Federal officials have confirmed outbreaks of H5N1 avian influenza at three dairy herds in California, likely caused by livestock transportation rather than exposure to sick birds.
Health officials announced last week that they suspected cows at three Central Valley dairy farms had contracted the disease and were awaiting tests to confirm it. On Tuesday, officials said those tests showed the strain of the virus infecting the California herds was nearly identical to the one in the Colorado dairy farms - suggesting the infections were the result of interstate transmission from cattle.
The B3.13 genetic sequence found in the infected cows was clearly the result of "anthropogenic movement; in fact, there's no chance that it was an independent leakage 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 wants to blame wild birds: No!"
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In a statement from the California Department of Food and Agriculture, officials said there have been no confirmed human cases of H5N1 bird flu in the state. Neither the U.S. Centers for Disease Control and Prevention nor state officials consider this development to be a significant threat to public health. The risk to humans is considered low.
"The greatest 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 dairy have been reported in other states, including Texas, Colorado and Michigan.
Officials also said the state's supply of milk and dairy products has not been affected. Contaminated milk cannot be sold and pasteurization inactivates the virus, "so there is no cause for concern for consumers of" pasteurized milk or dairy products.
"We have been preparing for this eventuality since the beginning of this year, [hightly pathogenic avian influenza] detections have been confirmed on 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 worker health and public health as our top priorities."
In hopes of stopping or slowing the spread of the virus, the U.S. Department of Agriculture in April restricted the movement of livestock between states. It required that dairy cattle used for milking be tested for avian influenza before each movement, and that producers report any positive cases before moving the animals across state lines.
Large-scale livestock transport is standard practice on U.S. dairy farms, with many sending day-old calves away to be raised on 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 feedlots, calf farms, or directly to slaughter.
In 2022, research from a team at Texas Tech University found that 1 in 10 calves born to dairy cattle were raised off-farm on "calf farms." That jumped to nearly 5 in 10 when researchers looked at farms with more than 500 dairy cows.
The researchers noted that these operations are often hundreds, if not thousands, of miles away from the dairy farms where the animals are born. "It is not uncommon to see operations feeding more than 20,000 unweaned calves in the Central Great Plains and Western regions," the authors wrote.
This large-scale movement of livestock is one of several biosecurity weaknesses that dairy industry observers and critics say are contributing to the spread of the disease.
According to a map from the U.S. Department of Agriculture, 197 herds in 14 states have been affected since March, when the virus was first identified in U.S. dairy cattle.
In a July interview, Maurice Pitesky, an associate professor with a research focus in poultry health and food safety epidemiology at UC Davis, noted that the dairy industry is "particularly susceptible to the potential for disease transmission from a single dairy," in part because of these livestock transfers.
Commercial poultry farms, which have struggled with avian influenza for decades, have the advantage of being closed systems, as most operations have physical barriers such as fences and walls that keep wildlife, including waterfowl, away from the commercial birds. Dairy operations, on the other hand, are open to the outside environment and in many cases-such as flushing dairy barns with lagoon water-intentionally introduce potentially contaminated water (from dairy lagoons housing waterfowl) into their facilities.
"When you go to a poultry farm, you have to fill out a form saying that you haven't touched any other birds in the last 48-72 hours, because they are very concerned about disease transmission," he said, stressing the biosecurity of these operations.
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Surveillance of the virus is also complicated by the fact that H5N1-infected cows show only subtle signs of infection: lethargy, reduced milk production, etc. Poultry, on the other hand, die.
Without mandatory on-site testing or milk pool testing (where farmers test samples of their cows' milk), it is difficult to know where the virus is.
Currently, dairy farmers in California are monitoring 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 potentially limit the spread of the disease.
Payne said the state has quarantined the three herds where infections have been confirmed, and all animal movements on or off those farms now require permits.
"There are no lactating cows leaving the herds at this time," he said, adding that "young cattle that need to be removed, or animals that need to go to slaughter, or dead animals that need to be moved... all of that is allowed through the establishment of a fairly comprehensive biosecurity program that is managed and developed with California CDFA staff, veterinarians and veterinary medical officers."
He said he has been on the phone for days answering calls from producers and helping them follow best practices when looking for signs of infection.
The location of the herds has not been disclosed. The federal government provides county data on infected poultry, but only statewide figures for dairy.
"We don't share that very often because sometimes there's only one or two dairies in a particular county, and for privacy reasons we often refer people to the state," said Will Clement, a spokesman for the USDA. "If the state Department of Agriculture wants to share that information, that's their prerogative. But we don't want to give anyone, if you will, away in a specific region," he said.
A spokesman for the Agriculture Department said his agency does not name specific counties.
This story originally appeared in the Los Angeles Times.