The outbreak of highly pathogenic avian influenza (HPAI, more specifically avian influenza type A H5N1), continues among U.S. dairy cattle after its initial discovery in that species this spring.

Since March, H5N1 outbreaks have been confirmed in dairy cattle in over 200 herds. This includes 34 dairy cow operations in California, making it 14th state with dairy herds that have tested positive for the virus.

Testing culled dairy cattle beef
The U.S. Department of Agriculture (USDA) has been testing raw milk from cows to identify the virus in herds. The Food and Drug Administration (FDA) has separately tested commercial milk supplies and cites federal agency research showing that pasteurization kills the virus.

The disease is unpredictable, however, as it has shown up in the milk of asymptomatic dairy cattle, as well as those that show a high number of clinical signs over an extended period of time, according to Dr. Sarah Tomlinson, associate deputy administrator for strategy and policy at USDA Animal and Plant Health Inspection Service’s (APHIS) Veterinary Services.

Now, the USDA’s Food Safety and Inspection Service (FSIS) has announced that it will add H5N1 influenza to its national surveillance program monitoring beef for pathogens and chemical contaminants.

Starting September 16, FSIS began testing for avian influenza type A H5N1 in dairy cattle carcasses as part of its National Residue Program (NRP) to ensure the safety of beef for human consumption and to detect the virus’s presence in asymptomatic cows, according to an August 13 FSIS notice.

The NRP is an interagency program that tests meat, poultry, and egg products for a wide range of chemical resides and contaminants, including veterinary drugs and pesticides.

The H5N1 testing program follows three separate studies conducted by the USDA demonstrating the safety of U.S. beef. In August, the FDA released the results of a second retail sampling survey that tested 167 dairy products for H5N1. According to an analysis coordinated by the USDA, no viable H5N1 virus was detected in any of the products, which included pasteurized milk, products made from pasteurized milk, and aged raw milk cheese.

Over the next year, FSIS will use the NRP to test 800 skeletal muscle samples collected from dairy cow carcasses at slaughter. Samples will be analyzed by FSIS laboratory staff members using polymerase chain reaction (PCR) testing, the notice states.

Carcasses will be held by slaughter facilities pending results. Testing will not require additional holding time for carcasses beyond current NRP protocols, according to the notice.

Carcasses that test positive for H5N1 will not be introduced into the food supply, the FSIS notice states. Instead, they will be purchased by the USDA for research purposes. APHIS may also conduct additional traceback activities to gather epidemiological data and prevent the spread of H5N1.

“AABP is supportive of any efforts that help us identify the scope of this disease outbreak,” said Dr. Fred Gingrich, executive director of the American Association of Bovine Practitioners.

“The other thing about that plan,” Dr. Gingrich continued, “is it will identify animals that are infected at the time of slaughter. That’s important because it helps our industry maintain consumer confidence, especially since any of those animals that are being sampled will not go into the commercial meat supply, even though FSIS has already determined cooking beef appropriately poses no risk to the consuming public.”

California cases growing
Federal and state governments have already implemented various measures to prevent the spread of the virus, including import restrictions and a negative H5N1 test for out-of-state transport.

However, the virus continues to spread, most recently in California, which is the nation’s top milk-producing state with more than 1,100 dairy farms housing 1.72 million milk cows.

Cattle at three dairies located in the state’s Central Valley began showing clinical signs consistent with HPAI beginning August 23-25, according to a statement from the California Department of Food and Agriculture (CDFA).

Following an investigation by the CDFA and rapid disease detection by California Animal Health and Food Safety Laboratory System (CAHFS), the U.S. Department of Agriculture’s National Veterinary Services Laboratories (NVSL) confirmed HPAI in these three dairies. Since then, a total of 34 affected dairies in Central California have also had confirmed cases. Seven of the newer HPAI-confirmed dairies were in a group targeted for testing because of elevated risks from their recent connections with the initial affected premises.

