Originally, it was suspected that the mosquito-borne Zika virus entered Brazil in 2014 with travelers to the FIFA World Cup or a championship canoe race. New research suggests that the virus actually entered Brazil a year earlier, in 2013.
Researchers wrote that they analyzed the Zika gene sequence from several samples collected in Brazil. Samples included one from a blood donor, one from a fatal case and one from a newborn with microcephaly (the congenital disease linked to the outbreak). They compared this information to airport flight data from all countries with reported Zika outbreaks between 2012 and 2014.
Among the samples, they found little genetic variability. The cases were closely related with a strain that circulated in French Polynesia in November 2013. This suggests that all of the cases were caused by viruses from a single origin, a single introduction into the country. Thus, someone from French Polynesia might have introduced the virus to Brazil.
Martin Hibberd, a professor of emerging infectious diseases at London School of Hygiene and Tropical Medicine, an expert who was not involved on this recent study says,
The introduction of one Zika virus leading to a widespread outbreak may seem surprising. However, the modeling of other Zika outbreaks, and also the highly-related Dengue outbreaks, suggest that this is not unusual. In the right conditions with sufficient mosquitoes and closely-packed humans, the virus can spread rapidly.
The Zika virus was first identified in rhesus monkeys in Uganda in 1947 with the first human cases being reported in 1952. According to the World Health Organization, outbreaks have occurred in Africa and Asia. Between 2013-2014, outbreaks of Zika were reported in several Pacific islands, including Tahiti and French Polynesia. Brazil became the first country in the Americas to report the Zika virus in May 2015. About 30,000 cases have been reported in Brazil, so far.
Zika rarely has any symptoms. Any symptoms that do occur mimic those of dengue fever or chikungunya. The virus has been reported in the southern United States as well.