Six possible human infections as H5N1 breaks into Californian dairy heartlands

Cattle at a state fair
Cases have been confirmed in dairy workers exposed to H5N1-infected cows - Jim Vondruska/REUTERS

As many as six people are believed to have caught H5N1 bird flu in California since the virus gained a foothold in the state’s vast cattle herds.

California confirmed its fourth case on Thursday in a farm worker who was exposed to sick cattle. All four cases were among dairy workers who were exposed to H5N1-infected cows at separate farms in Central Valley.

State authorities said they had been notified of two more presumed cases, which are currently undergoing testing by the US Centre for Disease Control (CDC).

Those who tested positive suffered mild symptoms, including eye redness or discharge and did not require hospital treatment, the California Department of Public Health said.

Bird flu has been spreading through US cattle since December 2023, and has since infected hundreds of dairy herds in 14 states and at least 18 people. All but one of the cases have been in dairy or poultry workers.

‘Heck of a lot of virus’ in cow milk

California – America’s largest dairy producer – managed to avoid an outbreak until September of this year, but over the last month the virus has spread rapidly between the state’s farms.

So far, at least 99 herds have been infected, according to state authorities, making California the state with the largest number of animal cases so far.

Some experts fear cases in California will climb further, owing to the fact the state has a sizeable community of people who drink unpasteurised milk for its supposed health benefits.

Mark McAfee, founder of the Fresno-based Raw Milk Institute, previously told The Telegraph he had received multiple calls from people in California requesting samples of H5N1-infected milk because they think it will give them immunity.

The Food and Drug Administration (FDA) has assured people that the country’s pasteurised milk supply is safe to consume, and has repeatedly warned the public to avoid the raw variety for fears it could lead to H5N1 transmission.

“There’s a heck of a lot of virus in the milk of an infected cow,” said Richard Webby, director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals.

“If the virus evolves to become effective at infecting humans – although we are not yet at that point – milk from a single cow would be enough to infect thousands of people,” he said.

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