“We have been preparing for this eventuality since earlier this year, when HPAI detections were confirmed at dairy farms in other states,” CDFA Secretary Karen Ross said in a statement. “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.”

As of September 20, H5N1 outbreaks have previously been confirmed in dairy cattle in 13 other states: 64 herds in Colorado, 31 in Idaho, 29 in Michigan, 26 in Texas, 13 in Iowa, nine each in Minnesota and New Mexico, seven in South Dakota, six in California, four in Kansas, two in Oklahoma, and one each in North Carolina, Ohio, and Wyoming. Those figures likely don’t represent a fully accurate total, as public health and veterinarians have acknowledged that producers are reluctant to report cases for fear of disrupting operations.

That’s despite the USDA’s Farm Service Agency (FSA) expanding its Emergency Assistance for Livestock, Honeybees, and Farm-raised Fish Program (ELAP) this summer to assist with a portion of financial losses resulting from reduced milk production when cattle are removed from commercial milking in dairy herds having a confirmed positive H5N1 test.

Lindsey Holstrom, national incident coordinator at USDA APHIS’s Veterinary Services, spoke at a panel discussion on HPAI during AVMA Convention 2024 this past June in Austin, Texas. She said biosecurity is key to mitigate the risk of disease.

Research by the agency found that 50% to 85% of affected premises had frequent visitors who had contact with cattle, 51% moved cattle using trucks or tailers shared with other farms, and 27% had received cattle within 30 days of clinical signs.

“What we have learned from poultry is you can have spread of indirect contacts. There’s not one thing we can change for how this virus spreads,” she said. “No one anticipated this would jump to dairy. We’re still learning about transmission and virology and pathogenesis in the cows themselves. … There’s still a lot we’re trying to learn and how to eliminate from dairy cattle. We’re trying to help farmers manage the disease on their premises.”

Vaccine testing
In related news, the USDA’s Center for Veterinary Biologics (CVB) is beginning field trials to determine the safety and efficacy of the first ever vaccine for H5N1 in cattle.

Before the USDA notice, the only way companies could test a vaccine for H5N1 was in biosafety level-3 laboratories.

“Field studies to support licensure of nonviable, non-replicating vaccines may be conducted outside of containment without terminal disposal of milk and other commodities,” the August 28 notice states, allowing companies that are developing H5N1 vaccines to perform studies quicker, and for USDA to get a vaccine approved sooner.

According to a June 12 report by Reuters, at least 24 companies were working to develop an avian flu vaccine for cattle—including Zoetis, which started developing a vaccine for dairy cattle in spring, and Merck Animal Health.

15th human case of avian influenza confirmed
Health officials are trying to determine how a human who had no connection to dairy or poultry farming became infected with avian influenza (HPAI, more specifically avian influenza type A H5N1). So far, 15 human cases have been reported since 2022.

The Centers for Disease Control and Prevention (CDC) confirmed on September 6 that a human case of H5N1 occurred in Missouri. The case was identified through that state’s seasonal flu surveillance system. An investigation into the potential exposure is ongoing by the Missouri Department of Health and Senior Services (DHSS), according to the CDC.

Missouri DHSS reported that the patient, who was hospitalized, had underlying medical conditions, was treated with influenza antiviral medications, and has recovered.

Of the 15 human cases reported in the U.S., 10 have been connected to exposure to poultry and four connected to exposure to dairy cows. This is the first case without a known occupational exposure to sick or infected animals. H5N1 outbreaks in dairy cattle have not been reported in Missouri but have been reported in commercial and backyard poultry flocks this year.

The agency still maintains that the risk to the general public from H5N1 remains low but says that, as always, circumstances may change quickly as more information becomes available.

“The results of this investigation will be particularly important in light of the current lack of an obvious animal exposure. It is important to note that, while rare, there have been novel influenza A cases where an animal source cannot be identified. The main concern in these situations is that no onward transmission is occurring. Findings from the ongoing investigation will inform whether guidance changes are needed,” the CDC announcement stated.