While highly pathogenic avian influenza H5N1 has been confirmed in 923 dairy herds in 16 states, California has been hit the hardest.

As of Jan. 9, infections have been confirmed in 706 of the state’s dairy herds, and California accounted for 149 of the 151 new herd detections in the last 30 days, according to USDA’s Animal and Plant Health Inspection Service.

“It’s slowing down, at least the rate of infection. I’m kind of holding my breath to make sure that’s not just a holiday testing delay,” said Anja Raudabaugh, executive director of Western United Dairies, which represents more than 75% of the milk produced in the state.

She’s hopeful some of the new biosecurity measures being rolled out by California Department of Food and Agriculture will prevent further dairies from getting infected — especially those on the North Coast, where no positive cases have been detected.

Early success

The virus was first detected in dairy cows in late March in Texas, steadily spreading to several other states in the spring. It wasn’t detected in California until Aug. 29.

“California locked down significantly on the animal movement side in March of 2024 as the virus started to spread out of Texas. So I do attribute our limited animal movement and our quarantines from out-of-state infected dairies for being the initial reason for the success,” Raudabaugh said.

But there was a breach in biosecurity when a small number of asymptomatic animals thought to be in recovery were shipped into California from Idaho in mid-August.

Significant heat waves in October added to the flu’s impact, resulting in a significant spike in cow mortality.

Heat relief

“Since then, it’s been cooler. We’ve been able to deal with the virus a little bit more … it has not been as stressful. The cows are getting sick, I don’t want to minimize it, it’s not a fun thing to go through,” Raudabaugh said.

The quarantine procedure involves additional restrictions and paperwork that are very stressful. There are quite a few additional steps on dairies that are being under quarantine or surveillance, she said.

But making sure workers are protected continues to be a huge priority — and making sure there’s a lot of education about what bird flu would look like in a human, she said.

With cooler weather, cows are generally responding to hydration treatments a lot more effectively. But there is a concern these animals are not coming back to full milk production. They’re typically coming back at about 70% of pre-flu production, Raudabaugh said.

Increased culling

That’s sending a lot of recovered cows to slaughter, which has been really difficult because it’s very hard to get any kind of replacement animals now, she said.

“The culling was definitely not like part of the business plan, and we did experience last month … almost a 10% drop in milk production, and we know it was due to bird flu,” she said.

That’s been really hard economically on the affected dairies, Raudabaugh said.

It’s difficult to estimate the number of direct mortalities from the flu. Some were due to comorbidities, older or maybe respiratory issues before this and additional culling because production didn’t come back.

“That’s all been kind of dramatic, but when you add in the heat element we had in October … it definitely was unusual. So it’s in the thousands,” she said.

Cow numbers

California traditionally hovered at around 1.7 million dairy cows. That’s now hovering closer to 1.47 million. Raudabaugh doesn’t know how much of the decline can be attributed solely to flu, but culling is higher than normal.

There are 735 dairies — 75% of all dairies in the state — under quarantine, and about 100 have come off of quarantine. Cases of reinfection have not yet been reported in California, but they are being reported in Texas, Idaho and Colorado, she said.

Bird flu in dairy herds

Number of infected herds and date of most recent detection

California — 706, Jan. 7

Colorado 64 — 4, Aug. 13

Idaho — 35, Oct. 22

Michigan — 30, Dec. 30

Texas — 7, Dec. 13

Iowa — 13, July 9

Utah — 13, Nov. 6

Minnesota — 9, July 19

New Mexico — 9, Aug. 28

South Dakota — 7, July 31

Kansas — 4, April 17

Oklahoma — 2, July 11

Wyoming — 1, June 6

Ohio — 1 April 2

Nevada — 1, Dec. 6

North Carolina — 1, April 9