New Delhi, Feb 14 (IANS): H5N1 bird flu has silently spread from animals to some humans who treat animals, according to a study, by the US Centers for Disease Control and Prevention (CDC) that suggests bird flu cases may be undercounted.

The study showed that veterinarians didn’t have any symptoms, and thus did not seek medical care, unlike the infected poultry workers. The findings, come as the US is battling bird flu, and reported about 68 human bird flu infections last year.

The new study is more evidence that the number “is likely a significant undercount”, said Dr. Gregory Gray, an infectious disease researcher at the University of Texas Medical Branch in Galveston.

"This means that people are being infected, likely due to their occupational exposures, and not developing signs of illness and therefore not seeking medical care," Gray added.

The researchers said tracking medical clinics that report bird flu cases may not be enough to fully understand bird flu transmission.

For the study, the team examined blood tests of 150 vets from 46 states in the US. While none of them suffered red eyes or other symptoms common with bird flu, the tests revealed about 3 or 2 per cent of the vets had antibodies of H5N1 infection.

Although the three worked with dairy cattle, as well as other animals, none were infected. One reported working with a flock of infected poultry. Previous studies suggest some dairy farmworkers experienced symptoms but were never properly diagnosed.

Being small-scale studies, they could not give a proper estimate of undiagnosed human infections.

But these can translate to hundreds or thousands of infections, Gray said Currently, there is nothing to be alarmed about, but changes or mutations in the virus can potentially make people very sick, or boost the spread of the H5N1 virus, said Jacqueline Nolting, an Ohio State University researcher.