Vultures could hold key to deadly bacteria
As millions of Americans tuck into the traditional turkey dinner this Thanksgiving, they might spare a thought for the turkey vulture.
Its meal will certainly be undercooked, lacking in trimmings, eaten at the side of the road and seething with toxic bacteria that would kill most other animals. (Despite sharing a name - and a similarly featherless head - turkey vultures aren't closely related to the Thanksgiving star.)
"They're coming into contact with almost every kind of pestilence you can think of, and apparently they're fine," says Gary Graves, curator of birds at the National Museum of Natural History in Washington, DC. "So what's happening in the guts of these birds?"
To help find the answer he joined forces with scientists in Denmark to study the digestive system of 50 turkey vultures and black vultures that were captured and killed in Nashville, Tennessee.
They took swabs from the birds' faces where they had come into direct contact with rotting carcasses and samples from the hind gut at the end of the large intestine. They then compared the two.
Using DNA sequencing they detected almost 13,500 species of bacteria in the 50 face swabs - while only around 1,500 different bacteria were found in the hind gut samples. Either the vultures' digestive juices had killed the bacteria or the environment in the hind gut was so hostile the microbes could not reproduce.
But two groups of extremely nasty bacteria did survive - Fusobacteria, which has been linked to colon cancer in humans, and Clostridia, which produces tetanus, gangrene and botulism.
"Botulism is one of the most toxic substances in the world, and their guts are filled with this bacteria, which apparently has no effect on them," says Graves.
"It's a red flag moment when you say: 'Wow! I can't believe that bacteria that has such a negative association with human health is so prevalent in the guts of vultures and they appear to be doing perfectly fine. Why is that?' We're in the early stages (of finding out)."
Vultures seem to suffer no ill effects even when they are exposed to leprosy. The disease is thought to have originated in Africa and was spread to the Americas by the slave trade. It is very rare among humans in the US - about 100 cases are reported every year - but it is carried by armadillos, which are often hit by cars, and form a regular part of a vulture's diet.
Vultures are often described as nature's garbage collectors. They clean up virtually all of the tens of thousands of deer that are killed every year on America's roads. "It's an ecosystem service that's underappreciated in the US," says Graves.
And like all birds, they are the living descendents of dinosaurs. Their systems have co-evolved with bacteria over 65 million years. This is the first major analysis of their microbiome - the community of microorganisms that inhabit the body space of animals.
But in spite of their impressive resistance to deadly germs, vultures aren't immune to everything.
Some species in Asia are critically endangered because they've consumed the carcasses of cattle treated with diclofenac, an anti-inflammatory drug.
"Farmers take the carcasses to cattle graveyards and put them out for the vultures to eat," says Yula Kapetanakos at the Cornell Lab of Ornithology. "But it turned out that the drug causes renal failure in the vultures. Vultures that once numbered in the tens of millions in India declined by over 99% in the span of 20 years."
Without vultures to consume the cattle carcasses, she says, other scavengers - such as feral dogs - moved in. That led to an increase in rabies infections among humans in India, where more than a third of global deaths from the disease occur every year.
And there have been other unforeseen consequences. Zoroastrians place their dead on cylindrical structures called towers of silence, where the corpses are picked clean by vultures. They are forbidden to cremate or bury bodies, but the decline in vultures has forced them to look for alternative funeral arrangements, says Kapetanakos, who has studied Asian vultures for many years.
"That's the irony," she says. "Vultures can handle anything - anything - that nature can throw their way, and humans presented them with one drug that has nearly destroyed them."
Graves hopes further research will help isolate the genes that protect vultures from deadly bacteria.
"We need to find out what genes are associated with resistance to botulism, for instance. If we can do that then we can find out if there are any genotypes in humans that can confer this type of immunity.
"And eventually that might lead to gene therapies or genetic engineering in humans to the point where we could eat rotten meat if we wanted to," he says.
But that's a long way off. For now, it's probably best to carrying on cooking that Thanksgiving turkey.