Vanishing Vultures: Bird deaths linked to vet-drug residues

The recent puzzling crash in vulture populations in Pakistan turns out not to be some new bird plague, as conservationists had first suspected. Instead, birds eating livestock carcasses are dying in response to consuming a veterinary drug, says an international research team.

AT RISK. Oriental white-backed vultures make short work of an animal carcass. M. Virani

Three species of vultures–oriental white-backed, slender-billed, and long-billed–have been dwindling in Pakistan and India since the early 1990s, says veterinarian J. Lindsay Oaks of Washington State University in Pullman. He and his colleagues eventually homed in on diclofenac, a veterinary drug widely used to treat ailing livestock, the researchers report in an upcoming issue of Nature.

It’s the first time a common therapeutic drug has been documented to cause an ecological disaster, says Oaks.

Without swift intervention, “all three [vulture] species will likely become extinct in the wild within 5 years,” says coauthor Rick Watson of the Peregrine Fund in Boise, Idaho, one of the sponsors of the study.

Vultures play an important part in curbing ominous diseases such as anthrax and foot-and-mouth because the birds dispose of animal carcasses quickly, says Watson.

Ornithologist André Dhondt at the Cornell Laboratory of Ornithology in Ithaca, N.Y., says that consequences of vulture loss are rippling through the ecosystem. Already, he says, without vultures competing for carcasses, foxes have boomed, and the incidence of rabies has increased.

The vulture decline also brings cultural change to people, says Oaks. Farmers who traditionally leave dead animals to the vultures have to develop new practices. Members of the Parsi group are struggling to modify their ancient practice of so-called sky burials, setting out human corpses for the vultures.

Eighty-five percent of dead vultures that the team studied had chalky deposits on their internal organs. This condition, called visceral gout, signals kidney failure, says Oaks.

The researchers checked for infectious diseases but concluded that the condition of the dead birds suggested that they had been poisoned. Testing for cadmium and other known threats to bird kidneys yielded nothing. Then, researchers surveyed local livestock drugs for something that destroys kidneys when given orally to a bird. Diclofenac stood out, says Oaks.

The team soon found the drug in tissue from dead vultures. Feeding dead, drug-dosed mammals to captive vultures produced kidney failure and the same type of internal deposits seen in dead birds in the wild. Drug residues in natural waterways wouldn’t produce fatal doses, Oaks says.

Diclofenac never caught on for veterinary use in North America, says Oaks, although people take it here as an anti-inflammatory drug.

Ornithologist Keith Bildstein of Hawk Mountain Sanctuary Acopian Center in Orwigsburg, Pa., says, “Smoking guns are an all-too-rare commodity in conservation science, but Lindsay Oaks and his colleagues appear to have uncovered one.”


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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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