Avian flu outbreaks confirmed at three California dairy farms

Federal officials have confirmed outbreaks of H5N1 avian influenza at three dairy farms in California, but the source is likely the transportation of cattle and not exposure to sick birds.

Health officials said last week that cows on three Central Valley dairies were suspected to be infected and were waiting for confirmatory testing. On Tuesday, officials said testing showed the strain of the virus in the California herd was nearly identical to one found in a Colorado dairy herd, suggesting the infection was caused by interstate movement of cattle.

The B3.13 genetic sequences found in the infected cows are clearly the result of “anthropogenic movement,” making “the likelihood of independent spillover from wild birds into these dairy farms essentially zero,” said Brian Richards, emerging infectious diseases coordinator at the U.S. Geological Survey's National Wildlife Health Center. “So if anyone's trying to blame wild birds, they're wrong!”

According to a statement from the California Department of Food and Agriculture, there have been no confirmed cases of human infection with H5N1 avian influenza in the state, and neither the Centers for Disease Control and Prevention nor state officials consider the outbreak to be a significant public health threat, and the risk to humans is considered low.

“The primary concern is dairy workers who have close contact with infected dairy cows,” the agency said in a statement. Four other cases of human transmission from dairy farms have also been reported in other states, including Texas, Colorado and Michigan.

Officials also said the state's supply of milk and dairy products would not be affected, that tainted milk is not permitted for sale, and that “consumers need not be concerned about pasteurized milk or dairy products” because pasteurization inactivates the virus.

“We have been preparing for this since the beginning of the year. [hightly pathogenic avian influenza] “Detections have been confirmed on dairy farms in other states,” said CDFA Secretary Karen Ross. “Our extensive experience with highly pathogenic avian influenza in poultry means we are well prepared and have the expertise to respond to this incident, with worker health and public health as our top priority.”

In an effort to stop or slow the spread of the virus, the U.S. Department of Agriculture in April restricted the movement of cattle between some states, required dairy cows to be tested for avian flu before being moved, and required livestock owners to report positive cases before moving animals across state lines.

Large-scale movements of cows are standard practice on American dairy farms, with many sending calves just a few days old to farms that specialize in raising veal calves. Once the calves are grown, the females are typically sent back to the dairy where they were raised or to another dairy farm, while the males are sent to feedlots, veal farms, or directly to slaughter.

In 2022, Research by a team from Texas Tech University A university study found that one in 10 dairy calves were raised off-site on these “calf farms,” ​​a figure that rose to nearly five in 10 when researchers looked at farms with more than 500 dairy cows.

The researchers note that these facilities are often located hundreds or even thousands of miles away from the dairy farms where the animals were born. “It is not uncommon to find facilities feeding more than 20,000 preweaned calves in the central Great Plains and Western regions,” the authors write.

This large-scale movement of cattle is one of many biosecurity weaknesses that dairy industry observers and critics say contribute to the spread of disease.

Since the virus was first reported in U.S. dairy cows in March, 197 cows in 14 states have been infected, according to a USDA map.

The dairy industry is “particularly vulnerable to the possibility of disease transmission from a single dairy farm” because of cow movement, Maurice Pithesky, an associate professor of poultry health and food safety epidemiology at the University of California, Davis, said in a July interview.

Commercial chicken farms, which have been battling avian influenza for decades, have the advantage of being closed systems, in that most farms have physical barriers such as fences or walls to keep wildlife, including waterfowl, away from the commercial birds. In contrast, dairy farms are open to the outside environment and often intentionally introduce potentially infected water (from the dairy farm's lagoon where waterfowl roost) into the facility, such as by rinsing dairy barns with lagoon water.

“When you go into a poultry facility, you have to fill out a form saying you haven't touched any other birds for 48 to 72 hours because we are very concerned about disease transmission,” he said, stressing the importance of biosecurity at such facilities.

Monitoring the virus is also complicated by the fact that H5N1-infected cows show only subtle signs of infection — lethargy and reduced milk production — while poultry die.

Without on-site testing or mandatory milk pool testing, where farmers test samples of milk they collect from their cows, it's difficult to know where the virus is.

For now, California dairy farmers are monitoring their cows for signs of the disease, while state and local health officials are working to understand and hopefully limit the disease's spread, said Michael Payne, a researcher and outreach coordinator at the Western Institute for Food Safety and Security at the University of California, Davis.

Payne said the state has quarantined the three confirmed infected herds and now requires permits for any movement of animals on or off those farms.

“Right now, no cows are leaving those herds,” he said, adding, “Any young cows that need to be moved, any animals that need to be slaughtered, any dead animals that need to be moved, all of that is permitted through the creation of a fairly extensive biosecurity program that is overseen and developed by California Department of Animal Husbandry staff, veterinarians and veterinary medical officers.”

He said he has been fielding calls from producers for days, helping them follow best practices while monitoring for signs of infection.

The locations of the infected poultry have not been disclosed. The federal government provides county data on infected poultry, but only statewide figures for dairy.

“We don't share that information because sometimes there may only be one or two dairy farms in a particular county and because of privacy concerns, they tend to contact the state,” said USDA spokesman Will Clement. “If state agriculture departments want to share that information, that's their prerogative, but we're not interested in exposing anyone in a particular area, so to speak,” he said.

A state Department of Agriculture spokesman said the department was not naming specific counties.