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Reemergence of yellow fever virus in southeastern Brazil, 2017-2018: what sparked the spread?

Background The 2017-2018 yellow fever virus (YFV) outbreak in southeastern Brazil marked a reemergence of YFV in urban states that had been YFV free for nearly a century. Unlike earlier urban YFV transmission, this epidemic was also driven by forest mosquitos. The objective of this study was to evaluate environmental drivers of this outbreak. Methodology/Principal Findings Using surveillance data from the Brazilian Ministry of Health of human and non-human primate (NHP) cases of yellow fever, we traced the spatiotemporal progression of the outbreak. We then assessed the epidemic timing in relation to drought using a monthly Standardized Precipitation Evapotranspiration Index (SPEI). Lastly, we evaluated demographic risk factors for rural or outdoor exposure amongst YFV cases. Both human and NHP cases were first identified in a hot, dry, rural area in northern Minas Gerais before spreading southeast into the more cool, wet urban states of Espírito Santo, São Paulo, and Rio de Janeiro. Outbreaks also coincided with drought in all four southeastern states of Brazil. Confirmed YFV cases had an increased odds of being male (OR 2.58; 95% CI 2.28-2.92), working age (OR: 2.03; 95% CI: 1.76-2.35), and reporting recent travel from an urban to a rural area (OR: 5.02; 95% CI: 3.76-6.69). Conclusions/Significance The 2017-2018 YFV epidemic in Brazil originated in hot, dry rural areas of Minas Gerais before expanding south into urban centers. An unusually severe drought in this region may have created environmental pressures that sparked the reemergence of YFV in Brazil’s southeastern cities. Author Summary In 2017-2018, cities in southeastern Brazil experienced an unusual outbreak of yellow fever virus. In the early 20th century, these cities had large outbreaks of yellow fever, spread by Aedes mosquitos. But until this recent outbreak, they had been free of yellow fever for nearly a century. While this outbreak was spread by Haemagogous forest mosquitos, the reemergence of yellow fever in densely populated urban areas raises serious concerns about it reestablishing ongoing transmission in cities, spread by urban Aedes mosquitos. Our study sought to understand how and why yellow fever virus remerged in this area. We traced the outbreak, finding that it started in hot, dry, rural areas and spread south into cool, wet urban areas. Additionally, the outbreak coincided with a severe drought; this extreme weather may have promoted the spread of yellow fever. Infection was also associated with rural and outdoor exposure, further suggesting this epidemic originated in rural areas.