You know something bad’s about to happen in a movie when the vultures show up. In Disney’s Snow White, the future princess is tricked when the Evil Queen visits her cottage with a basket of apples, the reddest and juiciest of which is poisoned.

“They sure look delicious!” Snow White says.

“But wait till you taste one, dearie!” the Evil Queen cackles.

Cue two enormous black vultures, hunched over menacingly in a nearby tree, practically salivating as they watch the scene unfold. Death, of course, is near. Snow White brings the poisoned apple to her lips and falls into her long slumber.

Vultures have long been reviled as harbingers of death, but contrary to their bad reputation, the birds are vitally important to humans. Found on every continent except Australia and Antarctica, they consume virtually every last bit of the rotting carcasses they find—sometimes even the bones. They are, in other words, nature’s best cleanup crew. They help farmers avoid paying for carcass disposal when their cattle die. And, they reduce the spread of disease; most other scavengers, like flies and rats, leave leftovers, which creates a breeding ground for bacteria.

But vultures are disappearing globally. Humans have long been culprits: The birds get poached, snagged in fishing gear, and poisoned when they eat carcasses contaminated with hunters’ lead shot; they lose their habitat when it’s developed. And now, a study published last month in Biological Conservation has confirmed what many people fear is their greatest threat: Vultures in Africa, Asia, and Europe are being poisoned with diclofenac, a non-steroidal anti-inflammatory drug. Experts worry that if the poisoning continues, the loss of the birds could upset the balance of life and death in ecosystems around the world.

Diclofenac, which was developed to treat arthritis and other painful health conditions, is restricted to human use in America, but that’s not the case elsewhere. Livestock farmers across Europe, Asia, and Africa have routinely used diclofenac to treat fever and immobility in cattle, sheep, and goats because it’s cheap and widely available. These farmers tend to overlook the drug’s danger to vultures because it’s such a boon for business, preventing the loss of their precious livestock to lameness and infection.

Vultures ingest diclofenac when they feed on carcasses of deceased livestock that have been treated with the drug. “It causes kidney failure,” says José Pedro Tavares, director of the Zurich-based Vulture Conservation Foundation, one of several vulture advocacy groups that has cropped up since the early 2000s. “Vultures die within two days with a very small dose.”

Farmers have used veterinary diclofenac since the early 1990s. At the end of that decade, Vibhu Prakash of the Bombay Natural History Society reported an unusually steep decline in Asian vulture populations in India and Pakistan. But the cause of the decline—diclofenac poisoning—wasn’t uncovered until early 2000s. In 2006, India, Nepal, and Pakistan banned the use of veterinary diclofenac after those countries saw their vulture populations take a nosedive. Bangladesh, concerned about its own dwindling vulture population, also implemented a diclofenac ban in 2010. Across Asia, one species, the oriental white-backed vulture, declined to just one-thousandth of its pre-1990 population by the mid-2000s as a result of diclofenac poisoning. Currently the drug is still approved for veterinary use in Europe, yet vulture-conservation groups like Tavares’ are pushing for a ban.

In Europe and parts of Asia, farmers have easy access to a different non-steroidal anti-inflammatory drug called meloxicam that’s as effective as diclofenac but not toxic to vultures, says Tavares. It hasn’t been widely adopted, however, because even though it ultimately alleviates animals’ pain, it also appears to cause pain when it’s first injected. In a 2014 study on goats, the veterinary meloxicam formulas used had a higher pH and drug concentration, which can cause a sharp stinging sensation. Agitated livestock aren’t easy to inoculate, and may try to kick or run when they are in pain.

Looking to avoid injury and save time, farmers often turn to two other NSAIDs, ketoprofen and aceclofenac, which appear to hurt livestock less. But, like diclofenac, recent studies reveal these drugs—which are chemically similar to diclofenac—also appear deadly to vultures.

“Right now, getting all veterinary drugs, at least the NSAIDs, safety-tested on vultures ahead of being licensed … is the single highest priority for vulture conservation,” says Chris Bowden, the program manager of Saving Asia’s Vultures from Extinction.

Beyond policy initiatives, SAVE also is using captive-breeding and release programs to help bring some of Asia’s most at-risk species—white-backed, Indian, and slender-billed vultures—back from the brink. The staff collects baby vultures from the wild and brings them to one of their three breeding centers across India, where babies are put in large aviaries that hold 30 to 40 birds. When sexually mature vultures pair off and breed, females typically lay one egg. But the staff has learned that by removing this egg and sticking it in an artificial incubator, the absence sometimes prompts a fertile female to lay a second. This improves the odds at least one chick will survive. Last year, SAVE successfully hatched, reared, and released 60 vultures, and is on track for even more birds this year, according to Bowden.

Unlike vultures, mammalian scavenger species, especially those that also hunt and are well adapted to cities—coyotes, raccoons, opossums—are thriving. But avian scavengers on the whole, which appear less adaptable to human-caused environmental change, are generally declining. With fewer vultures around, other scavengers—the ones who aren’t as good at picking bones clean, and who therefore are more likely to spread disease—proliferate.

“The loss or decline of [avian] scavengers can have cascading impacts within an ecosystem, ultimately affecting human health,” says James Beasley, a scientist at the University of Georgia’s Savannah River Ecology Laboratory who has extensively studied vultures and other scavengers. “For example, in Asia, following the collapse of vulture populations due to accidental poisoning with diclofenac, populations of feral dogs and rats increased, resulting in a substantial increase in human rabies cases.”

Still, some vulture populations do appear to be rebounding, says Beasley. These include black and turkey vultures in North America, whose populations had once declined as a result of lead poisoning. But since 1991, when lead shot became closely regulated in the U.S., these species “have fared surprisingly well in the face of expanding human populations.”

Most likely, he says, these vultures have a knack for taking advantage of manmade situations: eating road kill, picking around in landfills, and roosting in cellphone towers and utility poles. And that gives him hope. Once the threat of poisoning is taken off the table, vultures in other continents may be able to adapt to their changing environments, too.