Dairy cows infected with bird flu in five U.S. states have died or been slaughtered by farmers because they did not recover, state officials and academics told Reuters.

The reports of these deaths suggest that the outbreak of bird flu in cows may have a more serious economic impact on the livestock industry than experts initially thought. Farmers have been slaughtering poultry infected with the virus for years, but the cost of rearing cows is much higher compared to chickens or turkeys.

A spokesman for the Agriculture Ministry said his agency was aware of a few deaths but that most cows were recovering. Reuters was unable to determine the total number of cows that died or were slaughtered in South Dakota, Michigan, Texas, Ohio and Colorado.

Avian influenza has infected cows in more than 80 herds in 10 states since late March. Some animals have died from secondary infections because the disease weakened their immune systems. Other cows were killed by breeders because they did not recover from the virus.

The infected cows produce less milk, have digestive problems, fever and reduced appetite, according to farmers and veterinarians.

In South Dakota, a 1,700-cow farm sent about 10 animals to the slaughterhouse because they didn’t recover and killed as many more who contracted secondary infections, said veterinarian Russ Daly, a professor at State University.

Another farm in Michigan killed about 10 percent of 200 infected cows because they also showed no signs of recovery, said Phil Durst, a professor at the University of Michigan. That state has also recorded two of the three confirmed cases of avian influenza in farm workers.

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In Colorado, some farms have killed cows because after contracting the disease they stopped producing milk, said Olga Roback, a spokeswoman for the local Department of Agriculture.

In Ohio, a spokeswoman for the department said cows died mostly because of secondary infections.

Cows have also died or been killed in Texas and New Mexico.