The outbreak of Zika virus in the Americas could be down to just one passenger that stepped off a plane in 2013.
New research suggests that the virus could have arrived in Brazil more than a year before it was first reported in March 2015, scotching the notion that it was brought to Brazil during the 2014 World Cup, or arrived during the Va’a World Sprint Championship canoe race.
More than 30,000 cases of infection with the Zika virus have been reported in Brazil. While symptoms are in most cases mild, concerns have been raised about infections in pregnant women after a possible link was made between the virus and cases of microcephaly in babies. There is also a potential link to Gullain-Barré syndrome, a paralysing nerve disorder, in adults.
The new research describes how a large team of researchers from the US, Canada, UK and Brazil narrowed down a possible window in which the virus could have come to the Americas.
Published in the journal Science, the study reveals that the team took samples from seven individuals in Brazil infected with the Zika virus during the recent outbreak - including a newborn baby, diagnosed with microcephaly, who had died. They then carried out next-generation sequencing to generate the Zika virus genomes and compared these genomes to each other, as well as to other Zika virus genomes from across the Americas.
The results showed that the strains are more similar to those present in Asia than in Africa. What’s more, the scientists discovered that all the strains found in the Americas came from a single common ancestor - indicating that the virus could have been brought to the Americas by just one individual.
To work out where the virus could have come from, the scientists compared the Zika virus genomes from across the Americas with those from other affected countries. The best match was found with the strain of Zika virus found in French Polynesia - a collection of islands in the South Pacific which experienced a Zika outbreak during late 2013 and early 2014.
The suggestion was backed up when the researchers looked at the times at which the samples were taken from infected individuals, and compared the differences in the genomes of the virus. “You are doing a family tree or genealogy of the virus, but you are putting it on a real time-scale of months and years,” Professor Oliver Pybus, an author of the study from the University of Oxford, told the Guardian.
From their results the scientists found that the most likely time that Zika virus entered the Americas was between May and December 2013 - more than a year before the virus was first detected in Brazil.
“This new study highlights the important finding that Zika virus was in Brazil long before it was first detected,” said Dr Laith Yakob from the London School of Hygiene and Tropical Medicine, who was not involved in the study. “It was missed because symptoms are usually mild and similar to those of other mosquito-borne viruses like dengue and chikungunya,” he added. “There is a great need for inexpensive, rapid and sensitive tests to correctly diagnose the different viral infections.”
To find out how the virus could have entered the country, the authors of the new study examined records of air travel into Brazil from countries where the Zika virus had been reported between 2012 and 2014. Their results showed that numbers increased dramatically between early 2013 and early 2014, rising from 3775 passengers a month to 5754 passengers a month, suggesting that the virus could have been carried by a plane passenger.
But Pybus believes more work is needed before it is confirmed that the virus entered by this route, or that it came from French Polynesia. “We know that there have been reports of Zika infection in places like Thailand and the Philippines and Indonesia, but there is no systematic survey or data from these locations,” he said. “There is a chance that both the outbreak in French Polynesia and the outbreak in the Americas independently originated from somewhere in Southeast Asia.”
The authors also point out that the Zika virus could have entered the Americas elsewhere but note that the fact that Brazil was the first country to report infections. “Either it went into Brazil and then to lots of other places, or it has come into Brazil multiple times from other places,” said Pybus.
While the new study debunks the idea that the Zika virus arrived in Brazil during the 2014 World Cup, its authors tentatively suggest that it could have arrived during the 2013 Fifa Confederations Cup. But Pybus urges caution about running away with the idea. “It would fit the timeline better than the World Cup, but it is still anecdotal, circumstantial evidence,” he says. “These specific events aren’t really great hypotheses - they do make great stories but they don’t make great science because there is no way to test this.”
