A MARTÍNEZ, HOST:
Animals that eat the carcasses of other animals can seem kind of gross. However, a recent study shows that scavengers, such as vultures or hyenas, can actually be good for human health. That same study, though, reveals that scavenger populations are declining. NPR's Jonathan Lambert reports that could mean more disease for humans.
JONATHAN LAMBERT, BYLINE: Growing up in India, environmental economist Anant Sudarshan remembers the vultures.
ANANT SUDARSHAN: When I used to go to school, we were crossing this river, and you always saw these vultures in vast quantities, partly because they would feed on carcasses along the side of the river.
LAMBERT: But in the mid-1990s, the vultures nearly vanished, declining by over 95%. The culprit - a painkiller given to livestock that just happened to be toxic to vultures. Millions fewer vultures meant a lot more carcasses. Sudarshan, now at the University of Warwick, published a study last year finding that had deadly implications for humans.
SUDARSHAN: We find sort of large effects on mortality of the order of sort of 100,000 additional deaths a year and, crucially, effects that are sustaining many years after the vulture disappears.
LAMBERT: Animal carcasses are hotbeds of bacteria that can cause human diseases. Without vultures to quickly pick them clean, rotting flesh piles up. It can spread disease through close contact or by getting into water. And all that extra meat meant more calories for feral dogs, which spiked in number. Here's Chinmay Sonawane, a biologist at Stanford.
CHINMAY SONAWANE: Millions more feral dogs, millions of more people being bitten by these dogs. And it's estimated something like 50,000 additional people were dying from rabies.
LAMBERT: To Sonawane, vultures exemplify the enormous but often hidden benefits that scavengers provide. And it's not just vultures. Researchers have found that African hyenas pick clean cattle carcasses that can spread anthrax, and catlike civets in Malaysia can cut down on diarrhea-causing bacteria by scarfing up rancid meat.
SONAWANE: In the last, like, five or so years, there had been a burst of case studies looking at this relationship between scavenging species and human health.
LAMBERT: Sonawane and his colleagues analyzed all those case studies and came away with a worrying picture. They found that 36% of scavenging species are declining or threatened with extinction. Larger scavengers were especially threatened.
SONAWANE: When we lose these large wildlife, smaller wildlife tend to replace them.
LAMBERT: The study, published in the journal PNAS last month, found that while these smaller scavengers, like rats or dogs, can pick up some of the slack, they're just not as good at cleaning up a carcass.
SONAWANE: Therefore, there's more carcass waste, therefore more pathogens in the environment. And then, therefore, people are more likely to pick up disease from these sources.
LAMBERT: People are also more likely to pick up diseases from smaller scavengers themselves, which tend to carry more pathogens. While there's still a lot to learn about the links between scavengers and specific diseases, Maastricht University biologist Christopher O'Bryan says this study represents a good start.
CHRISTOPHER O’BRYAN: The take-home message is that we need to be always factoring in nature into the equation of human health. And we can't ignore it.
LAMBERT: The Indian vultures offer a cautionary tale. Even after restricting the medication that sparked their decline, the vultures still haven't recovered.
Jonathan Lambert, NPR News.
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