Long-billed and slender-billed vultures have each fallen by almost 97 percent: 45,000 and 1,000 individuals, respectively, remain in India.
The disappearances may have a direct impact on public health and even social customs.
A 2001 article in the British Medical Journal linked two outbreaks of anthrax in humans to an absence of vultures, which safely scavenge anthrax-infected cattle in a country where facilities for incineration and efficient carcass processing are rare.
In addition the absence of vultures has also been a blow to India's small Parsi ethnic minority.
Parsis, who are prohibited by their religion from burying, burning, or submerging their dead, lay bodies in physical structures, called "Towers of Silence," to be consumed by vultures and other carrion-eating birds.
Without vultures, Parsis have been forced to turn to other alternatives, such as giant solar reflectors, to hasten decomposition.
Banned but Cheap
Efforts to eradicate the use of diclofenac in cattle have faltered due to poor enforcement and the widespread use of the human version of the drug in livestock.
"The main challenge is the huge stocks still available with distributors because of [its] long shelf life," said Vibhu Prakash, the study's lead author.
While manufacturing the drug is illegal, state-level regulators "have not said anything about the existing stocks," Prakash said.
"The drug inspectors have to make sure that the drug manufactured after August 2006 should not be sold on the market."
"It is a tall order to enforce this in our country."
Even worse, human diclofenac—which is comparable to the painkiller ibuprofen—is less expensive than the banned veterinary version of the drug.
"This gets filtered into the veterinary sector a lot," Prakash said. "So, [until] human diclofenac use is banned, its continued use in livestock is a distinct possibility."
Authorities have also introduced a substitute drug for livestock called meloxicam.
But meloxicam costs three to five times more than diclofenac, said Anil Kumar Chhangani, a research scientist at Jai Narain Vyas University in Jodhpur.
While more than 20 drugmakers have flooded the market with diclofenac for years, "there are just a few companies making meloxicam," Chhangani said.
Conservationists' last hope is to gather as many vultures as possible into captive breeding programs for a decade or more.
This would give time for the Indian government to completely eradicate diclofenac from the market.
A 2004 conservation plan calls for the creation of six breeding centers in South Asia, each producing 25 pairs from each of the three critically endangered species.
According to the plan, a center would be able to release a hundred pairs of vultures in about 15 years. Three centers are up and running in India, and a fourth center was established in Pakistan in 2006.
"We still have a long way to go," study author Prakash said.
Of course, there are other factors behind the vultures' demise, including the loss of habitat and nesting grounds to urbanization.
So diclofenac "may be a major cause," research scientist Chhangani said, "but it is not the cause of the decline."
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