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Epidemiology

Virus behind yellow fever outbreak in São Paulo probably came from Amazonia

James Gathany/CDC Mosquitoes of the genera Haemagogus and Sabethes (photo) transmit the yellow fever virus to monkeys and humans in forest areasJames Gathany/CDC

The yellow fever virus now circulating in northern São Paulo State most likely came from the Amazon. Virologist Renato Pereira de Souza and his team at the Adolfo Lutz Institute (IAL), in the city of São Paulo, reached this conclusion after partial sequencing of the virus extracted from monkeys found dead in the São José do Rio Preto and Ribeirão Preto regions in the latter half of 2016. “The viruses that infected these animals bear greater genetic resemblance to those circulating in Amazonia than to those from earlier outbreaks in São Paulo,” says Souza, director of the institute’s Division of Vector-Borne Diseases, who also stated that “Amazonia may be the source of the viruses behind the current outbreak in São Paulo and other states.” As part of his doctorate, completed in 2013, Souza analyzed changes in the genetic characteristics of the yellow fever virus in South America from 1930 to 2008. When he reconstructed the genetic evolution of the virus across time and space, he noted that the point of origin was always Amazonia, where the virus circulates among humans and monkeys in forest areas, transmitted by mosquitoes of the genera Haemagogus and Sabethes. Once in a while, the virus is exported to other regions – probably catching a ride in the blood of asymptomatic carriers – where it survives for short periods. “I believe this picture is repeating itself,” says Souza. The 2017 outbreak has been the biggest in 14 years. In January alone, 127 cases were confirmed in four states, with 47 reported deaths (42 in Minas Gerais, three in São Paulo, and two in Espírito Santo). In late 2016 and early 2017, the IAL team observed that the virus has been striking more species of monkeys in São Paulo. It had previously been found in howler monkeys and has now been detected in black capuchins and marmosets. Researchers are concerned the virus might spread into regions home to an abundance of the Aedes aegypti mosquito, which also transmits dengue, Zika, and chikungunya. If yellow fever reaches the Aedes mosquito, it might become an urban illness. “We’re not prepared for this,” says Souza.

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