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.

"It almost seems like a pandemic unfolding in slow motion," said Scott Hensley, a professor of microbiology at the University of Pennsylvania. "Right now, the threat is pretty low ... but that could change in a heartbeat."

The earlier the warning of a jump to humans, the sooner global health officials can take steps to protect people by launching vaccine development, wide-scale testing and containment measures.