In the NCEAS SNAPP Ecological Levers for Health working group, we’re collecting examples of local or regional interventions that can have direct, measurable benefits for human health (via reduced infectious disease) and the environment – win-win solutions. The case studies that we’ve collected thus far are so cool that we just can’t wait to share them! So today I’m going to share a story about vulture conservation and human infectious disease. The bite-sized, Tweetstorm version of the story is available at @parasiteecology.
Let’s start with the obvious: vultures are crazy awesome birds. They have a gross/creepy reputation because they’re a bit funny looking, they eat dead stuff, and they have some odd habits, like defecating on their own legs to increase evaporative cooling. But they also have some animal superpowers: they can smell things that are kilometers away, they can fly despite being huge, and their stomach acids are so brutal that they can literally eat anthrax for breakfast.
But even superbirds have their Kryptonite. In the past few decades, millions of vultures have died after consuming human-sourced poisons. One such poison is Diclofenac, an NSAID that is used in veterinary medicine. Because a single carcass is typically visited by many vultures, contamination with the drug in discarded livestock carcasses can have huge impacts on vulture populations. And it did. For instance, in India, populations of three vulture species (Gyps indicus, G. tenuirostris, and G. bengalensis) plummeted by 97-99% in just one decade!! Globally, the majority of vulture species are facing extinction (critically endangered, endangered, or threatened), but the vulture extinction crisis in India is especially notable.
With local and global conservation efforts and funding already stretched thinly over thousands of endangered species, why should we care about vulture conservation, specifically? Well, for starters, vultures have been spiritual and cultural icons forever. Ever seen a Western movie? Watched the Jungle Book? Gone for a walk or a drive in the wilderness? Yeah, life without vultures would be weird. It’d also smell terrible. As obligate scavengers, vultures’ unique adaptations allow them to find carcasses much sooner than many facultative scavengers (e.g., dogs, raccoons, rodents). And vultures tend to pick carrion bones clean – and might even eat the bones! – whereas other scavengers often only eat specific tissues. That means that in a world without vultures, putrefying carrion would be more common. And not just “in the wild.” Many cities around the world have rudimentary waste management, at best, and vultures are a major player in waste removal/reduction.
But in a world overrun by carrion, the stench would be the least of our problems. Carcasses repulse us because they are hotspots of disease risk – sources of exposure to anthrax, botulism, and other infectious agents. And more subtly, abundant carrion might also increase populations of animal reservoirs for disease, like rodents and feral dogs. For instance, when vulture populations drastically declined in India in the 1990s – and carrion availability hypothetically increased – the feral dog population increased by millions, despite ongoing sterilization programs. We can’t be sure that vulture declines caused the increase in feral dog populations, because many other things changed in India during that same period (e.g., urbanization). But vulture declines are one possible driver of increased feral dog populations, and during the same period, the risk of feral dog bites increased, as did the number of human deaths due to rabies (Markandya et al. 2008).
Rabies kills 59,000 people per year – mostly in rural Asia and Africa, where access to treatment is limited – and almost all of these human rabies infections come from feral dog bites. Asia and Africa are also the hotspots of global vulture declines, and this spatial correlation suggests that adding an ecological intervention – in the form of vulture conservation – to ongoing dog sterilization and public health interventions might be a successful way to reduce rabies transmission. But interventions won’t be supported by the public and policy makers unless they are demonstrably cost-effective. So Markandya et al. (2008) figured out the economic cost of rabies in India and the cost of vulture conservation in India, and concluded that the benefits of reduced rabies outweighed the costs of vulture conservation. This could be a practical win-win!
But how do we conserve vultures? In addition to captive rearing programs to immediately buffer vulture populations, the most important conservation action was to switch from the lethal vet med Diclofenac to a vulture-friendly vet med, like Meloxicam. India, Nepal, and Pakistan all banned Dicofenac in 2006, and since then, vulture population declines seem to have slowed or even reversed (see below – Prakash et al. 2012)! But because the vulture populations are so small, the most recent populations estimates are admittedly rather uncertain, so these trends should be viewed cautiously.
If Diclofenac is banned, why aren’t vulture populations growing like crazy? For starters, vultures are K-selected species, so their populations grow slowly even under the best conditions. And despite the ban, Diclofenac is still readily acquired, so some contaminated carcasses are still finding their way into the food chain. It’s also possible that the sheer number of feral dogs in India is hampering vulture recovery, if the vultures are being outcompeted by dogs for available carrion.
Since Indian vulture populations haven’t rebounded yet – they’ve only (hopefully) stopped declining – we wouldn’t actually expect that available carrion, dog populations, and the incidence of human rabies have decreased. So it’s too soon to say whether this ecological intervention successfully reduced human infectious disease, as predicted. To further complicate measuring the public health success of this intervention, rabies isn’t a notifiable disease in India, so human rabies cases and deaths often go unreported. Therefore, if the human health impacts of vulture conservation in India are ever going to be decisively evaluated, some intensive surveying of vulture populations, dog populations, and human rabies cases will be required in the near future.
In conclusion, education/policy initiatives for vulture conservation are predicted to be #Levers4Health – mutually beneficial solutions for human infectious disease and conservation. But enacting these interventions can be tricky, and measuring their long term success might be prohibitively difficult. We’ll be eagerly awaiting more news and data on vulture conservation, feral dog populations, and human infectious diseases from Asia and Africa.
Do you know of other examples of potential win-win solutions for reducing human infectious diseases and advancing conservation goals? If so, we’d love to hear about them! You can let us know in the comments, on Twitter, or by email!
If you’d like to learn more about the vulture conservation crisis and it’s impacts on human health, check out these references:
Balmford, A. 2013. Pollution, politics, and vultures. Science 339: 653-654.
Buechley, E.R, and Ç.H. Şekercioğlu. 2016. The avian scavenger crisis: Looming extinctions, trophic cascades, and loss of critical ecosystem functions. Biological Conservation 198: 220-228.
Gangosa, L., R. Agudo, J.D. Anadón, M. de la Riva, A.S. Suleyman, R. Porter, and J.A. Donázar. 2012. Reinventing mutualism between humans and wild fauna: insights from vultures as ecosystem services providers. Conservation Letters 6(3): 172-179.
Green, R.E., J.A. Donazar, J.A. Sanchez-Zapata, and A. Margalida. 2016. Potential threat to Eurasian griffon vultures in Spain from veterinary use of the drug diclofenac. Journal of Applied Ecology 53: 993-1003.
Ogada, D.L., et al. 2016. Another continental vulture crisis: Africa’s vultures collapsing toward extinction. Conservation Letters 9(2): 89-97.
Markandya, A., T. Taylor, A. Longo, M.N. Murty, S. Murty, and K. Dhavala K. 2008. Counting the cost of vulture decline – an appraisal of the human health and other benefits of vultures in India. Ecological Economics 67:194-204.
Prakash, V, et al. 2012. The population decline of Gyps vultures in India and Nepal has slowed since veterinary use of Diclofenac was banned. PLoS ONE 7(11): e49118. doi:10.1371/journal.pone.0049118
Photo/figure credits from the Tweetstorm can be found on the figures, or at these locations:
(4, 6, 8, and 15) BirdLife South Africa has a bunch of great fact cards that are worth sharing. You can check out the rest here.
(7) Thanks, Disney, for our childhoods.
(10) Photo credit to Corrinne
(12) Figure credit to Steven Vanek
(13) Find this and other excellent illustrations here
(16) From here
(18) The LA times has a great series of condor release photos here
(19) Thanks to Ginger at NCEAS for being our photographer!