One person probably sparked Brazil’s Zika crisis
Study of molecular evolution points to single event rather than mass gathering
Among Zika’s many mysteries is how the virus slipped into Brazil, the first country hit by the epidemic. Was it ferried in by competitive rowers from French Polynesia? Did it stow away among soccer-crazed fans who arrived en masse for the World Cup?
According to the first genomic analysis of the epidemic, neither theory appears to be true. The study, published Thursday in the journal Science, suggests the once-obscure virus arrived in Brazil through a “single introduction,” probably between May and December 2013, more than a year before its detection.
This window of time coincides with a nearly 50-per-cent surge in air travel to Brazil from Zika-affected countries, the paper found.
“By the time we really picked up on Zika in 2015, it was probably cooking for a while within Brazil and within South America,” said Dr. David Fisman, an epidemiologist with the University of Toronto’s Dalla Lana School of Public Health, who was not involved with the study.
He noted the study highlighted a potential problem with outbreak investigations, which often fixate on mass gatherings.
“The general environment in terms of travel and global connectedness may be a lot more important than these super-events,” Fisman said. “We focus on pilgrimages and sporting events . . . but the background noise is that there’s just millions and millions of people moving around all the time.”
The Zika virus, first confirmed in Brazil in May 2015, is circulating in 38 countries and territories. Once considered a “mild medical curiosity,” the epidemic has exploded in Latin America and the Caribbean. There is now “scientific consensus” implicating Zika in serious neurological disorders, the World Health Organization said this week.
Brazil and Panama have confirmed cases of Zika-linked microcephaly, a birth defect associated with underdeveloped brains. A dozen countries affected by the epidemic have also reported a rise in Guillain-Barré syndrome, an autoimmune disorder.
The new paper, written by nearly 60 researchers from the United Kingdom, Brazil, United States and Canada, is the first to analyze the outbreak’s molecular evolution.
Researchers sequenced the genetic code for viral strains from seven Zika patients, including a baby with mi- crocephaly. The sample size is small — and the findings are not conclusive — but the sequences allowed researchers to deduce when the virus may have arrived in the Americas.
“This analysis of virus genomes is a very, very powerful tool,” said Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh, who was not involved with the paper. “It does seem to mostly confirm what we already suspected — but that’s valuable.’
One of the suspicions supported by this work is that the virus is most closely related to a strain from French Polynesia, where Zika caused an outbreak in 2013 and 2014. That outbreak has also been linked to a rise in microcephaly cases.
The Science paper found that in Brazil, microcephaly cases were most closely correlated with Zika infections around the17th week of pregnancy. Although some have speculated Zika may have mutated to be- come more dangerous or easily spread, the Science study found no evidence of that.
“All the virus had to do was enter a new country where the mosquito was present, with lots of people to infect,” he said. “It was always very likely to take off. It doesn’t need to change or mutate.”
Using a tool called “molecular clock analysis,” the researchers found that Zika probably arrived in Brazil before the World Cup and Va’a canoe competition in 2014 — international events that have been blamed for delivering the virus to the Americas.
They noted another sporting event — the 2013 Confederations Cup soccer tournament, which included a French Polynesian team from Tahiti — coincided with Zika’s probable debut in Brazil. But that event ended before French Polynesia confirmed its first cases, the researchers noted.
Amore probable explanation is that Zika came to Brazil through routine travel, said paper co-author Dr. Kamran Khan, an infectious disease physician and scientist at Toronto’s St. Michael’s Hospital.
The researchers identified a dozen countries, including French Polynesia and the Philippines, where Zika was either endemic or causing outbreaks between May and December 2013 — and according to data from commercial airlines, travel to Brazil from these countries grew from nearly 4,000 passengers per month in early 2013 to nearly 6,000 a year later.
For Khan, the study supports his observations over the past several years: Mass gatherings are rarely to blame for sparking outbreaks.
“All of the ingredients were there for us to recognize,” Khan said.