For every death researchers know about, there could be many more Sergio Mendes, Projeto Muriqui-ES.

Rare monkeys in the forests of Brazil are being decimated by yellow fever.

The outbreak started in late 2016 and, as is often the case in South America, it has spread to humans, killing at least 50 since the start of 2017. The authorities have rushed vaccines to hospitals, where long queues await inoculation.

But there is no vaccine for monkeys who are dying en masse in Espírito Santo and Minas Gerais, the two states so far worst hit.


“Some 80 to 90 per cent of the brown howler monkeys are infected or have already died,” says Sergio Mendes at the Federal University of Espírito Santo in Vitoria, Brazil. “This is a true catastrophe. These outbreaks happen periodically, but this is the worst I’ve ever seen.”

Mendes knows of 400 howler monkey deaths in the state, and he believes this is likely to be only 10 per cent of the total, with the greatest losses happening largely unseen in remote forested areas.

Atlantic titis and geoffroy’s marmosets found dead last week in Espirito Santo are also being tested for yellow fever. Both are unique to the Mata Atlantica, one of the world’s most species-rich and most-endangered tropical forests.

Other endemic primate species affected by the outbreak include the endangered buffy-headed marmoset and crested capuchin, and the critically endangered muriqui. There are only about 1000 muriqui individuals left in the wild, and their slow breeding time means numbers would take a long time to recover from yellow-fever deaths.

There are also unconfirmed reports of capuchin monkeys dying of suspected yellow fever in neighbouring Minas Gerais and in São Paulo states.

The virus is normally found in several forest-dwelling mammals, from marsupials to monkeys, and is transmitted by Haemagogus and Sabethes mosquitoes.

Marco Almeida, a veterinary epidemiologist from Rio Grande do Sul state’s health agency, says the current outbreak is unlikely to be caused by a new, more virulent form of yellow fever virus, as it is known to mutate very slowly.

Instead, he thinks recent prolonged and torrential rains provided ideal conditions for mosquitoes. Often delivering a week’s rain in a day, the deluges lasted over a month and may have weakened the monkeys by cutting the times when they can feed and challenging their immune systems.

“The mosquitos can disperse across forest for up to 6 kilometres from their breeding point,” says Júlio-César Bicca-Marques, a primatolologist at Pontifical Catholic University of Rio Grande do Sul, “but they’ll also get rides in trucks. Plus, infected hunters, tree-cutters and agricultural workers can spread the virus.”

It’s well known from lab tests that howlers are the most vulnerable to yellow fever of all of South America’s monkeys. “But with these current high infection levels, the virus could spread to all of the region’s 14 other primates,” says Almeida.

“Part of the problem is forest fragmentation,” says primate conservationist Karen Strier of University of Wisconsin-Madison. “Only 5 per cent of Mata Atlantica remains. So Mata Atlantica primate populations are small and isolated. Wipe one out, and natural recolonization is very difficult.”

With monkeys being key seed-dispersers, the prognosis for both forest and primates is not good. Meanwhile, as the epidemic increases, ill-informed individuals have started attacking the region’s monkeys, in the erroneous belief that they can spread yellow fever to humans directly.