How bird flu spreads among dairy cows remains mystery

By Ching Lee 

With cases of highly pathogenic avian influenza in dairy cows rapidly rising in California, animal health officials continue to try to contain the spread, even as they remain flummoxed by how the virus moves among herds.

Based on genetic information of the virus, officials say it appears migratory waterfowl initially introduced H5N1 bird flu to dairy cows in late winter of 2023 in a single, isolated event. But genomic experts say wild birds are not continuing to spread the disease in cattle.

“The truth is that we don’t really know how this virus spreads exactly,” said Deniece Williams, head clinician at the University of California, Davis, School of Veterinary Medicine, during a UC Cooperative Extension webinar last week.

California is the newest of 14 states affected by highly pathogenic avian influenza, or HPAI, in dairy cows, and infections have soared since the first three cases were confirmed on Aug. 30. As of Sept. 30, 43 dairy herds—all in the Central Valley—have been confirmed with H5N1. The Golden State ranks second in the nation—behind Colorado—with the most bird flu cases in dairy cows.

The disease first showed up in Texas dairy cows in March. Researchers determined that lactating cows moved the virus out of the Texas panhandle to other states. But once it got to those states, the virus spread multiple ways, said Jason Lombard, a Colorado State University livestock epidemiologist who has been studying the disease in dairy cattle.

Though scientists have ruled out certain infection sources, such as wild birds, he said, “we still don’t really know for sure” how H5N1 moves from cow to cow, herd to herd and different dairy premises. It remains unclear whether younger cattle and nonlactating cows are involved in disease spread. Scientists also don’t know if reinfection has occurred in affected dairies.

Lombard noted 50% of the affected dairies in Michigan, for example, did not move their cows or introduce new cattle into their herds, but their cows still became infected. While many of the farms did acquire cattle from out of state, he said, some of the cattle came from states that had no reports of H5N1.

As of Sept. 30, the U.S. Department of Agriculture has confirmed 242 total cases of H5N1 in dairy cows nationwide.

“How do we stop this? I think that is still the biggest question to answer,” Lombard told webinar participants.

Since April, there have been 14 human cases of H5N1 reported in the U.S., according to the U.S. Centers for Disease Control and Prevention. Four were associated with exposure to dairy cows, and nine were linked to exposure to poultry. The most recent case, confirmed Sept. 6, had no immediate known animal exposure. Experts maintain that workers who milk cows are highest at risk of being exposed to the virus.

A case in Michigan indicates there has been cow-to-poultry transmission. It is suspected that employees who worked at both a poultry and dairy operation may have spread the disease. That some of the employees also shared housing presents another possible transmission route, Lombard said.

Meanwhile, USDA confirmed on Sept. 18 a new case of H5N1 in a commercial turkey flock in Merced County, with some 64,800 birds affected. The new case represents the first detection of the virus in commercial poultry since the state declared itself HPAI-free on June 28, according to the California Department of Food and Agriculture.

State Veterinarian Annette Jones said lab results show dairy and poultry farms within 6 miles of the turkey flock all tested negative for H5N1, indicating the turkey flock likely was not infected by a local asymptomatic flock or dairy herd. Early investigation also has ruled out obvious links to other poultry flocks and dairy herds through shared equipment or animal movement, she added.

It will take more time to better understand other potential means of spread, Jones said, such as through farm employees, service providers that visit the farm and peridomestic wildlife.

Since the start of the outbreak in February 2022, more than 7.2 million commercial and backyard birds in California have been affected by H5N1, according to USDA. Nationwide, more than 100.7 million birds have been affected, with the disease detected in 1,176 flocks in 48 states. Of those flocks, 509 were commercial poultry.

Cows can shed the virus in different secretions, with milk being the most common. It was initially thought that the virus spread largely through milk and the milking parlor, such as through milking equipment. But Lombard said experts are now less sure about milk being the primary transmission route.

Milk trucks were a prime suspect for spreading the disease, but after sampling 18 of them in Michigan with only one testing positive, they are no longer considered high on the list, he added.

While birds secrete a lot of the virus into aerosol that can move in plumes, scientists have not yet been able to demonstrate the same in cattle. Until they have air sampling results, Lombard said it remains unclear if HPAI is spreading through virus-laden aerosol.

“We really need to figure out how this is moving farm to farm,” he said. “Without that, I think we’re doomed to have this continue to circulate.”

Infections have ranged from 7% to 22% of the herd in some cases. In California, Williams of UC Davis said some dairies have needed to treat up to 50% of the herd.

The disease’s impact on cows can be severe. In one example, a cow that was ruminating about 500 minutes a day had dropped to about 50 minutes. Her milk production dropped from about 100 pounds to less than 20.

After recovery, some cows produced 30% less milk for the remainder of their lactation, Lombard said. One farm shipped 22% less milk after it was hit with the virus and saw a “dramatic” increase in somatic cell count, an indicator of udder health and milk quality.

“I think the big question is, what are these cows going to do when they go through a dry period and come back the next lactation?” he said.

State Veterinarian Jones said the virus has an asymptomatic incubation period of about three to 10 days and can travel in cows on trucks.

Unlike with poultry and backyard birds, CDFA has not disclosed in which counties HPAI-positive dairy farms are located, saying producers should focus their vigilance on the Central Valley in areas where dairies are most concentrated.

“Further, we highly recommend all producers in California take actions to protect their herd regardless of location,” Jones said.

In discussing biosecurity and biocontainment during the UCCE webinar, Williams stressed the importance of creating a separation line around the dairy and limiting traffic to one or two access points. There should also be a vehicle cleaning and disinfection station at the access points.

“We really want to limit access to vehicles on the dairy as much as possible and only let those people that really need to cross that line cross it,” she said. “We have people that walk all over these dairies, and we really need to stop that.”

Dairies should set up a foot bath and/or supply shoe coverings for anybody crossing the separation line. Employees should be provided on-site coveralls and boots so that no outside clothing enters the dairy.

Even though it remains unclear how much the milking parlor plays a role in transmission, producers should milk sick cows last, to help prevent spread through milking machines. Milkers should wear gloves, wash their hands between turns and change gloves between pens.

If possible, Williams said dairies should stop purchasing cattle or buy only animals that have tested negative for the virus. Producers who must purchase cattle should isolate the new animals for 30 days.

For more biosecurity recommendations, go to www.cdfa.ca.gov/AHFSS/Animal_Health/docs/Biosecurity_Protect_Your_Herd_HPAI.pdf.

(Ching Lee is an assistant editor of Ag Alert. She may be contacted at clee@cfbf.com.)