Federal officials have confirmed that three California dairy herds have suffered outbreaks of H5N1 bird flu, due most likely to the transportation of cattle and not exposure to diseased birds.

Health officials announced last week that they suspected cows at three Central Valley dairies had contracted the illness, and were awaiting testing for confirmation. On Tuesday, officials said those tests revealed that the strain of virus that infected California herds was nearly identical to that found in Colorado dairy herds — suggesting the infections were the result of interstate transfer of cattle.

The B3.13 genetic sequence found in the infected cows was clearly the result of “anthropogenic movement; essentially zero chance it was an independent spill from wild birds into these dairies,” said Bryan Richards, the Emerging Disease Coordinator at the U.S. Geological Survey’s National Wildlife Health Center. “So, if anyone is trying to blame wild birds: Nope!”

In a statement from the California Department of Food and Agriculture, officials said there were no confirmed human cases of H5N1 bird flu in the state, and neither the U.S. Centers for Disease Control and Prevention nor state officials see this development as a significant public health threat; the risk to humans is considered low.

“The primary concern is for dairy workers who come into close contact with infected dairy cows,” said officials in the statement. Four cases of human infection from dairy have been reported in other states, including Texas, Colorado and Michigan.

Officials also said the state’s supply of milk and dairy foods is not affected. Contaminated milk is not permitted to be sold and pasteurization inactivates the virus, “so there is no cause for concern for consumers from” pasteurized milk or dairy items.

“We have been preparing for this eventuality since earlier this year when [hightly pathogenic avian influenza] detections were confirmed at dairy farms in other states,” said CDFA Secretary Karen Ross. “Our extensive experience with HPAI in poultry has given us ample preparation and expertise to address this incident, with workers’ health and public health as our top priorities.”

Hoping to stop or slow the spread of the virus, the U.S. Department of Agriculture in April limited the movement of some interstate cattle transfers, mandating that lactating dairy cattle get tested for bird flu before any transfer, and that livestock owners report any positive cases before moving the animals across state lines.

Large-scale cattle movement is standard practice among U.S. dairy farms, and many send days-old calves away to be raised at farms that specialize in rearing calves. Once the calves are grown, the females are generally sent back to the dairy where they were raised — or to another dairy — while males are sent to feeding lots, veal farms or straight to slaughter.

In 2022, research from a team out of Texas Tech University showed that 1 in 10 dairy-born calves were raised off-site at these “calf ranches.” That rose to almost 5 in 10 when researchers looked at farms that had more than 500 lactating cows.

The researchers noted that these operations are often located hundreds, if not thousands, of miles from the dairy farms where the animals were born. “It is not uncommon to see operations feeding over 20,000 pre-weaned calves in the Central Great Plains and West regions,” wrote the authors.

This large-scale transfer of cattle is one of many biosecurity weak spots that observers and critics of the dairy industry say is contributing to the spread of the disease.

According to a USDA map, 197 herds have been affected in 14 states since March, when the virus was first reported in U.S. dairy cattle.

In an interview from July, Maurice Pitesky — an associate professor with a research focus on poultry health and food-safety epidemiology at UC Davis — noted that the dairy industry “is uniquely susceptible to the potential for disease transmission from a single dairy” in part because of these cattle transfers.

Commercial poultry farms, which have been contending with avian flu for decades, have the advantage of being closed systems in that most farms have physical barriers such as fences and walls that keep wildlife, including waterfowl, away from the commercial birds. In contrast, dairies are open to the outside environment and in many cases — such as the flushing of dairy stalls with lagoon water — purposely introduce potentially infected water (from dairy lagoons where waterfowl roost) into their facilities.

“When you go on to a poultry facility, you have to fill out paperwork that says you haven’t touched any other birds for 48-72 hours, because they’re so concerned about disease transmission,” he said, underscoring the biosecurity of these operations.

Surveillance of the virus is also complicated by the fact that H5N1 infected cows show only subtle signs of infection — lethargy, decreased milk production, etc. Poultry, on the other hand, die.

Without mandating on-site testing, or milk-pool testing — in which farmers test samples of the milk they’ve pooled from their cows — it’s hard to know where the virus is.

For now, California dairy farmers are working to keep an eye on their cows for signs of illness. And according to Michael Payne, a researcher and outreach coordinator at the Western Institute for Food Safety and Security at UC Davis, state and local health officials are working to understand and perhaps limit the movement of the disease.

Payne said the state has quarantined the three herds where infections have been confirmed. And all animal movement on or off those farms now requires permits.

“There are no lactating cows leaving those herds right now,” he said, adding that “any younger stock that need to go, or animals that need to go to slaughter, or dead animals that need to be moved … all of that is being permitted through the creation of a pretty extensive biosecurity program that is being overseen and developed with California CDFA employees, veterinarians, veterinary medical officers.”

He said he’s been on the phone for days fielding calls from producers and helping them follow best practices as they watch for signs of infection.

The location of the herds has not been divulged. The federal government provides county data on infected poultry, but only statewide figures for dairy.

“We do not tend to share that because sometimes there are only one or two dairies in a certain county, and due to privacy concerns, we tend to refer folks to the state,” said Will Clement, a spokesman for the USDA. “If the state department of ag wants to share that information, that is their purview. But we don’t want to out anybody, if you will, in any specific region,” he said.

A spokesman for the state department of agriculture said his agency is not naming specific counties.