Zika Spreads Through the Americas and Is Linked to Birth Defects

Late in 2014, clinicians in northeastern Brazil began investigating an unfamiliar illness marked by low-grade fever, flat pinkish rashes, bloodshot eyes, and joint aches. Although the complaints looked similar to dengue, standard evaluations eliminated it and other known conditions. By March 2015 the outbreak had reached Camaçari, near Salvador in Bahia, where patients' blood samples were sent to virologists at the Federal University of Bahia, who identified Zika virus using polymerase chain reaction testing. The virus was first formally detected in Brazil in May 2015. First discovered in a Ugandan rhesus monkey in 1947, Zika had been linked to only 14 documented cases of human disease worldwide before an outbreak on Yap Island in Micronesia in 2007.

Genetic analyses showed the virus circulating in Brazil was an Asian lineage closely related to a strain observed during an epidemic in French Polynesia. A 2016 molecular clock analysis published in Science indicated that the pathogen probably arrived in Brazil between May and December 2013, more than a year before it was detected, a period that matched a rise in air travel to Brazil from Zika-affected regions. That ruled out theories blaming visitors to the 2014 World Cup or an August 2014 canoe race; some researchers suggested the 2013 Confederations Cup, in which a team from Tahiti played. Transmission accelerated across Brazil due to an unexposed population, dense urban hubs, basic sanitation shortfalls, and widespread populations of the principal carrier, the Aedes aegypti mosquito. Unusually high temperatures linked to the 2015-2016 El Nino further favoured transmission, and one climate model found the transmission risk in South America in 2015 was the highest since 1950.