Friday, October 7, 2011

Vultures curb spread of disease

With the recent news headlines in northern Botswana focused on a possible anthrax outbreak, I become nervous lest some misguided person try to implicate vultures in spreading the bacteria. This has happened in the past, so it is worth reading the views of anthrax expert, Peter Turnbull, below:

Nature's clean-up squad in action (Photo: J Bestelink)

"At the time of death in an animal dying of anthrax, the blood and body fluids are loaded with anthrax bacteria: there are more bacteria than red blood corpuscles in the blood. On exposure to the oxygen of the air, these start to form spores - the hardy forms of the anthrax agent which can survive for decades in the soil where blood and body fluids from such an animal have spilled. Vultures, in demolishing the dead animal before much of this sporulation can occur, contribute to reducing the environmental contamination left by the carcase. In theory, the faster the carcase is consumed, the less the chance of sporulation and residual contamination. And, at least to a point, more vultures = faster consumption. Faster consumption aldo reduces the chance of spread of the disease by flies.

The vultures do, of course, get the infected blood and gore on their beaks and feathers and they will go to nearby water to wash this off. However, unsporulated bacteria are fragile and die off quickly in water (they are also quickly killed within the vulture's digestive system). So, although the water may derive some anthrax spore contamination from bacteria that have started to sporulate, this will be low - bacteriological testing has proved how difficult it is to detect anthrax spores in such bodies of water. By the time the birds are airborne again, numbers of spores on their feet, beaks, feathers and in their guts is back to zero or very low.

Vultures are very clean, and bathe regularly (Photo: P Hancock)

Contrary to what you might expect, knowing the infamy of anthrax, it is not a highly infectious disease. That is to say, an animal generally needs to ingest or inhale a large dose of spores to become infected. So, small numbers of residual anthrax organisms on the feet, beaks, feathers or in the guts of vultures constitute essentially zero risk or threat to animals elsewhere. There is again scientific evidence for this.

The decline in the numbers of vultures around is probably already apparent to you where you live and you are probably well aware of the particularly catastrophic decline over the past two decades on the Indian sub-continent largely from inadvertent diclofenac poisoning, but also from other human-made causes also at play in Africa - habitat destruction, deliberate poisoning, capture for traditional medicines, drowning in reservoirs, powerline collisions and electrocution and so on. The apparent consequences of this population crash are not nice. Substantial increases in numbers of rats and of feral dogs have been attributed to the decline in vultures and with that, vastly increased numbers of cases of at least rabies and distemper. In addition to the health issues, there have been consequent costs to the economy estimated at billions of dollars. One British Medical Journal article has even suggested a link (albeit unproven) between absence of vultures and a rise in human anthrax cases.

In Africa, loss of our vulture populations can be expected to lead to similar disturbances in other scavenger popul;ations, again with possible concomitant increases in rabies, distemper and plague. Botulism from decaying carcasses is another strong possibility.

It also needs to be remembered that vultures rely on other vultures to detect carcasses; when vulture populations decline, their ability to find carcasses deteriorates at an accelerating rate.

So, as well as helping to reduce residual contamination at the site of an anthrax carcass and minimizing fly-borne transmission, thereby playing an important role in curtailing the spread of anthrax, the importance of vultures to the overall health of the ecosystem should not be underestimated.

Also not to be forgotten is that circling vultures have always been, and continue to be, the best signal to game managers and farmers that deaths have occurred within their boundaries, enabling prompt action as needed.

In summary, persecuting vultures as an approach to anthrax control is folly, and in the long run, can be expected to increase problems associated with both anthrax and other diseases."

Peter Turnbull, with thanks to Orr Spiegel, Hebrew University of Jerusalem, Steve Bellan, University of California, Berkeley, and Kerri Wolter, VulPro South Africa, for most helpful comments.

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