Specialists from the US federal Centers for Disease Control and Prevention (CDC) have discovered a hidden spread of the H5N1 bird flu virus among people. The trend, however, has been masked by a seasonal increase in respiratory infections and a resurgence of tuberculosis. The respective report was posted on the CDC's official website.

According to the report, H5N1 was found in three veterinarians who worked with cattle. In both cases, the source of infection was not clear. Moreover, none of these veterinarians had flu-like symptoms. Also, there was no evidence of human-to-human transmission.

The infections were detected through antibody tests among 150 veterinarians in 46 US states, indicating that the virus is spreading beyond the boundaries of known outbreaks.

Health officials have stressed that many cases of this infection remain undetected.

"If cases are occurring more frequently than detected in humans, we risk missing small changes that allow the virus to begin to spread much more easily in humans," said University of Nebraska infectious disease researcher Lauren Sauer.

The first case of human H5N1 in the US state of Nevada was reported last week, bringing the total known human cases in the US up to 68. Nevada has also just detected a new H5N1 strain in cows, D1.1, that may be better suited to replicating within mammal cells.

Transmission of H5N1 through cow milk has now been experimentally confirmed, which has raised concerns among health officials.

They are urging people to keep themselves and pets away from raw dairy products, avoid feeding pets raw meat, avoid interacting with wildlife, and wear protective gear if handling potentially infected livestock.