Fifteen farms across six US states are battling bird flu outbreaks — amid growing concerns the virus could spread to people.

Texas, Kansas, New Mexico, Michigan, Idaho and Ohio have all reported the virus in herds, raising concerns over the milk supply.

At the same time, America's largest egg producer — in Texas — has also had to cull nearly 2million birds after detecting the disease in its flock, which could drive up the price of eggs.

The first case in cattle was reported on March 25 in Texas, with cases then reported in herds across the country as cattle were shipped from the state.

A Texas farm worker has tested positive for the virus, and three cats in the state are reported to have died from the disease.

Health agencies say the risk to the public is still low, but farm workers in close contact with infected animals and equipment are thought to be at higher risk.

Scientists are not certain how the virus is spreading between cattle, but experts suggest it may be via infected milk droplets on dairy workers clothing or gloves or on the suction cups from milking machines.

Tests so far have only picked up the virus in the cow's milk, while samples from the nose and blood of infected animals have tested negative.

'We haven't seen any true indication that the cows are actively shedding virus and exposing it directly to other animals,' said the agency's Dr Mark Lyons at a briefing yesterday, reports Science.

Cats are thought to have been infected after consuming infected milk from cows.

Texas — America's biggest cattle state — is the most affected by the outbreaks, with seven farms reported to have cows infected with bird flu.

Three farms in Kansas are also reporting infected cattle, as well as two in New Mexico and one each in Michigan, Idaho and Ohio.

Many of these have been linked to Texas, having recently received cattle from infected farms in the state. It is not uncommon at this time of year for dairy herds to be sent to more northern areas.

Diseased cows are reported to be eating less and producing less milk at present, although they are not dying from the disease. 

Experts are concerned by infections in mammals, because each one gives the virus an opportunity to gain mutations making it better able to infect mammals and spread between them.

Some have also raised concerns over the infection in a human, saying it is so mild that it risks other human cases being missed. 

Dr Angela Rasmussen, a virologist at the University of Saskatchewan, said: 'Particularly worrisome is the fact that this human case was limited to mild conjunctivitis.

'That's good for the person who got H5N1. But it's not good for preventing spillover. Mild cases may not be recognized and isolated.'

Officials say they first became concerned bird flu may have spread to cattle in Mid-February after dead domestic birds and cats were spotted at farms.

This led to cattle being tested for the virus, which ultimately revealed the virus.

The human case was then also revealed after they came in for treatment for an eye infection.

The CDC says it is taking the bird flu case in the US 'very seriously' amid growing fears the virus could jump to humans and cause an epidemic.

Mandy Cohen, the agency's director, told NPR that they were 'monitoring the situation very closely'.

She added: 'That just means more opportunity for this virus to mutate and change. And that's what we want to make sure we are continuing to stay ahead of.'

Experts are concerned by bird flu because of the apparent ease with which it is infecting a wide range of mammals — now being detected in more than 40 different species.

They worry that the more infections there are in mammals, the higher the risk of the virus gaining mutations allowing it to infect and spread between them.