SILVA JARDIM, Brazil (AP) — In a small lab nestled in Brazil’s Atlantic Forest, researchers with gloved hands and masked faces cradle four tiny golden monkeys so a veterinarian can delicately slide a needle under the thin skin of each sedated animal’s belly.

The next morning, biologist Andréia Martins brings them to the precise spot where they were caught. She opens the wire cages and the monkeys dart out, hopping to a tree or the ground, ascending the canopy and regrouping as a family. They chatter noisily as they vanish into the rainforest.

This brief, strange encounter with humanity has been for the sake of their own health – and the survival of their kind. These endangered wild monkeys, called golden lion tamarins, have now been vaccinated against yellow fever, part of a pathbreaking campaign to save a threatened species.

“Vaccinating wild animals for the sake of animals, not to protect humans, is novel,” said Luís Paulo Ferraz, president of the nonprofit Golden Lion Tamarin Association.

When yellow fever began to spread in Brazil in 2016, resulting in more than 2,000 human infections and around 750 deaths, it also quickly killed a third of the highly vulnerable tamarins, the majority of them in just a few months. So scientists in Brazil customised a yellow-fever vaccine for the endangered monkeys.

The inoculation campaign started in 2021, and already more than 300 tamarins have been vaccinated. The first such effort in Brazil — and one of the first worldwide — raises vital questions about how far to go to save a species from extinction.

One of the traditional adages of conservation is “Leave it be.” But in an age when every corner of the globe is touched by human influence – from melting icebergs to fragmented forests to plastic-filled oceans – a new generation of scientists and environmentalists is increasingly calling for more interventionist approaches to save wild animals and ecosystems.

“There are people who say we shouldn’t touch nature, that we shouldn’t alter anything. But really, there are no pristine natural habitats left,” said Tony Goldberg, a disease ecologist and veterinarian at the University of Wisconsin-Madison, who supports vaccinating wildlife when it’s safe and practical. “People are waking up to the magnitude of the problem and realising they have to do something.”

Carlos R Ruiz-Miranda, a conservation biologist at State University of Northern Rio de Janeiro, is among the scientists who have worked for more than three decades to protect the golden lion tamarins, twice going to their rescue when extinction threatened. He said vaccinations are the only option left: “Is it too extreme? Give me another alternative.”

“We have to intervene when it’s a human-borne conservation risk, if you’re going to have an environment with wildlife,” said Ruiz-Miranda.

Viruses have always abounded in nature. But humans have drastically changed the conditions and impacts of how they spread in wildlife. Epidemics can travel across oceans and borders faster than ever, and species already diminished by habitat loss and other threats are more at risk of being wiped out by outbreaks.

“Human activity is absolutely accelerating disease spread in non-human populations,” said Jeff Sebo, an environmental researcher at New York University, who was not involved in the Brazil project.

But there are risks. It’s tough to decide which species get the attention and resources needed for survival. In Brazil, a political climate of anxiety about the COVID-19 pandemic and misinformation about vaccines in general has caused delays. Yet if the scientists get it right, they could be pioneers to show what’s possible to save threatened wildlife.