Zika may have arrived in the American continent in 2013, more than a year before the virus was first detected in Brazil, a new study has claimed.
Researchers found that the Brazilian outbreak may have started from a single introduction of the virus into the Americas, in mid-2013.
Airline data from that time show an upsurge in the number of people travelling to Brazil, particularly from areas where Zika was circulating, researchers said. The timing coincides with when Zika virus outbreaks were occurring in the Pacific islands, they said.
According to Oliver Pybus, an evolutionary biologist at the University of Oxford in the UK and a co-author of the study, someone infected during the 2013 Zika epidemic in French Polynesia might have introduced the virus to Brazil.
The researchers found that the timing of the virus’s likely introduction to Brazil coincided with a surge in the number of travellers arriving in Brazil from Zika-affected countries.
The findings suggest that, contrary to previous speculations, fans who attended the FIFA World Cup or a championship canoe race, held in Brazil in 2014, are not to blame for bringing the virus into the country.
Most likely, the virus arrived before these events, and was circulating in Brazil for months without being recognised, the researchers found.
The high degree of genetic similarity among the samples that the researchers looked at in their study points to a single introduction of the virus, Live Science reported.
By considering both the small genetic differences among the virus samples, and the average rate at which such genetic changes are expected to happen, the researchers were able to calculate that the introduction happened sometime in 2013.
“If the Zika virus epidemic in Brazil did, indeed, arise from a single introduction, then the virus must have circulated in the country for at least 12 months prior to the first case being reported in May 2015,” the researchers said. Zika virus was first identified in rhesus monkeys in Uganda in 1947, and the first human cases were reported in 1952. Since then, Zika outbreaks have occurred in Africa and Asia, according to the World Health Organization.
In May last year, Brazil became the first country in the Americas to report a Zika virus outbreak. So far, about 30,000 cases have been reported in the country.
The study was published in the journal Science.
