When it comes to the potential of H5N1 avian flu, otherwise known as bird flu, picking up mutations that may lead to human-to-human spread, that "train has already left the station," warns one infectious disease expert.

An emerging strain of highly pathogenic avian influenza has spilled over from wild birds to dairy cows in the state of Nevada, and officials at the US Department of Agriculture (USDA) and some scientists are concerned by a mutation they are seeing.

Nevada has confirmed its first human case of bird flu in a farm worker who was exposed to infected dairy cattle, the Central Nevada Health District said on Monday, after authorities last week reported a second strain of the virus in cows in the state.

Nevada's first human case of avian flu was confirmed Monday morning. The person infected is a worker at a dairy farm in Churchill County, the Central Nevada Health District said.