By Maci Guay
The detection of highly pathogenic avian influenza, HPAI, subtype H5N1 in dairy cattle was confirmed in March 2024 by the U.S. Department of Agriculture, USDA. This finding was the first of its kind and shocked the global scientific community.
The official confirmation came on the heels of testing performed at the Texas A&M Veterinary Medical Diagnostic Laboratory’s Canyon and College Station laboratories.
Following reports from local dairy practitioners and hundreds of tests to rule out other potential pathogens, the Canyon lab tested a nasal swab sample from dairy cows.
The positive result was the first indication that HPAI had crossed species, an unexpected discovery as cows were not considered at high risk for infection with this particular virus strain.
Ongoing efforts to answer questions
After a year of ongoing surveillance and diagnostic services, TVMDL continues to safeguard animal and human health as experts proceed with finding answers to the many questions related to HPAI in nonavian species.
To date, 17 states have confirmed cases of avian influenza in cattle, and the viral infection has been detected in at least 989 dairy herds.
Since its discovery in 2024, TVMDL has conducted over 12,000 avian influenza tests and continues to help monitor this threat through state-of-the-art diagnostic services.
“TVMDL runs these tests for the sake of the animals, but also because we care about all the people who interact with those animals,” said Alexis Thompson, DVM, Ph.D., DACVPM, resident director for the TVMDL laboratory in Canyon.
According to the Centers for Disease Control and Prevention, CDC, pasteurization kills the virus in milk, so commercial dairy products are safe for human consumption. However, certain HPAI virus strains are zoonotic, meaning they can be transmitted from animals to humans. Individuals who work directly with potentially infected animals may be at higher risk of infection. To date, there have been 70 cases of dairy and poultry workers infected due to the current outbreak.
“We do it for the dairy workers, livestock owners, veterinarians, people working in wildlife sectors and even shelters,” Thompson said. “All of them are at a higher risk of infection, so it’s our duty to let people know where this virus is.”
Background of avian influenza
Avian influenza is a contagious viral disease caused by influenza type A virus infections in bird species. Infected birds can shed the virus through their mucous, saliva or feces. Humans and animals can be infected with the virus by direct contact with an infected animal, touching contaminated surfaces or airborne transmission.
The current avian influenza outbreak began in early 2022 and for over two years, largely remained in avian species. Although most viral outbreaks tend to wane over time, this outbreak has spilled over into approximately 30 mammalian species across the U.S.
Despite the virus infecting different species over its two-year run, the spillover into dairy cattle in the Texas Panhandle took experts by surprise.
“We have known that birds have been a host for avian influenza for decades,” Thompson said. “I was shocked by the positive tests in cattle because they are known to get influenza type D but not influenza type A.”
The discovery of HPAI in dairy cattle
In late February 2024, TVMDL began to receive notices of sick cattle with reduced milk production and fever, among other varying clinical signs, on dairy farms across the Texas Panhandle. Working alongside dairy practitioners, TVMDL compiled a battery of testing options to determine the cause of illness.
“In response to all of this, our lab began testing a plethora of samples to rule out possible etiologies,” Thompson said. “We ruled out lots of common diseases in dairy cows, which helped the veterinarians and other laboratories working on this unusual event narrow down possible causes.”
Although the ongoing negative test results were insightful, diagnostic laboratories did not have a specific suspected disease to investigate.
The case finally broke following reports of unexplained morbidity events of several wild birds and cats from the same dairies.
Connecting cat and bird hosts to dairy cattle
TVMDL received the first reports of deceased birds on March 15, 2024, and cat specimens began arriving three days later.
Kiril Dimitrov, DVM, Ph.D., TVMDL assistant agency director for microbiology and research, oversees avian influenza testing at both the Canyon and College Station lab locations.
“Positive results for avian influenza in cats were not surprising,” Dimitrov said. “For more than 25 years, we have known that cats are susceptible to the virus. It was the big numbers that were surprising.”
Veterinarians on dairies communicated their findings with the diagnostic laboratory, including that one dairy lost 20 cats in 24 hours. Based on this information, veterinarians began to submit samples from cats for testing. Testing not only revealed the cats as positive for HPAI but that they were carrying significant viral loads.
Questioning a possible connection between the dead birds, cats and the sick cows, Thompson threw avian influenza into the list of potential pathogens to investigate.
From March 19-22, 2024, multiple samples from cats tested positive for HPAI. On March 22, 2024, TVMDL tested a nasal swab sample from a sick cow living on the same dairy as the cats infected with avian influenza. Remarkably, the swab tested positive for HPAI subtype H5N1.
“The current hypothesis is that the disease jumped from birds to cattle, and then from the cattle to the cats,” Thompson said. “There is always some worry when a virus jumps from one species to another because the bridge becomes smaller at each new host.”
What happens when the pathogen jumps species?
Initially, Thompson feared that dairies would face a similar fate as the poultry industry when highly pathogenic avian influenza was first discovered particularly, the mass depopulation on affected operations and the deterioration of the dairy industry, which employs more than 3 million Americans.
“I was thinking about the future and how this pathogen would affect this large community,” Thompson said. “This is a pathogen jumping into a new species. We had a lot of unknowns at the time, and we still do.”
The week of March 25, 2024, when the USDA announced the confirmation of HPAI type H5N1 in dairy cattle, TVMDL tested 138 samples from 12 Texas, New Mexico and Kansas dairy farms. HPAI-positive samples were detected in dairy cattle on every farm with submitted samples.
Now, after a year of nationwide testing, experts have increased their knowledge of the outbreak. For example, affected herds tend to have a low mortality rate despite the concerns of clinical signs associated with the disease.
Protecting the dairy industry from avian influenza
Since the initial discovery of HPAI in dairy cattle, testing at TVMDL and other diagnostic labs has paved the way for the establishment of multiple federal orders and programs aimed at protecting the dairy industry:
- In April 2024, the USDA announced a federal order for all lactating dairy cows to be tested for HPAI before being transported across state lines, hoping to limit the spread of the contagious disease.
- In late May 2024, USDA launched the Dairy Herd Status Program, a voluntary program for dairy producers to monitor herd health and promote transportation efficiency.
- In December, the USDA announced the National Milk Testing Strategy, a federal order requiring the testing of raw milk at the state level to work toward freedom from disease.
As a member of the USDA’s National Animal Health Laboratory Network, TVMDL has provided testing for federal orders and routine surveillance testing for concerned dairies.
Source : tamu.edu