White-backed vulture numbers have fallen by 95% in India. Credit: © GettyImages

A massive vulture die-off in India may be caused by a veterinary drug present in cattle flesh, hints a new study.

For over 200 years, the vultures disposed of the dead at the Towers of Silence, a Parsi burial site that sits atop Malabar Hill in Mumbai, India. But in the last decade, the population has plummeted by more than 95 percent, boosting the population of rabies-riddled feral dogs.

Now bird virologist J. Lindsay Oaks of Washington State University in Pullman is proposing that an anti-inflammatory drug called diclofenac may be poisoning the vultures. Vets in India use the painkiller in cattle. By eating cattle carcasses, birds might be building up toxic levels of the drug.

In a survey of shrinking vulture colonies in Pakistan, Oaks and his colleagues from The Peregrine Fund found that vultures had died of kidney failure, which could be caused by diclofenac poisoning, and that their tissues contained the drug. Birds that had died of other causes did not test positive for diclofenac, Oaks told the 6th World Conference on Birds of Prey and Owls last month in Budapest, Hungary.

But wildlife epidemiologist Andrew Cunningham of the Zoological Society of London's Institute of Zoology is not convinced that the mystery has been solved. He and his colleagues have built a vulture care centre in Haryana state in India, where they are also searching for the cause of the birds' death.

Cunningham argues that birds on the Indian subcontinent may be suffering from something different to those in Pakistan. "The signs point to it being an infectious agent".

Indian vultures are sick for three to five weeks before they die, and have inflammation in their nervous system, a mark of infection. In Pakistan, birds die quickly and their organs are covered with a chalky white paste of uric acid, characteristic of renal gout.

"There may be a combination of things going on," says Debbie Pain, head of international research at the UK's Royal Society for the Protection of Birds in Bedfordshire. She too says that an infectious agent cannot yet be ruled out.

Cunningham's team is surveying veterinarians, farmers and villagers to understand how diclofenac is used, and analysing cattle tissues for concentrations of the drug. "We are trying to prove or disprove the involvement of this drug on a scientific basis," he says.