Bangkok - A second drug used to treat cattle for pain could be deadly to endangered vultures and should be prohibited as part of a campaign to prevent their extinction, according to a study released Wednesday.

Millions of long-billed, slender-billed and oriental white-backed vultures have died in South Asia - mostly in India - after eating cattle carcasses tainted with diclofenac, an anti-inflammatory painkiller given to sick cows.

Now, researchers writing in the Royal Society journal Biology Letters have found that a second drug, ketoprofen, has proven toxic to vultures and should no longer be used to treat livestock in Asia.

"Surveys of livestock carcasses in India indicate that toxic levels of residual ketoprofen are already present in vulture food supplies," the researchers wrote. "Consequently, we strongly recommend that ketoprofen is not used for veterinary treatment of livestock in Asia and in regions of the world where vultures access livestock carcasses."

Tens of millions of vultures played a key role in South Asian ecosystems before the introduction of diclofenac in the early 1990s. Now, populations of the three species of the critically endangered, griffon-type vultures are thought to have dropped by as much as 97 percent, according to Britain's Royal Society for the Protection of Birds.

Vultures play a vital role in disposing of carcasses, keeping down populations of stray dogs and rats that also feed on dead cattle and can spread disease among humans.

"From millions of individuals in the 1980s, vultures have simply disappeared from large swaths of India, Pakistan and Nepal, and at least three species have been brought to the brink of extinction," said Richard Cuthbert, one of the study's authors and a scientist with the Royal Society.

"The rate of decline of these magnificent birds is staggering," he said. "For the oriental white-backed vultures, for every two birds alive last year, one will now be dead, and this is all because of the birds' inability to cope with these drugs in livestock carcasses, the birds' principal food source."

The swift demise of the vultures prompted authorities in India to ban diclofenac more than two years ago. Conservationists are hoping authorities take similar action with regards to ketoprofen and require farmers to use only meloxicam, which has proven safe.

"We would like to see other safe alternatives, but it should be the responsibility of the Indian pharmaceutical industry to test these to determine their safety to vultures," said Vibhu Prakash, director of the vulture program of the Bombay Natural History Society in India.