The bird flu outbreak in California is spreading, and dairy farmers say the situation is worse than they expected.

A third California farmworker has tested positive for avian flu, according to the state health department. If confirmed by the US Centers for Disease Control and Prevention, it would be the 17th human case of H5N1 influenza in the US since March, when the virus was first detected in cows.

Farmers and veterinarians in California are warning that the infection appears to be hitting herds harder than in other states, leading to a higher percentage of sick and dead cows in affected herds.

There are no signs of a more serious illness in people. Like the first two human cases in California, this third case involves a farm worker who was exposed to sick dairy cattle. Investigators see no connection between the latest case and either of the first two, suggesting this is another case of animal-to-human transmission, the California Department of Public Health said in a press release. In all three cases, symptoms were mild and included red, bloodshot eyes, a sign of conjunctivitis.

CDC Principal Deputy Director Dr. Nirav Shah said Friday the cases were not a surprise.

“Additional cases may continue to be identified as additional herds continue to test positive,” Shah said.

While public health officials remain calm about the infections, outside experts say each new human infection is a sign that the outbreak is not under control and that people who work with livestock and other sick animals are not adequately protected.

“These reports only heighten my fears that this virus, if left unchecked, will cause serious harm to human health,” said Dr. Jennifer Nuzzo, who leads Brown University’s Pandemic Center.

“Many people are too dismissive of the threat posed by H5N1 on farms, as the few mild cases have mistakenly suggested. But epidemiologists know that the more people become infected, the more likely we are to see severe consequences,” Nuzzo wrote in an email.

“I have seen little that would lessen my concern that this virus will ultimately lead to hospitalization or death,” she added.

Dozens of California Herds Affected

More than 80 flocks in California have been affected by avian influenza since late August, according to the latest USDA data. California is the largest milk producer in the country.

The California Department of Food and Agriculture is sampling milk tanks in areas where herds are testing positive.

As in Colorado, where milk tank testing was mandated by the state, testing helped quickly identify more affected herds, said Dr. Eric Deeble, USDA’s acting senior adviser for the H5N1 response.

Deeble said the USDA has sent a strike force to help California track the spread of the outbreak, and the state is looking at ways to expand milk testing beyond areas with known infections.

Even as investigators ramp up their efforts to track the spread of the virus in California, local farmers are warning that H5N1 infections in their herds are more severe than previously reported.

In other states, about 10% of cows in affected herds showed symptoms, resulting in fewer than 2% of the animals dying, according to the American Veterinary Medical Association.

In contrast, dairy farmers in California are reporting the virus is infecting 50% to 60% of their herds, and 10% to 15% of cows are dying from the infections, according to the California Dairy Assurance Program. The higher infection and death rates in California were first reported by the Los Angeles Times.

The increasing severity of cases has raised questions among scientists, who wonder whether environmental factors such as heat may play a role, or perhaps the virus is changing as it adapts to cows and causes more severe infections.

They said scientists could better tell whether genetic changes in the virus might play a role if state and federal officials publicly shared more information about the genetics of the viruses they discovered, and did so more quickly.

The first infected herds in California were discovered in late August. On Monday, the CDC shared the gene sequences of viruses isolated from the first two farm workers in the state to test positive on a widely used data-sharing site called GISAID.

The same database also contains apparently recently published sequences from dairy cattle in the United States that were shared by the USDA’s Animal and Plant Health Inspection Service, but those sequences are missing vital information needed to place them into the context of the larger outbreak, including which state they came from and the date they were collected.

The USDA says it continues to follow the same process it has throughout the outbreak, uploading raw sequences on Fridays when they become available and often within two weeks of sample collection. The agency adds additional information about the sample, including the state where it was collected and the date of collection, after the epidemiological investigation is completed, approximately six weeks later.

Other countries, including some in Africa operating with basic equipment and minimal resources, share virus sequences within days and usually with more information than the USDA provides, a GISAID spokesperson told CNN.

Without this information, it is difficult for scientists to track the evolution of the virus and understand whether it is changing and becoming a more serious threat.

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