After three more herds in California’s Central valley tested positive for bird flu, questions have been raised about the whether true extent of the outbreak in the US is much wider than recognized, given inadequate or nonexistent biosecurity and a persistent lack of testing.
The new cases, revealed by officials at the US Department of Agriculture (USDA), bring the total in California to eight affected herds discovered this month.
“We are really not testing enough,” said Meghan Davis, an associate professor at Johns Hopkins Bloomberg school of public health. “The lack of testing has actually been one of the most startling things for me, in terms of watching the response to this outbreak unfold.”
California is the 14th state to announce H5N1 cases in dairy cows since the outbreak was first identified in March. New Mexico and Michigan also recently announced more cases in dairy herds.
Yet several states and localities have resisted testing – among animals and people.
In Missouri, the first patient to test positive after no known contact with animals had a close contact fall ill at the same time – but the contact was not tested for the flu and has not undergone a blood test to check for H5N1 antibodies.
In Colorado, nine cases were discovered in July among 109 poultry workers who reported symptoms and consented to testing – only a fraction of the 663 workers who were exposed to H5N1-positive chickens, according to a recent report from the US Centers for Disease Control and Prevention (CDC).
Continued low levels of testing even among workers with confirmed exposures underline the limits of federal agencies in outbreak response.
Experts worry the virus is much more widespread than is being reported – and every newfound case in an animal raises the risk of more people getting sick.
In California, farms near the affected herds will do bulk milk sampling, Eric Deeble, deputy under secretary for marketing and regulatory programs at USDA, said on Thursday.
Bulk testing of milk can reveal cases among cows that seem healthy. After Colorado mandated this type of testing, officials discovered positive cases in 11 more herds.
“It’d be really great to see USDA step in with some very strong recommendations” around efforts like bulk milk testing, Davis said.
US officials have weighed bulk-testing mandates “since day one” but have not instituted them, Deeble said in a call last month. He said that Colorado’s successes were “probably unique to Colorado, and extrapolating out to the rest of the country is not entirely appropriate”.
Testing “everything is a lot”, said Steve Grube, chief medical officer for the center for food safety and applied nutrition at the US Food and Drug Administration (FDA), at an August briefing.
The USDA requires H5N1 testing only for lactating dairy cows moving across state lines.
Deeble added that he had a “high degree of confidence” that the existing testing was accurately capturing the status of animals moving between states, though movement within states was not being monitored as closely. Even so, he said: “I do feel like the response is adequate.”
It’s not clear how H5N1 came to California. The cases could represent a lapse in interstate testing, or the virus may have begun circulating before the order went into place in April, or the virus could have spilled from cows into wildlife and then back into cows in California – or people and contaminated equipment, like trucks, may have played a role.
A USDA report in June found the virus is most likely being spread by human activity through transportation vehicles that are not decontaminated between herds, rearing and milking practices, or through contaminated clothing and equipment.
Other biosecurity lapses may include open-air dairy operations and practices like washing cows’ stalls with reused water from lagoons that may have been contaminated by other cows and wildlife. Major dairies also frequently send calves to “calf ranches” where thousands of young cows from different states may be raised together and then sent back home or on to other farms.
Sequencing of samples from the first three California herds reveals the virus is closely related to the strain circulating among dairy cows in other states, Deeble said on Thursday – indicating the infections were not caused by a new spillover event from the bird flu strain that has circulated among wild birds in North America since 2022.
The strain discovered in cows has also spilled back into other animals, including wild and domesticated birds, cats and mice, which could then move to infect other animals with the bovine-adapted virus.
H5N1 vaccines for cows are now undergoing testing, and vaccines for people are rolling off production lines now – though they have not yet been authorized for use.
No human cases have been reported in California, and the state issued a health alert for providers to look out for possible H5N1 cases in people. No poultry flocks have been affected by this outbreak, the state said in a statement.
In June, bird flu was detected in San Francisco wastewater, but the source was not clear.
California is the leading dairy producer in the nation, responsible for 20% of the US milk supply, and a widespread outbreak could have significant economic effects for farmers – especially the longer the virus is able to circulate.
“Thinking about worker protections and guidance for workers is incredibly important,” Davis said. “Setting up things now would be the more proactive way to go about it, and would be in keeping with our principles of how we manage outbreaks, which is to be as aggressive as possible within the space of early detection and early response.”
Farm workers are at the highest risk of contracting the virus from sick animals. And if their illnesses – and the illnesses of animals they care for – are going undetected, it could create new risks for others, Davis said.
“If this would get into the general public, we don’t know what we would see.”
