The discovery of three dead monkeys in a forested area of Colombia’s Putumayo department became the unexpected but vital warning signal that led to the early detection of a yellow fever outbreak, according to PAHO.
The dead primates — a howler monkey, a white-faced monkey, and a spider monkey — were found in close proximity, prompting immediate investigation. “Thanks to the training we received on proper sample collection in primates, we were able to detect a yellow fever outbreak after discovering the three dead monkeys at the same time,” said Wilder Pérez of the Putumayo Health Secretariat’s Vector-Borne and Zoonotic Disease Program. “Without that knowledge, the animals might have been buried without analysis — and we would have missed a critical opportunity to act.”
The training, provided under the Pan American Health Organization’s PROTECT initiative and funded by the Pandemic Fund, included instruction in safe wildlife sampling, vector surveillance, and cross-sectoral coordination. Pérez had participated in the first PAHO/WHO workshop on early warning signs, held months earlier in Tolima — one of the departments now hardest hit by yellow fever.
After the monkey deaths were reported, swift containment measures followed: environmental workers were vaccinated, entomological studies were launched, and surveillance teams analyzed the surrounding area. These efforts confirmed the virus’s presence and helped prevent its spread to urban areas.
Colombia has reported 114 confirmed human cases of yellow fever, including 49 deaths, between 2024 and the end of June 2025 — a fatality rate of 42.9%. Tolima alone has confirmed 95 of those cases, including 35 deaths, making it the worst outbreak outside the Amazon region in the country’s recent history. Meanwhile, at least 56 confirmed epizootics — monkey deaths due to yellow fever — have been recorded nationwide.
All human cases have been linked to the sylvatic transmission cycle, where mosquitoes in forested areas transmit the virus from infected monkeys to humans. The outbreak underscores the importance of maintaining strong vaccination coverage in rural areas and using a One Health approach that integrates human, animal, and environmental surveillance.
The latest training session — the Second Workshop on Epizootic and Vector Surveillance — took place in May 2025 in Mocoa, Putumayo. It brought together health and environmental professionals from more than ten territories, as well as Colombia’s Ministry of Health, the National Institute of Health, the Ministry of Environment, National Parks, PAHO/WHO, PANAFTOSA, and Brazil’s Ministry of Health. Together, they shared strategies for improving early detection and outbreak response.
“These experiences help share knowledge between regions, standardize protocols, and support more timely interventions,” said Mauricio Vera of Colombia’s Ministry of Health and Social Protection.
The strengthened surveillance system now in place may prove to be one of Colombia’s most important tools in preventing future outbreaks of yellow fever and other zoonotic diseases.
