WASHINGTON, July 11 (Reuters) - Turkeys at a farm in Virginia had antibodies to a low-risk strain of bird flu but direct evidence of infection has not been found, the U.S. Department of Agriculture said on Wednesday.

None of the birds became ill but 54,000 were being slaughtered as a precaution, said Dr. John Clifford of USDA's National Veterinary Services Laboratories.

The turkeys had antibodies to a low pathogenic form of the H5N1 avian influenza virus, Clifford said. This strain of H5N1 does not usually make birds ill, although it could potentially change into a more dangerous form if allowed to spread.

"Every indication is that the virus detected is consistent with the North American strain of low pathogenic H5N1, which is not a human health concern," Clifford said in a statement.

"The turkeys showed no signs of illness, and there was no mortality. Thus far, there is no evidence the virus is actually present in the samples collected. The testing detected only antibodies, which indicate possible past exposure to the virus."

Officials around the world are monitoring birds for all forms of avian influenza. The highly pathogenic H5N1 strain is the cause of greatest worry because of its occasional deadly spread to people.

It has been found in 59 countries in Asia, Europe and Africa but has yet to be detected in the Americas. It is deadly to chickens and sometimes infects people.

H5N1 has killed 192 of the 318 people known to have been infected, according to the World Health Organization.

According to the world animal health organization OIE, outbreaks of highly pathogenic avian influenza have been reported in birds in France, Germany, Bangladesh, and Vietnam this month. Malaysia, the Czech Republic and Togo have also been fighting outbreaks.

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