Experts have warned that human transmission of bird flu may be far more widespread than thought, as farmers in Texas and Wisconsin are reported to have symptoms of the virus but are avoiding testing.

Dr Barb Petersen, a dairy veterinarian in Amarillo, Texas, explained that workers at a local farm where cattle have tested positive for the virus are suffering tell-tale symptoms. 

She said: 'People had some classic flu-like symptoms, including high fever, sweating at night, chills, lower back pain,' as well as upset stomach, vomiting and diarrhea.

They also tended to have 'pretty severe conjunctivitis and swelling of their eyelids'.

Meanwhile, veterinary researchers in Wisconsin — where the virus has infected cows — have reported multiple cases of local farmers suffering bird flu-like symptoms. 

But farmers are notoriously reluctant to seek medical help, meaning 'a lot of cases  are not documented', according to Dr Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory. 

To date only one person has tested positive for the virus — a farmer in Texas who suffered from eye inflammation.

But the CDC says at least 44 others are under monitoring for potential infections with the bird flu virus H5N1.  

It comes as the USDA announces plans to test samples of beef taken from slaughterhouses for cattle that aren't used for human consumption.

They will also be running studies on whether cooking beef to the recommended internal temperature of 145F (62C) kills the bird flu virus.

No tests have so far shown bird flu inside animal tissues, but last week the USDA said they had detected the virus in cattle's lungs.

Testing on grocery store milk carried out last week by officials suggested one in five of the products contain the virus.

Officials insisted this did not pose a risk to humans, however, saying pasteurization had 'deactivated' the virus.

A total of 34 dairy herds across nine states — with many in Texas — have tested positive for the disease so far.

Dr Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, told NBC News that he had heard reports of people with the infection.

He added that farmers are not cooperating with demands to test in part because of their long hours and because of fears they may be asked to cull their herds — as poultry farmers are to their flocks.

It comes amid concerns that eight people in India may also be infected with bird flu.

The two doctors and six workers at a poultry farm were all exposed to birds that had H5N1, officials said, and are being tested for the virus.

The local administrative office in Ranchi, east India, has also opened its own 'bird flu ward' to hold those in quarantine.

Nearly 2,000 chickens in the area have also been culled after the infections were confirmed.