Another spillover of the H5N1 bird flu virus from wild birds to dairy cattle appears to have occurred, this time in Arizona.

A version of the avian influenza virus different from the one rampaging through herds since last spring has been detected in dairy cows in Nevada, USDA says.

Three of America’s top milk-producing states are not participating in the U.S. Department of Agriculture’s program designed to catch the spread of the virus.

The finding indicates that the virus, known as H5N1, has spilled from birds into cows at least twice — leading to these two sets of infections — and that it could continue to do so. It also suggests that the virus may pose a persistent risk to cows and to the people who work closely with them.

Three U.S. dairy veterinarians unknowingly had H5N1 infections, otherwise known as bird flu. This included one person who worked only in states without known outbreaks in cows, according to a new study from the U.

Bird flu has been detected in Arizona dairy cattle milk, and a dairy farm has been placed under quarantine as a precaution, according to officials.

Arizona agricultural officials say they now have the first detection of H5N1 avian influenza in milk produced by a dairy herd within the state.