Once upon a time, the vulture was an abundant and ubiquitous bird in India.

The scavenging birds hovered over sprawling landfills, looking for cattle carcasses. Sometimes they would alarm pilots by getting sucked into jet engines during airport take-offs.

But more than two decades ago, India’s vultures began dying because of a drug used to treat sick cows.

By the mid-1990s, the 50 million-strong vulture population had plummeted to near zero because of diclofenac, a cheap non-steroidal painkiller for cattle that is fatal to vultures. Birds that fed on carcasses of livestock treated with the drug suffered from kidney failure and died.

Since the 2006 ban on veterinary use of diclofenac, the decline has slowed in some areas, but at least three species have suffered long-term losses of 91-98%, according to the latest State of India’s Birds report.

And that’s not all, according to a new peer-reviewed study. The unintentional decimation of these heavy, scavenging birds allowed deadly bacteria and infections to proliferate, leading to the deaths of about half a million people over five years, says the study published in the American Economic Association journal.

“Vultures are considered nature’s sanitation service because of the important role they play in removing dead animals that contain bacteria and pathogens from our environment - without them, disease can spread,” says the study’s co-author, Eyal Frank, an assistant professor at University of Chicago’s Harris School of Public Policy.

“Understanding the role vultures play in human health underscores the importance of protecting wildlife, and not just the cute and cuddly. They all have a job to do in our ecosystems that impacts our lives.”

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They found that after anti-inflammatory drug sales had risen and vulture populations had collapsed, human death rates increased by more than 4% in districts where the birds once thrived.

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The authors estimated that between 2000 and 2005, the loss of vultures caused around 100,000 additional human deaths annually, resulting in more than $69bn (£53bn) per year in mortality damages or the economic costs associated with premature deaths.

These deaths were due to the spread of disease and bacteria that vultures would have otherwise removed from the environment.

For example, without vultures, the stray dog population increased, bringing rabies to humans.

Rabies vaccine sales rose during that time but were insufficient. Unlike vultures, dogs were ineffective at cleaning rotting remains, leading to bacteria and pathogens spreading into drinking water through runoff and poor disposal methods. Faecal bacteria in the water more than doubled.

“The vulture collapse in India provides a particularly stark example of the type of hard-to-reverse and unpredictable costs to humans that can come from the loss of a species,” says Mr Sudarshan, an associate professor at the University of Warwick and co-author of the study.