Men work on the production of yellow fever vaccine at a laboratory in Rio de Janeiro. (RICARDO MORAES/REUTERS)

An outbreak of yellow fever has claimed the lives of more than 600 monkeys in Brazil’s Atlantic rain forest region, threatening the survival of rare South American primates, a zoologist said.

The monkeys, mostly brown howlers and masked titis, are falling out of trees and dying on the ground in the forests of Espirito Santo state in Brazil’s southeast.

“The number of dead monkeys increases every day,” said the zoologist, Sergio Lucena. “We now know that the rare buffy-headed marmoset is also threatened by the yellow fever virus and dying.”

The howler’s sounds closely resemble grunts or barks. It was the silence that fell on the forests that first alerted farmers that something was amiss, sparking specialists to investigate.

The masked titi is considered “vulnerable” by the Swiss-based International Union for Conservation of Nature, which has placed the animal on its Red List of Threatened Species.

No evidence has so far surfaced of the affliction that is felling woolly spider monkeys, considered by the IUCN to be one of the world’s most endangered types of monkey.

Brazil is suffering the worst yellow fever outbreak among humans in decades. At least 69 people have died, nearly all in the central state of Minas Gerais, where the problems began. Most people recover from yellow fever after the first phase of infection, which usually involves fever, headache, shivers, loss of appetite and nausea or vomiting, according to the World Health Organization.

Millions of Brazilians have been vaccinated as health authorities scramble to prevent the outbreak from turning into an epidemic. There is no such protection available for monkeys.

Yellow fever, a viral disease found in tropical regions of Africa and the Americas, mainly affects humans and monkeys and is transmitted by the same type of mosquito that spreads dengue and the Zika virus.

Hundreds of thousands of people died of it in the Americas before a vaccine was developed in 1938.

— Reuters