SILVA JARDIM, Brazil — At a small lab in Brazil’s Atlantic rainforest, researchers with gloved hands and masked faces weigh four tiny golden monkeys so a veterinarian can gently insert a needle under the thin skin of each sedated animal’s abdomen.
The next morning, biologist Andréia Martins takes them to exactly where they were caught. She opens the wire cages and the monkeys scoot out, hop to a tree or onto the ground, scale the canopy and regroup as a family. They chat loudly as they disappear into the rainforest.
This brief, strange encounter with humanity was for their own health – and the survival of their species. These endangered wild monkeys, known as golden lion tamarins, have now been vaccinated against yellow fever, part of a groundbreaking campaign to save an endangered species.
“Vaccinating wildlife for the good of animals, not to protect humans, is novel,” said Luís Paulo Ferraz, president of the nonprofit Golden Lion Tamarin Association.
When yellow fever began spreading in Brazil in 2016, resulting in more than 2,000 human infections and around 750 deaths, it also quickly killed a third of high-risk tamarins, most of them within months. That’s why scientists in Brazil have developed a yellow fever vaccine specifically tailored to the endangered monkeys.
The vaccination campaign started in 2021 and more than 300 tamarins have already been vaccinated. It was the first such attempt in Brazil and raises important questions about how far one has to go to save a species from extinction.
One of the traditional conservation adages is “let it be”. But at a time when every corner of the earth is being affected by human impact — from melting icebergs to fragmented forests to plastic-filled oceans — a new generation of scientists and environmentalists are calling for increasingly more intrusive approaches to saving wildlife and ecosystems.
Carlos R. Ruiz-Miranda, a conservation biologist at the State University of Northern Rio de Janeiro, is among the scientists who have worked to protect golden lion tamarins for more than three decades, twice rushing to their rescue when they were threatened with extinction . He says vaccination is the only option left: “Is it too extreme? Give me another alternative.”
Viruses have always existed in abundance in nature. But humans have drastically altered the conditions and impacts on how they spread throughout wildlife. Epidemics can spread across oceans and borders faster than ever before, and species already declining from habitat loss and other threats are at greater risk of being wiped out by outbreaks.
“Human activities absolutely accelerate the spread of disease in nonhuman populations,” said Jeff Sebo, an environmental researcher at New York University who was not involved with the Brazil project.
Southeast Brazil was once covered by rainforest, but today the rolling landscape is an uneven checkerboard of dark green jungle and grassy cow pastures – only 12% of that rainforest remains.
Yet it is the only place on earth where wild golden lion tamarins live.
Long-standing efforts to save the charismatic monkeys — famous for their copper-colored fur and small, inquisitive faces framed by silky manes — have included a pioneering captive-breeding program coordinated by some 150 zoos worldwide. Many of these animals were then carefully released in Brazil starting in 1984 in collaboration with local landowners.
Then came yellow fever.
After the first laboratory-confirmed death of a tamarin from the virus in 2018, a monkey census found that the population of wild tamarins had dropped from about 3,700 to 2,500.
“This epidemic spread very quickly from north to south, across the country — no wildlife does that,” Ruiz-Miranda said. “It’s people. They overcome long distances in buses, trains, planes. They bring the disease with them.” Yellow fever is transmitted by mosquitoes, he explained, but highly mobile infected people spread the disease much further and faster than insects alone.
“We realized that if we don’t do something, we could lose the entire population in five years,” said Ferraz of the Golden Lion Tamarin Association.
At the time of the yellow fever outbreak, Marcos da Silva Freire was deputy director for technological development at Brazil’s Oswaldo Cruz Foundation, which oversees vaccine diagnostics and production in the country.
Freire arranged with the Rio de Janeiro Primate Center to begin trials of various doses of yellow fever vaccine on about 60 monkeys, close relatives of tamarins, in January 2018.
A year later, he checked the antibody levels in her blood. The vaccine seemed to work, with no negative side effects. When the team received government approval to begin vaccinating wild monkeys, Freire oversaw the initial rounds of vaccinations.
So far they have vaccinated more than 300 tamarins and found no adverse side effects. When they caught monkeys and retested them, 90% to 95% showed immunity – similar to the effectiveness of human vaccines.
The outbreak appears to have subsided and the monitored monkey population has stabilized overall and even increased slightly within the Poço das Antas Biological Reserve.
While authorities elsewhere have vaccinated animals to protect human health, it’s still very rare for scientists to give vaccine injections to directly protect an endangered species.
“What are the unintended consequences of vaccination? You can’t always be sure,” said Jacob Negrey, a biologist and primatologist at Wake Forest University’s School of Medicine.
However, scientists are increasingly studying the merits of vaccinating endangered wildlife and are making plans to potentially vaccinate tigers against canine distemper in Asia, chimpanzees against respiratory disease in Africa and koalas against chlamydia in Australia.
“There are people who say we shouldn’t touch nature, that we shouldn’t change anything. But really, there are no more pristine natural habitats,” said Tony Goldberg, a disease ecologist and veterinarian at the University of Wisconsin-Madison who supports vaccination of wild animals when it’s safe and practical.
“People are becoming aware of the scale of the problem and realizing that they need to do something.”