The Zika virus may have originated in Polynesia and been taken to South America by Pacific Island canoeing teams competing in Rio de Janeiro in 2014, new research has shown.
The study, published in PLOS Neglected Tropical Diseases, suggests the Zika virus sweeping through Latin America may have been introduced to Rio de Janeiro during the 6th World Sprint Championship canoe race in August 2014.
The race included teams from four Pacific countries French Polynesia, New Caledonia, Cook Islands and Easter Island where the virus circulated during 2014.
Zika is believed to cause defects in the brains of fetuses if women become infected during pregnancy.
As part of the study, researchers observed 119 Brazilian patients who tested positive for Zika. None of those studied had travelled in previous months, confirming the virus was acquired locally.
The researchers looked at 10 randomly selected positive samples and found similarities in the Brazilian strain of the virus and Asian strains.
Professor Michael Baker, of the University of Otago in Wellington, said the timing was plausible because of the intense outbreaks in 2014 in several Pacific countries and then a few months later that same virus in January 2015 in Rio.
"They speculate that there were quite large teams of canoers who came from Pacific specifically to Rio so it does seem like a plausible hypothesis."
Professor Baker said although the hypothesis was speculative, it was a reminder of how easily viruses could "hitchhike" to different parts of the world.
"However, it is a reminder that emerging infectious diseases will frequently 'hitchhike' across the globe in infected humans, who may be asymptomatic [show no symptoms]."
University of Otago professor Philip Hill said the assertion that the virus had originated from the Pacific was purely speculation.
"My guess is that this study shows the virus they have found in this small population in Brazil likely originated from Asia-Pacific somewhere and it found its way into this group of people around 2014/15, but it could easily have started off in Asia a long time before then and come from another part of Brazil in 2014/15. You can't be more specific than that."
The World Health Organisation has said the virus has been reported in 61 countries and territories, mostly in Latin America and the Western Pacific region.
Most people who get the virus have no symptoms. Others may suffer from a mild and brief illness, but infections in pregnant women have been strongly linked to fetal deaths and to potentially devastating birth defects such as babies born with abnormally small heads.
Q&A: Zika virus
What are the classic signs of Zika?
The most commonly reported disease symptoms are a rash, itchiness, exhaustion, headache and joint pain. Only about one third of confirmed patients recall a brief fever at the start of their symptoms.
Should we be worried?
University of Otago in Wellington professor Michael Baker said on a global scale we should be because it was another problem for people often living in developing resourced countries. But in New Zealand the general consensus is that Zika cannot be established here because we do not have mosquitoes capable of transmitting it to other people.