Veterinary epidemiologist Jason Lombard, DVM, Colorado State University, recently provided a review of what’s known, and unknown, about H5N1 in dairy cattle.

“This outbreak has been in the U.S. for about three years,” said Lombard. “Multiple genotypes of influenza A have been circulating in livestock and poultry sectors, and most of the reservoir for this is wild migratory birds.”

The first detection of the current H5N1 outbreak was in February 2022 when USDA confirmed disease presence in a commercial turkey flock in Indiana. Multiple outbreaks also occurred in backyard poultry flocks. All 50 states have reported cases of H5N1 in poultry, with 174.8 million birds affected. Most of those birds were depopulated.

“In early 2024, we had an uncharacterized cattle morbidity event in the Southwest U.S. with reports of large bird die-offs and farm cats dying with neurologic signs,” said Lombard. “In March 2024, USDA confirmed H5N1 in goats on a farm with infected poultry in Minnesota; later that month, USDA confirmed H5N1 in dairy cattle from Texas and Kansas. That was thought to be a single-time spillover from wild migratory birds into dairy cattle.”

In April 2024, Texas reported the first human case in a dairy worker. USDA enforced the Federal Order pertaining to H5N1 in cattle, which required testing of lactating cows for interstate movement. The order also required labs to report positive test results to USDA and state and federal authorities to better understand H5N1 spread.

In May 2024, USDA confirmed H5N1 in alpacas on a farm that had infected poultry. That month, FDA confirmed that H5N1 in dairy products is inactivated by current pasteurization methods. In October, USDA confirmed H5N1 on an Oregon pig farm with infected poultry.

Other species that have tested positive for H5N1 including wild cats, domestic cats, marine mammals, raccoons, rodents, skunks and wild canines. “The one that stands out is domestic cats,” Lombard said. “We heard early on that cats were dying even before farms saw clinical signs in cows.”

How a virus that affects poultry and wildfowl jumps to cattle and other mammals remains unknown.

“When this first started … my understanding is that some dairies were green chopping at the time,” said Lombard. “Someone said ‘Those cutters aren’t going to pick up a dead bird from the ground.’ My gut says that’s one potential route. There’s enough vacuum produced from green chopping that you could suck some dead birds in and feed it directly to cattle if oral exposure would lead to disease.”

Milk has many receptors for influenza, and the mammary gland is a metabolically active gland that produces a lot of virus. “It seems like it found the perfect host to propagate in milk,” he said. “The question is ‘How is it moving from cow to cow via milk?’ I think a percentage of cows are infected through the milking process. But when studies try to replicate that, they haven’t been able to move it from cow to cow with a contaminated milking machine. They’ve dipped teats in contaminated milk and couldn’t get disease to spread.”

Researchers found out quickly that milk tends to have the highest viral load. The Federal Order requiring testing of lactating cows was meant to stop the spread of disease from states that were already impacted. That strategy worked well.

The National Milk Testing Strategy is silo or bulk tank testing to make sure H5N1 isn’t circulating in herds. Forty-five states are enrolled.

Cattle with clinical signs had fevers of 4º to 5º F above normal for several days followed by a sharp decrease in rumination, water and feed intake and activity. One confusing aspect was variation in clinical signs among farms. Some cows completely dried off, although some of those dry-offs may have been management decisions based on pregnancy status. Some cows were euthanized or died; some herds had numerous abortions. In general, infected cows did not return to normal production.

There are known risks for spreading the virus in lactating cattle, and people are also a risk. “One of the first to be infected with a bovine strain was a poultry facility in Michigan,” said Lombard.  “People from the poultry facility were working on the dairy on weekends, and we presumed they were probably carrying the virus back to the poultry facility.”

The virus appears in cows’ urine occasionally. Some poultry farms that had dairy manure spread on adjacent fields may have become infected from the manure.

“Other than lactating cattle, which are a proven risk, there are other risks that are low but still spread disease,” said Lombard. “That makes it tough to come up with biosecurity practices. If you can help it, don’t move any cattle, especially lactating cattle. All the normal biosecurity practices are good, but I don’t know if they’re going to stop the spread of H5N1.”

Cattle exposure to H5N1 can occur through eyes, nasal passages/respiratory systems, fly bites, rectally/vaginally, orally (feed and water), feet, injections and teats/udder.

“This disease is spread in the milking parlor,” said Lombard. “It’s spread like other contagious mastitis pathogens.” Liner squawks and reverse jetting of milk into quarters from the previous cow can spread H5N1 if milking personnel aren’t doing a good job post-milking.

Many influenza viruses are transmitted by aerosol, but cattle don’t have as many influenza receptors in their respiratory systems as other animals. However, in California, the virus was detected in the air of milking parlors and in the exhalations of cows. Infectious H5N1 virus was also detected in air and wastewater streams.

Lombard said people become frustrated when they’re told to do a lot of biosecurity practices and those practices don’t work. This is why he and others believe aerosol is a potential route of transmission.

For most herds, bulk tank testing is negative about 30 to 45 days after initial clinical signs. Most farmers don’t know when their bulk tank went positive, but they know when they first saw clinical signs. Some herds are out up to 90 days and still have positive bulk tanks. Lombard knows of one herd that is still testing positive after one year, which he attributes to naïve animals coming into the herd and chronically infected cattle that continue to shed.

An H5N1 vaccine is on the horizon. Lombard said the efficacy is good but the regulatory aspect is likely holding up the process.

“We may never figure out exactly how this moves from farm to farm,” Lombard admitted. “Once it gets into an area, it seems to move fairly easily. Last I saw from USDA, almost 50% of infected farms didn’t have any cattle movement. It’s moving in other ways and trying to figure out what those are has been a challenge.”