What Caused Brazil's First Urban Yellow Fever Outbreak in 80 Years?

Rural and outdoor exposure increased risk of yellow fever infection during the 2017-2018 epidemic in Brazil, begging the question of what caused the sudden spike in cases, a researcher said.

After controlling for yellow fever vaccination status, month, and year, characteristics associated with increased odds of being a confirmed case of yellow fever were male sex, ages 16 to 65 ("working age"), and recent travel, reported Joelle Rosser, MD, of Stanford University in California.

Farmers and rural workers appeared to be the first to be infected, with three times higher odds of yellow fever in the first year of the outbreak compared with the second year.

Based on these and other environmental factors, Rosser presented her hypothesis that drought was the cause of the epidemic at a presentation during the virtual American Society of Tropical Medicine & Hygiene (ASTMH) annual meeting.

She detailed the history of yellow fever, and why this outbreak was so unusual. She noted that there had not been a yellow fever outbreak in large Brazilian cities, such as Rio de Janeiro, since the 1940s, thanks to aggressive vector control.

Rosser said the outbreaks used to be driven by urban-dwelling Aedes Aegypti mosquitoes, but had since been driven by the forest-dwelling Haemagogus breed. In fact, research found that Haemagogus, not Aedes Aegypti, drove the 2017-2018 epidemic.

Dehydration increases the mosquito biting rate, and parasite load in howler monkeys -- which are the usual hosts of yellow fever -- increases in fragmented environments, she explained. Her group postulated that the drought drove the mosquitoes, which can travel long distances quickly, and the howler monkeys to the "rural/urban interface," where they were when the rainy season started.

"Once the rainy season started, mosquitoes proliferate in concentrated areas. This allows for amplification of transmission between forest mosquitoes and non-human primates, spilling over into humans," Rosser said.

Not only that, but she also noted decreased competition from Aedes Aegypti mosquitoes in urban areas, due to aggressive vector control following outbreaks of dengue fever and Zika virus.

"The drought brought howler monkeys, forest mosquitoes, and humans into closer contact at the rural/urban interface," she concluded.

The clinical data supporting this theory were from a database of "confirmed versus discarded cases" of yellow fever, including 2,097 confirmed cases and 6,999 discarded cases. A higher proportion of men and people ages 16 to 65 comprised the confirmed versus discarded cases:

• Men (82.3% vs 63.4%, respectively)
• Ages 16-65 (88.1% vs 78.3%)

Likewise, no recent travel was reported in a smaller proportion of the confirmed versus discarded cases (81.0% vs 94.1%).

Rosser said the next steps of her work would be to examine the interaction between drought and land use practices, in the hopes of helping to prevent another severe yellow fever outbreak.

"Drought is a very complex variable, and a number of factors affect drought beyond temperature and precipitation," she told MedPage Today. "Water management and land management are really important factors in drought and in habitats available for non-human primates and mosquitoes."

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