Like a warning call from the wild, howler monkeys in Argentina alerted health officials to an outbreak of yellow fever.

In November 2007, researchers found four wild monkeys in a population that they had been studying dead. The biologists, park rangers, and veterinarians quickly searched both within and outside of the research area to see if other monkeys were dying as well.

They were. At least 59 howler monkeys died during the austral springs and summers of 2007-2008 and 2008-2009. About two months after the first monkeys were found, tests conducted at the Instituto Nacional de Enfermedades Virales Humanas confirmed the culprit: yellow fever.

Colonists and the slave trade brought the virus that causes yellow fever to the Americas from Africa. South American primate species did not evolve with the yellow fever virus, and thus never adapted defenses against the disease. Howler monkeys, along with all other primates found in the New World, are very vulnerable to yellow fever. This mosquito-born disease is also deadly to humans.

The primate researchers notified Argentina’s National Health Authority, which launched a vaccination campaign for the people of Misiones province for yellow fever. What began as an ecological study of South American monkeys helped save lives.

“This study shows the importance of wildlife monitoring as a means of early detection for pathogens that could affect both animals and humans,” said Dr. Pablo Beldomenico of the Wildlife Conservation Society.

Because most howlers die suddenly after becoming infected with the yellow fever virus, a team of Argentine scientists and WCS health experts concluded that these populations do not serve as reservoirs for the disease. In other words, when members of these populations do get sick, they signal to humans that an outbreak is afoot. The researchers recently published their findings in the American Journal of Primatology.

Other strategies for yellow fever transmission between humans and wildlife include encouraging people to report any monkey deaths they encounter, as well as educating those who live near protected areas of the disease risks of capturing animals for pets.

“The outbreak has tragic conservation implications for the endangered brown howler monkey, one of the two species affected,” said added Beldomenico. “This primate is highly threatened primarily by habitat destruction, hunting, and now disease.”