Zika already arrived in Latin America in 2013, according to a genetic study that disproves the theory that the virus was imported to Brazil during the 2014 football World Cup.
In a study published Thursday by the US journal Science, researchers at Oxford University and the Brazilian Evandro Chagas Institute analyzed the genetic code of the virus and estimated that it was imported to Brazil between May and December 2013.
The scientists looked at flight data and found that this time window coincided with increasing numbers of travellers who arrived from regions where the virus had previously been present, and with outbreaks on Pacific islands.
Zika may have been introduced during the 2013 Confederations Cup football tournament, in which French Polynesian players from Tahiti took part, but this hypothesis has yet to be proven, the study said.
"Although the American outbreak virus is most closely related to a strain from French Polynesia, it's also possible that Zika was introduced separately to the Americas and French Polynesia from South East Asia," Oxford biologist Oliver Pybus said.
The virus, which is mainly spread by Aedes aegypti mosquitoes, causes only mild flu symptoms in most patients.
However, the outbreak in Latin American countries has raised international concerns because it has been linked to birth defects in babies, as well as a neurological disorder called Guillain Barre syndrome.
World Health Organization chief Margaret Chan pointed out this week that more than half of the world population lives in areas where the Aedes aegypti mosquito is present.
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