Federal officials have confirmed that an outbreak of H5N1 bird flu in three California dairy herds is likely caused by cattle transportation, not contact with sick birds.
Health officials announced last week that they suspected the disease had spread to cows at three Central Valley dairies, and were awaiting testing to confirm. On Tuesday, officials said those tests showed the virus strain infecting the California herds was nearly identical to the one found in Colorado’s dairy herds — indicating the infections were the result of interstate transfer of cattle.
The B3.13 genetic sequence found in infected cows was clearly “the result of anthropogenic activity; there’s basically zero possibility that it was spread independently from wild birds to these dairies,” said Brian Richards, emerging disease coordinator at the U.S. Geological Survey’s National Wildlife Health Center. “So, if anyone is trying to blame wild birds: No!”
In a statement from the California Department of Food and Agriculture, officials said no human cases of H5N1 bird flu have been reported in the state, and neither the U.S. Centers for Disease Control and Prevention nor state officials view the incident as a significant public health threat; the risk to humans is considered low.
“The main concern is for dairy workers who come into close contact with infected dairy cows,” officials said in the statement. Four cases of human infection from dairies have been reported in other states, including Texas, Colorado and Michigan.
Officials also said the supply of milk and dairy foods in the state has not been affected. Contaminated milk is not allowed to be sold and pasteurization inactivates the virus, “so there is no cause for concern” for consumers about pasteurized milk or dairy products.
“We have been preparing for this eventuality since earlier this year, when (highly pathogenic avian influenza) was detected on dairy farms in other states,” said CDFA Secretary Karen Ross. “Our extensive experience with HPAI in poultry has given us adequate preparation and expertise to respond to this eventuality, with worker health and public health being our highest priorities.”
In hopes of stopping or slowing the spread of the virus, in April the U.S. Department of Agriculture restricted some interstate cattle movements, mandating that dairy cattle be tested for bird flu before any move, and that ranchers must report any positive cases before moving animals across state borders.
Large-scale cattle movements are standard practice on American dairy farms, and many send calves as young as a few days old to be raised at farms that specialize in raising calves. Once the calves are grown, females are typically 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 done by a team from Texas Tech The university showed that 1 in 10 dairy-born calves were raised off-site on these “calf farms.” When the researchers studied farms with more than 500 milking cows, that number rose to 5 in 10.
The researchers found that these operations are often located hundreds, or even thousands, of miles from the dairy farms where the animals were born. “It is not uncommon to see operations in the Central Great Plains and West regions feeding more than 20,000 weaned calves,” the authors wrote.
The mass movement of cattle is one of several biosecurity vulnerabilities that dairy industry observers and critics say are contributing to the spread of the disease.
According to a USDA map, 197 cattle in 14 states have been affected since March, when the virus was first reported in U.S. dairy cattle.
In an interview in July, Maurice Pitesky — an associate professor at UC Davis who researches poultry health and food safety epidemiology — noted that the dairy industry is “uniquely susceptible to the possibility of disease transmission from a single dairy,” and one reason for this is the transfer of cattle.
Commercial poultry farms, which have struggled with avian flu for decades, have the advantage of being closed systems because most farms have physical barriers such as fences and walls that keep wildlife, including waterfowl, away from commercial birds. In contrast, dairies are open to the outside environment and in many cases – such as flushing dairy stalls with lagoon water – intentionally introduce potentially infected water (from dairy lagoons where waterfowl roost) into their facilities.
“When you go to a poultry facility, you have to fill out paperwork saying 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.
Monitoring the virus is also complicated by the fact that cows infected with H5N1 show only mild symptoms of infection – lethargy, decreased milk production, etc. Chickens, on the other hand, die.
Without mandatory on-site testing, or milk-pool testing — in which farmers test milk samples collected from their cows — it’s hard to know where the virus is.
Right now, California dairy farmers are working to track signs of the disease in their cows. And state and local health officials are working to understand and possibly limit the movement of the disease, according to Michael Penn, a researcher and outreach coordinator at the Western Institute for Food Safety and Security at UC Davis.
The state has quarantined the three herds where infections were confirmed, Penn said, and now requires permits for all animal movements on or off those farms.
“There are no milk cows going out of those herds right now,” he said, adding that “any young animal, or any animal that needs to go to slaughter, or dead animal that needs to be transported … all of that is being allowed through the creation of a very comprehensive biosecurity program that is being overseen and developed with California CDFA staff, veterinarians, veterinary officers.”
He said they have been talking to growers on the phone for several days and helping them follow best practices to monitor for signs of infection.
The location of the flocks has not yet been revealed. The federal government provides county data on infected poultry, but only statewide figures for dairy.
“We don’t share it because sometimes there’s only one or two dairies in a certain county, and because of privacy concerns, we refer people to the state,” said USDA spokesman Will Clement. “If the state agriculture department wants to share that information, that’s their jurisdiction. But we don’t want to single out anybody, if you will, in any specific area,” he said.
A spokesman for the state Department of Agriculture said his agency was not naming specific counties.
