The Problem - Diagnosing the problem
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Populations declines that were first recorded bu BNHS Principal Biologist Dr Vibhu Prakash at Keolodeo National Park in India. These declines were found to be occurring nationwide across India (Prakash et al 2003) and in neighbouring Pakistan and Nepal, triggered urgent investigationw to uncover the cause of the declines. Research biologists from the Bombay Natural History Society, Bird Conservation Nepal and the Ornithological Society of Pakistan were joined by international partners from the Royal Society for the Protection of Birds (UK), Zoological Society of London (UK) and The Peregrine Fund (USA). Because of the rapidity of the decline simple population modelling established that the declines had to be caused by a major reduction in adult survival, as reduced breeding success could not account for declines of nearly 50% a year.
Through collecting carcasses of dead and dying vultures researchers quickly established that dead birds were often characterised by the presence of extensive visceral gout, and of 284 post-mortems carried out on vultures in Pakistan, India and Nepal gout was found in 84% of birds (Oaks et al 2004; Shultz et al 2004). Visceral gout is caused by a build up of uric acid in the body, which at very high levels crystallises in the body coating all internal organs in a white paste-like coating (see image below). Uric acid is the white substance found in the guano of all birds, and the characteristic presence of visceral gout in vultures suggested the cause of death was likely to be related to kidney failure.
For several years researchers battled to understand what might be the cause of the deaths. Dead birds were tested for pesticides, herbicides, toxic heavy metals and other environmental pollutants. While trace levels of some of these compounds were detected, in the majority of cases they were they at insufficient levels to cause physiological damage and there was no link between these compounds and the gout found in most dead birds. Because of the geographic range and speed of the declines one initial strong hypothesis was that a novel infectious disease agent was responsible for mortalities.
The key diclofenac breakthrough was made in 2003 by researchers working for the Ornithological Society of Pakistan, The Peregrine Fund, and led by Professor Lindsay Oaks from Washington State University, USA. Lindsay recognised that the class of pain killers known as Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) had been linked to kidney failures and cases of visceral when some of these drugs were given to birds. Visiting pharmaceutical shops in Pakistan the team found that a new NSAID, diclofenac, had recenly come on sale and was commonly available. Investigating the carcasses of vultures collected in Pakistan revealed that every carcass that had visceral gout also had traces of the NSAID diclofenac, whereas those carcasses with no gout had no diclofenac. The team then gave diclofenac to vultures, either by injecting birds or by feeding them tissues from buffalo and goats injected with diclofenac: birds that received a high dose of diclofenac died within days of treatment and with extensive visceral gout. In 2004 the results of this work were published in the journal Nature, revealing that diclofenac was the cause of the vulture declines (see Oaks et al 2004)
Extensive research has followed up on this work, establishing the same correlation between gout and diclofenac in birds from India and Nepal (Shultz et al 2004). modelling the amount of diclofenac required in the environment to cause the observed decline rates (Green et al 2004), measuring the prevalence of diclofenac in cattle carcasses available for vultures and finding that this was present in 10% of carcasses (Taggart et al 2007), and modelling the observed prevalence of diclofenac and determining that diclofenac was on its own responsible for the vulture population crash (Green et al 2007). These results, have now established without doubt that diclofenac is the main, if not the only, cause of the vulture declines.
Other hypotheses put forward for the vulture declines include reduced food availability, increased numbers of dogs, and habitat destruction. Follow this link for discussion on why these are not credible causes for the collapse in vulture numbers.
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