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Yellow Fever — Once Again on the Radar Screen in the Americas
Over the past several weeks, a fifth arbovirus, yellow fever virus, has broken out in Brazil, with the majority of the infections occurring in rural areas of the country. These are referred to as sylvatic, or jungle, cases, since the typical transmission cycle occurs between forest mosquitoes and forest-dwelling nonhuman primates, with humans serving only as incidental hosts. In this ongoing outbreak, health authorities have reported 234 confirmed infections and 80 confirmed deaths as of February 2017. Confirmed infections have occurred in the Brazilian states of Minas Gerais, Espírito Santo, and São Paulo, and hundreds of additional cases remain under investigation. The high number of cases is out of proportion to the number reported in a typical year in these areas.
Although there is currently no evidence that human-tohuman transmission through Aedes aegypti mosquitoes (urban transmission) has occurred, the outbreak is affecting areas in close proximity to major urban centers where yellow fever vaccine is not routinely administered. This proximity raises concern that, for the first time in decades, urban transmission of yellow fever will occur in Brazil.
Yellow fever is the most severe arbovirus ever to circulate in the Americas, and although vaccination campaigns and vector-control efforts have eliminated it from many areas, sylvatic transmission cycles continue to occur in endemic tropical regions. The most recent outbreak in Brazil highlights this phenomenon. If the current outbreak leads to urban spread through A. aegypti mosquitoes, clinicians should adopt a high index of suspicion for yellow fever, particularly in travelers returning from affected regions. As with all potentially reemerging infectious diseases, public health awareness and preparedness are essential to prevent a resurgence of this historical threat.
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