Agricultural workers have discovered bird flu in more than three dozen California cow herds in the past month, according to a report issued Sept. 20 by the U.S. Department of Agriculture (USDA).

California is one of 14 states to confirm finding the virus in dairy cows since March. The state reported cases found in 40 herds. In the last 30 days, Idaho and Michigan have also reported finding the virus in a single cow herd each, according to the USDA.

The highly pathogenic avian influenza, or bird flu, is a highly contagious virus that often kills poultry, according to the Food and Drug Administration (FDA).

Though the virus usually only kills about 2 percent of the cows infected, farms in several states this year have culled hundreds of dairy cows after they failed to return to milk production, the USDA said.

The USDA has worked with state officials to detect the virus in poultry since February 2022. In March, the agency received its first report of the infection in a dairy cow.

Since then, the USDA has worked to assess the prevalence of the virus in U.S. dairy herds, recording a total of 238 cases across 14 states: California, Texas, Idaho, Michigan, New Mexico, Colorado, South Dakota, Minnesota, Oklahoma, Iowa, Wyoming, North Carolina, Ohio, and Kansas.

Once infected, dairy cows usually lose their appetites and produce less milk, and their milk is thickened and discolored, but most of the cows recover, according to the American Veterinary Medical Association.

Cows and other mammals can be infected with bird flu viruses when they eat infected birds and poultry, or are exposed to environments contaminated with the virus, according to the Centers for Disease Control and Prevention (CDC).

“Spread of [bird flu] viruses from mammal to mammal is thought to be rare, but possible,” the CDC reported this year.

The CDC confirmed one human infection on April 1 in a person who was exposed to dairy cows in Texas. This was the first instance of likely mammal to human spread of the virus.

In May, the CDC began reporting other cases of humans who had been exposed to infected dairy cows.

Cows at a California dairy. The risk of dairy cows spreading bird flu to humans is low, according to the Centers for Disease Control and Prevention. (Rich Pedroncelli/AP Photo)

The CDC said the current risk to the public of bird flu viruses remains low, but people who are exposed to infected animals or birds through work or recreation run a greater risk of becoming sick.

At least 35 cats have tested positive for the bird flu virus in states where the virus has also been found in dairy cattle, according to the American Veterinary Medical Association. The illness in cats can be severe and have a high mortality rate.

Reuters contributed to this report.