New test results released by the US Food and Drug Administration on Friday found that bird flu virus is making its way from dairy farms and into milk processing plants but also confirmed that the commonly used flash pasteurization method fully neutralizes the virus.

The FDA collected and tested 275 bulk samples of raw milk collected from farms in four states where herds had tested positive for H5N1, or bird flu. The samples were collected between April 18 and 27.

Scientists tracking the spread of bird flu are increasingly concerned that gaps in surveillance may keep them several steps behind a new pandemic, according to Reuters interviews with more than a dozen leading disease experts.

Many of them have been monitoring the new subtype of H5N1 avian flu in migratory birds since 2020. But the spread of the virus to 129 dairy herds in 12 U.S. states, opens new tab signals a change that could bring it closer to becoming transmissible between humans. Infections also have been found in other mammals, from alpacas to house cats.

The U.S. Department of Agriculture said Tuesday that the strain of bird flu currently circulating among dairy cattle appears to be only transmissible via mammary glands, or udders, rather than respiratory droplets, a finding that gives officials greater optimism about managing the risks facing dairy workers.