Brazil probably experienced a single Zika virus introduction a year before illnesses were detected, and air traffic records hint that its arrival coincided with a heavy travel period, researchers who analyzed the gene family tree reported yesterday.
How and when the Asian strain of Zika virus arrived in Brazil has been one of the many mysteries surrounding the outbreak. Complex genetic sequencing studies now allow scientists to find clues about virus transmission history in the wake of outbreaks, as they did during West Africa's Ebola outbreak.
A group led by scientists from the University of Oxford and Brazil's Evandro Chagas Institute reported their findings about how the virus came to Brazil yesterday in an early online edition of Science.
Air travel–linked trigger suspected
The outbreak was first detected in May 2015, but next-generation gene sequencing of seven Zika viruses suggest one introduction that probably occurred some time between May and December of 2013. The viruses the researchers studied included samples from a blood donor, an adult whose infection was fatal, and a newborn with microcephaly and other birth defects.
The authors found little variability among the seven samples, and the comparison with other Zika viruses pointed to the last half of 2013 as the likely entry into Brazil. A comparison with airline patterns revealed the timing coincided with an increase in travelers to Brazil from Zika-endemic areas at a time when outbreaks were occurring in the Pacific Islands.
One possible scenario the team noted was virus introduction during the Confederations Cup soccer tournament, which brought players from French Polynesia, where there was a Zika outbreak between 2013 and 2014.
Oliver Pybus, DPhil, a biologist at Oxford University, said in a statement from the school that from late 2012 onward, there was a 50% rise in passengers coming to Brazil from countries with Zika virus. He said that although the Americas outbreak strain is most closely related to the one that fueled French Polynesia's outbreak, "It's also possible that Zika was introduced separately to the Americas and French Polynesia from South East Asia."
Pybus added that more information on the epidemiology and genetic diversity of Zika viruses from Southeast Asia will help shed more light on the virus's transmission history.
Nuno Faria, PhD, with the University of Oxford and the Evandro Chagas Institute, said, "There is a lot of work still to be done in terms of tracking and predicting the spread of Zika in Brazil. We will have a much better picture of the virus later this year."
Infection and microcephaly timing estimate
In an effort to better define the Zika-microcephaly risk window, the group's city-level analysis found Zika-linked microcephaly cases best track with Zika infection at around week 17 of pregnancy, or week 14 for severe microcephaly.
The team cautioned that while they did find a temporal and spatial connection between Zika virus transmission and microcephaly, the pattern does not show causation. They wrote that further studies are under way to more definitively establish if Zika infection plays a role in microcephaly or other poor birth outcomes.
Mar 24 Science study
Mar 24 University of Oxford press release
Mar 24 American Association for the Advancement of Science press release