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The 2014 World Cup isn't to blame for the spread of the Zika virus in Brazil

Scientists estimate the virus arrived in Brazil in 2013

Soccer: World Cup-Germany vs Brazil Witters Sport-USA TODAY Sports

World Cup travelers who came to Brazil in 2014 probably weren't the source of the country's Zika virus outbreak, a study published today in Science reveals. A DNA analysis of seven Zika virus samples suggests the virus arrived in Brazil sometime between May and December 2013 — a full six months before the June soccer tournament.

For months now, scientists and reporters have speculated that the 2014 World Cup facilitated the arrival of the Zika virus in Brazil. But a comparison of virus samples from Brazil with virus samples from other countries points to an earlier introduction in mid-to-late 2013 — a period when the country experienced a 50 percent rise in the number of airline travelers from Zika-afflicted areas, researchers note. Today's report also shows that the Brazilian virus is most closely related to Zika from French Polynesia. This could mean that air passengers from that region brought the virus to Brazil more than two years ago.

Zika arrived a full six months before the World Cup

The Zika virus was first detected in Brazil in March 2015. It's relatively harmless in most cases; only 20 percent of infected persons develop symptoms, which are similar to the flu and last no more than a week. But there’s no treatment or vaccine for the virus, which is concerning given that Zika has been linked to microcephaly — a birth defect that can affect the brain size of newborns — as well as Guillain-Barré Syndrome, a severe neurological disorder. That's why researchers want to figure out how the virus arrived in Brazil; figuring out the source and the speed of its spread could help create better interventions and reduce the virus' impact.

To decipher the timing of Zika's introduction in Brazil, scientists sequenced the entire genome of seven Zika virus samples taken from patients in Brazil after the virus was detected in 2015. Among them was a sample taken from a baby with microcephaly who died after birth. By comparing the sequences to virus samples from other countries, the researchers found that the Brazilian form of the virus is closely related to the Zika virus that's found in French Polynesia. The researchers also used small molecular changes in the virus' DNA to create a timeline of its introduction in Brazil. They estimate that it was likely introduced during a 7-month period at the end of 2013.

It makes more sense to look at patterns of travel than single sporting events

The new timeline could mean that the virus was introduced in Brazil by travelers during the Confederations Cup soccer tournament in 2013 — a tournament that involved French Polynesian participation from Tahiti. But that tournament occurred before cases of Zika were reported in Tahiti, Oliver Pybus, an evolutionary biologist at the University of Oxford and a study co-author, told Science magazine. Because of this, it might make more sense to look at larger patterns of travel rather than pointing at a single sporting event, he said. In addition, the researchers were limited in their conclusions because they didn't have samples from all the countries that have reported cases of Zika. It's therefore still possible that a traveler from a country that wasn't sampled brought the virus to Brazil.

The study is significant because it builds on years of mapping the Zika genome, says Lee Norman, an intelligence officer in disaster medicine planning in the United States Army National Guard, who didn’t work on the study. "I think of it as the most recent addition to understanding the 'march across time and geography' the Zika virus is taking." And that could be very useful for researchers because understanding the genetic make-up of the newest cases can help predict and model the future, he says. For instance, knowing the speed with which the virus changes on a molecular level could aid the search for an effective vaccine or treatment. It can also help scientists create budgets for interventions and plan how they might limit travel to affected areas, Norman says.

How did Zika persist without causing a large number of cases?

For Andrew Pekosz, director of the Center for Emerging Viruses and Infectious Diseases at Johns Hopkins Bloomberg School of Public Health, the study prompts more questions than it provides answers. For instance, if the virus was in Brazil for months before human cases were detected, how did it persist without causing a large number of cases? "That would be an important thing to investigate," he says. "Perhaps there were low numbers of infections in humans for that year which never registered with public health authorities." If that's the case, then Zika cases may have been misdiagnosed as Chikungunya or Dengue, Pekosz says. It's also possible that the virus infected other animal species or that it was maintained in mosquitoes by sexual transmission or passed from an infected female mosquito to her eggs. In short, the delay in the outbreak means that something researchers don't yet understand occurred in Brazil.

Even the timeline itself is "a bit more of a guess" because the study doesn't include virus sequences from the 12 to 18 months between the predicted introduction and the appearance of large numbers of cases in Brazil, Pekosz says. Overall, the timeline is "quite interesting," but it's still "very speculative."

Since the outbreak was first reported, around 30,000 people in Brazil have been infected with the Zika virus. Given all the steps health officials are taking to control Zika's spread, figuring out how the virus' genetic make-up has changed in the past "may or may not be a good predictor of what will occur," Norman says. But building a timeline of the virus' introduction is valuable nonetheless. "It is the best we have for prognosticating the future and doing public health and public policy planning," he says.