Bird flu likely circulated in U.S. dairy cows on a limited basis for about four months before federal officials confirmed the disease that has now spread to nine states, according to a new federally funded research paper.

A US farmworker who caught bird flu after working with dairy cattle in Texas appears to be the first known case of mammal-to-human transmission of the virus, a new study shows.

Doctors in Texas are describing the only known human case of H5N1 avian flu connected to the ongoing outbreak of the disease in dairy cows. Bird flu in humans remains extremely rare, but in the hundreds of cases documented worldwide over the past few years,

Bird flu, or avian influenza, can be deadly in birds but normally doesn’t infect people. While some strains have caused serious disease and even death in humans, most cause relatively mild symptoms. In the U.

The new hybrid variant of bird flu spreading in American cows is no more dangerous to human health and the public should be “reassured” it is unlikely to impact Britain, the chief vet said. Some experts have called for screening of cattle and milk in the UK for signs of any avian influenza in domestic herds amid the US outbreak,

With little incentive for US farmers to test their cattle, and many undocumented laborers on dairy farms, the full scale of the outbreak is unclear.