In a "tour de force genetic investigation of the outbreak" according to a related Perspective, scientists have shown how the recent yellow fever outbreak in Brazil originated in nonhuman primates in the forest and spilled over into human populations. The study also demonstrates the power of the various techniques involved to monitor infectious disease outbreaks in real time, critical for developing countermeasures. Despite the fact that there's an effective vaccine for yellow fever, the virus causes 29,000 to 60,000 deaths annually in South America and Africa. Brazil has recently experienced its largest-recorded yellow fever outbreak in decades, with more than 2,000 confirmed cases and 676 deaths between December 2016 and March 2018. To understand how this outbreak occurred, Nuno Faria et al. analyzed epidemiological, spatial, and genomic data from the region. First, they compared a time series of confirmed cases in humans and nonhuman primates (NHPs), finding that that human cases lagged behind those in NHPs by four days. As well, they found that the risk of yellow fever was greatest for people who reside or work in forested areas, where the mosquitoes that usually only feed on NHPs may occasionally bite and transmit the virus to humans. The results are striking: near the origins of the outbreak, 85% of cases are in men, who are more likely to travel through remote areas of the jungle than women. The researchers also sequenced 62 yellow fever genomes from infected humans and NHPs from the most affected Brazilian states, comparing these genomes to previously published ones. The data suggest that the 2017 outbreak was more likely caused by a strain introduced from an endemic area, possibly northern or central-western Brazil, rather than by the reemergence of a lineage that had persisted in the outbreak region of Minas Gerais. Although the epidemic was likely initiated in NHPs, the spread of the virus appears to have been aided by human activity, for example by transport of infected mosquitoes in vehicles or by illegal trade of NHPs, the authors say. Alan Barrett highlights these findings in a related Perspective, in which he discusses the value of modeling disease outbreaks for developing public health countermeasures.
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Journal
Science