Diclofenac – the mystery solved

Pat Benson/The Peregrine Fund The drug diclofenac appears to be the cause of these vulture deaths Zoom In

Although the drug, diclofenac, has been used in human medicine for decades, it has only recently been introduced for veterinary use in India and Pakistan. Vultures appear to have been exposed to the drug while scavenging livestock carcasses.

Several anomalies remained with the new theory: experiments suggested that diclofenac had a rapid effect on birds, with death following a few days after exposure, while birds observed in the field lingered for several weeks. Diclofenac is also toxic to dogs (though at much higher levels), but dog populations were increasing at sites where Gyps vultures had declined. Lesions more characteristic of infectious disease than a contaminant had been found in dead birds; and diclofenac is excreted by mammals within a few days, which means that a very high proportion of the cattle scavenged by vultures would need to have been treated with diclofenac within a few days of death for the drug to be present in quantities toxic to the birds.

Concerned that the decline of the three species could still be caused by an infectious disease, rather than the drug, BirdLife announced that it was keeping an open mind, pending the results of tissue sample analysis in India.

"The decline of Asian vultures is one of the steepest declines experienced by any bird species." —Dr Debbie Pain, RSPB

However, new research published in the scientific journal Nature in January 2004 has confirmed that veterinary use of diclofenac is responsible for the recent devastating declines in south Asian vulture populations.

These findings are the result of a three-year study by The Peregrine Fund and Ornithological Society of Pakistan (BirdLife in Pakistan) investigating vulture mortalities in the Pakistan Punjab. The study found that 85 percent of 259 vultures examined had died of visceral gout, a condition caused by renal failure. Exhaustive testing failed to find evidence of viral or bacterial infectious disease, pesticides, poisons, heavy metals, or nutritional deficiency sufficient to explain the renal failure observed in dead vultures.

Having eliminated the classic causes of renal failure, researchers tested the theory that vultures were encountering a toxin while feeding on livestock carcasses (their main food source). Surveys of veterinarians and pharmacists identified diclofenac as a recently introduced and widely used non-steroidal anti-inflammatory drug (NSAID), known to be toxic to the kidneys of mammals.

Residue testing found diclofenac in all the analysed vultures that had died with visceral gout (25 birds), while vultures that had died of other causes (including trauma, gunshot and lead poisoning) tested negative for diclofenac residues.

As few as one in 760 carcasses containing diclofenac at a dose lethal to vultures would be sufficient to cause the observed decline in vulture numbers (30% per year). Clearly, even small-scale usage of the drug can have catastrophic consequences.

Further investigation showed that diclofenac was fatal to vultures at 10 percent of the recommended mammal dose. Tissue residues in livestock treated at the labelled dose rate were sufficient to cause gout and death in vultures. These findings, coupled with the high incidence of visceral gout in wild vultures found dead in Pakistan, India and Nepal confirm that diclofenac is the primary cause of the Asian vulture decline.

Diclofenac is widely used in human medicine globally, but was introduced to the veterinary market on the Indian subcontinent during the early 1990s. The drug is cheap (less than US$1 for a course) and widely used in the treatment of inflammation, pain and fever in livestock. In Pakistan 92 percent of 84 veterinary stockists surveyed sold the drug on a daily basis.