So, in all, Missouri’s case count in the H5N1 outbreak will stay at one for now, and there remains no evidence of human-to-human transmission. Though both the household contact and the index case had evidence of an exposure, their identical blood test results and simultaneous symptom development suggest that they were exposed at the same time by a single source—what that source was, we may never know.

California and Washington

While the virus seems to have hit a dead end in Missouri, it’s still running rampant in California. Since state officials announced the first dairy herd infections at the end of August, the state has now tallied 137 infected herds, and at least 13 infected dairy farm workers. California, the country’s largest dairy producer, now has the most herd infections and human cases in the outbreak, which was first confirmed in March.

In the briefing Thursday, officials announced another front in the bird flu fight. A chicken farm in Washington state with about 800,000 birds became infected with a different strain of H5 bird flu than the one circulating among dairy farms. This strain likely came from wild birds. While the chickens on the infected farms were being culled, the virus spread to farmworkers. So far, two workers have been confirmed to be infected and five others are presumed to be positive.

As of publication time, at least 31 humans have been confirmed infected with H5 bird flu this year.

With the spread of bird flu in dairies and the fall bird migration underway, the virus will continue to have opportunities to jump to mammals and gain access to people. Officials have also expressed anxiety as seasonal flu ramps up, given influenza’s penchant for swapping genetic fragments to generate new viral combinations. The reassortment and exposure to humans increases the risk of the virus adapting to spread from human to human and spark an outbreak.

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The H5N1 bird flu appears to be stampeding through dairy farms in California, the country’s largest milk producer. Over the weekend, the total number of confirmed infected cow herds stunningly doubled, going from 17 last Thursday to 34 Monday morning, according to state and federal officials.

With the new tally, California now ranks second among all affected states for having the most herds with avian influenza. Only Colorado, which has adopted bulk milk-tank surveillance, has more, with 64 herds confirmed. California’s high ranking is despite the fact that it only reported its first three infected herds on August 30, while the dairy outbreak was first confirmed on March 25 and thought to have begun late last year.

To date, 232 herds in 14 states have been infected with the bird flu.

In an announcement last week, California officials said the batch of herds that tested positive were “targeted for testing due to elevated risks from their recent connections with the initial affected premises.” They called the positive results “not unexpected” and reported that they fit with the state’s plans of finding infections as early as possible. “Early detection provides the opportunity to work with farms to quickly implement enhanced biosecurity, cow care, and employee protection.”

When the state’s first infected herds were announced, California Department of Food and Agriculture Secretary Karen Ross touted the state’s preparations and readiness. “Our extensive experience with [highly pathogenic avian influenza] in poultry has given us ample preparation and expertise to address this incident,” Ross said. She further assured dairy farmers that they are approaching the outbreak with the “utmost urgency.”

Troubling transmission

The news comes as public health experts are concerned about a puzzling case of H5 bird flu in a person in Missouri, who had underlying medical conditions but no known exposure to animals. The case is the 14th human infection amid the dairy cow outbreak this year. All of the 13 prior infections were in farmworkers known to be exposed to infected animals: nine in poultry workers and four in dairy workers. But Missouri has not reported H5N1 in dairy herds and hasn’t had recent outbreaks in poultry facilities, either.

While the Missouri case has recovered, health officials are stumped about how that person became infected. In a press briefing earlier this month, an official with the Centers for Disease Control and Prevention suggested the case could be a “one-off.” But, the CDC has reported that another person in the household with the Missouri case fell ill at the same time, suggesting a possible common exposure to the virus. That household contact was not tested for flu and has also since recovered. The CDC is now conducting serological testing to see if the household member has developed antibodies against the bird flu, indicating a previous infection.

The CDC has also reported that a health care worker who interacted with the patient fell ill, but tested negative for flu. On Friday, the CDC reported that a second health care worker became ill with a mild respiratory illness after interacting with the Missouri case but was not tested for flu and recovered before the investigation began. The CDC will offer serological testing for that health care worker, too.

While there’s no evidence of human-to-human transmission yet, health experts fear that there is not enough testing—of case contacts, animals, or farmworkers—and transmission is going unseen. As the wily influenza virus spreads across the country to new mammalian species, it gains new opportunities to adapt to humans and cause more severe disease. Influenza experts have long feared that the virus has pandemic potential.

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A person in Missouri with no reported exposure to animals was confirmed to have been infected with H5-type bird flu, the Missouri Department of Health and Senior Services (MDHSS) announced late Friday.

MDHSS reported that the person, who has underlying medical conditions, was hospitalized on August 22 and tested positive for an influenza A virus. Further testing at the state’s public health laboratory indicated that the influenza A virus was an H5-type bird flu. The Centers for Disease Control and Prevention has now confirmed that finding and is carrying out further testing to determine if it is the H5N1 strain currently causing a widespread outbreak among US dairy cows.

It remains unclear if the person’s bird flu infection was the cause of the hospitalization or if the infection was discovered incidentally. The person has since recovered and was discharged from the hospital. In its announcement, MDHSS said no other information about the patient will be released to protect the person’s privacy.

The report marks the 15th human case of an H5-type bird flu infection in the country since 2022. But, the case stands out—and is quickly generating alarm online—because the man reported no contact with animals. All 14 of the previous cases occurred in farmworkers who had contact with either dairy cows or poultry that were known to be infected with H5N1.

The finding in a person without such an exposure raises the possibility that the H5N1 virus is spreading from person to person, undetected, or is spreading via an undetected animal source.

But, while the case raises concern, some infectious disease experts are cautious not to sound the alarm without more data on the case and potential exposures.

“[U]ntil such data is collected and analyzed, my level of alarm is only mildly heightened,” Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security and founding associate director of the Center for Forecasting and Outbreak Analytics at the CDC, said online.

“I am encouraged that this case was detected through existing surveillance systems, which bodes well for our ability to identify any additional cases in the future,” she added. “Federal, state, and local health officials maintained flu surveillance through the summer months in response to the H5 situation, and that was definitely the right move.”

But Rivers, like many of her colleagues, has long worried about H5N1’s potential to jump to humans and spark a pandemic.

To date, H5N1 is known to have infected 197 herds in 14 states. Missouri has not reported infected herds, but has reported infected poultry farms.

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