Yellow fever is spreading fast among Brazilian howler monkeys, killing thousands of those living in protected reserve. Experts are worried about the disease because they have never seen it took the lives of so many monkeys. Yellow fever is endemic in South America and began spreading in late 2016.

Even health officials in Brazil are getting worried about potential human repercussions of yellow fever. There are reports that as of mid-March, 400 human cases were recorded, including 150 casualties. To make it worse, there are another 900 suspected cases of human yellow fever that are yet to be confirmed, according to StatNews.

According to an animal biology professor, Sergio Lucena Mendes of Universidade Federal de Espirito Santo, howler monkeys is much sensitive to the disease compared to humans. However, they are still baffled of how yellow fever can transfer from one forest to another. The disease is supposed to be contained in a land-locked island and affected monkeys should have been isolated.

As of today, yellow fever virus is advancing from forest to forest even those that are 100 meters apart. Mendes said that he has never seen an outbreak of this magnitude, Science Daily reported. The pace of its advance can be described as a mass culling of howler monkeys.

Meanwhile, anthropology professor Karen Strier of the University of Wisconsin said that they still don't know what could be the repercussion of these howler monkeys dying en masse. It is also unsure if other primates living in Brazilian forests like the black capuchin, buffy-headed marmoset, and northern muriquis will be affected with yellow fever as well. The muriquis are particularly in trouble since they are already classified as critically endangered.

Strier described the current situation of brown howler monkeys as "tensed and scary." She revealed how the supposed noisy forests are getting quieter due to yellow fever attack. Brazilian forests are also threatened with imbalance, which could make other primates as the new dominant territorial species after the howler monkeys succumb to yellow fever.