whether the case suggests that the virus is now traveling between humans.

"This is the 14th human case of H5 (avian flu) reported in the United States during 2024 and the first case of H5 without a known occupational exposure to sick or infected animals," the Centers for Disease Control and Prevention wrote in a press release Friday, Sept. 6. The agency added that the risk of avian flu to the general public remains low.

Details about the patient have been sparse to protect their identity, but some experts have found the state and federal response to be frustratingly slow. For instance, while CDC labs confirmed the avian flu diagnosis, the agency's investigators can't look into the infection further unless state authorities request their help. So far, Missouri hasn't made such a request.

"We have not had a need for more extensive on-site assistance at this time, as we are still limited to one case with low risk of sustained transmission," DHSS spokesperson Lisa Cox wrote in an email.

CDC spokesperson Nick Spinelli did not respond to follow-up questions about the agency's further involvement in the Missouri case after its nationwide surveillance program detected it.

Here's what we know—and what we don't—about the unusual infection.

How did a Missouri resident contract bird flu?

Until now, direct contact with infected livestock has caused all the human infections with the H5N1 flu virus during the current outbreak. The Missouri case reported on Sept. 6 is notable because it seems to involve somebody who is not a livestock worker.

"The patient has reported no exposure to animals," the department's press release reads. But the patient's level of contact with non-livestock animals, like pets or wild birds, remains unknown.

The flu can infect pets like outdoor cats when they come into contact with infected birds or other wild animals. It has already been found in red foxes in Missouri and skunks in Kansas, as well as domestic cats in at least 13 other states, according to the USDA's Animal and Plant Health Inspection Service.

Dr. Dana Hawkinson, the medical director for infection prevention and control at the University of Kansas Health System, said that a pet could be responsible for the Missouri infection, but it's hard to say without knowing more about the patient's circumstances.

"I think household pets are probably a fairly good, plausible explanation for this," he said. "In general we just don't have a lot of information about this case."

He added that the most likely vector for the disease is high-contact surfaces where the virus can linger.

"Humans in general just touch their face on a regular basis, and don't always do the best hand hygiene," he said. "I have to believe there was some contamination where this person picked it up by high-touch surfaces, rather than airborne."

Can bird flu spread between humans?

Historically, there have been some examples of the H5N1 virus spreading between humans, Hawkinson said. However, this type of transmission isn't nearly as likely between people as it is between animals.

"You need to have the receptors and you need to have the virus, which is able to bind to the receptors on our cells to cause infection," he said. "Humans tend not to have as much of the receptor for this avian influenza in their body compared to other animals, especially birds."

He added that transmission of the avian flu virus between humans has been limited to just a few people during past outbreaks. While it's possible the recently reported Missouri infection was contracted from another human, the CDC maintains that the virus's risk to the public remains low.

"The main issue is just making sure you're washing your hands after any possible interaction or touching of contaminated surfaces," Hawkinson said. "Try to avoid touching your hands to your face."

While human-to-human transmission isn't currently a major concern, the avian flu virus has already mutated enough to make the jump from birds to mammals—and it will likely continue to evolve in the future.

This year's flu shots are not formulated to protect against the H5N1 virus, Hawkinson said. The government does have a supply of H5N1-specific vaccines, but they currently aren't recommended for most people to use.

2024 The Kansas City Star. Distributed by Tribune Content Agency, LLC.