Brazil's First Urban Yellow Fever Outbreak in Nearly a Century May Have Been Fueled by a Surprising Culprit

Between 2016 and 2018, more than 2,000 people fell ill with yellow fever in Brazil, and nearly 750 of them died. It was the country’s first urban outbreak of the disease in decades. Now, scientists have proposed a possible explanation of how the deadly epidemic unfolded.

In a study published August 14 in the journal Science Advances, they make the case that an extreme, once-in-a-century drought may have helped the disease proliferate. Based on computer simulations, the scientists suspect that the drought changed the behavior of forest-dwelling howler monkeys, marmosets and mosquitoes, which are all vectors of the virus that causes yellow fever. With little water available in the forest, these nonhuman primates and insects likely increasingly headed toward urban areas in search of the vital liquid.

Additionally, the mosquitoes may have started biting more to avoid drying out. Together, these behavioral changes probably contributed to the spread of yellow fever among humans, according to the researchers. The paper “makes a really strong case” for this scenario, Noah Rose, an ecologist at the University of California, San Diego who was not involved with the research, tells Science’s Ailie McWhinnie.

However, in addition to very wet conditions, the new study hints that extremely dry conditions may also influence the spread of disease—an important consideration in the face of human-caused climate change, which is contributing to more frequent and more severe droughts. Yellow fever is caused by a virus that typically spreads among humans via bites from infected Aedes aegypti mosquitoes. Most people show no symptoms or experience a mild illness that can involve fever, chills, severe headache and body aches, nausea, vomiting, fatigue and weakness.

Symptoms typically improve within a week, although fatigue can linger for months. However, a small percentage of people develop a more severe version of the disease, with symptoms including high fever, jaundice, bleeding, shock and organ failure. Severe yellow fever is fatal between 30 and 60 percent of the time.

Fortunately, a single dose of a vaccine can provide lifelong protection. Yellow fever remains a persistent public health threat in Africa and South America, where an estimated 67,000 to 173,000 severe infections and 31,000 to 82,000 deaths occur each year. Before Brazil’s most recent outbreak, the country’s last urban bout with yellow fever occurred in 1942.