NEW YORK — The worrisome Zika virus apparently has been in Brazil at least a year longer than experts previously thought, according to new research.
Some experts have speculated the virus first came to the Americas sometime in 2014. But the new study — led by Brazilian researchers — concludes Zika landed in Brazil a year earlier.
The researchers coupled cutting-edge genetic sequencing of the virus with an analysis of human travel patterns.
They succeeded in piecing together "a very compelling story about both the route and the date of introduction of this virus into the Americas," said Dr. Ian Lipkin, a prominent infectious disease specialist at New York's Columbia University.
The sequence information may also help in future understanding of how the virus causes disease, Lipkin said. But it remains a mystery why the virus is increasingly linked to serious birth defects and other health problems, he added.
The study was published online Thursday by the journal Science.
Zika virus was first identified in Uganda in 1947. It is spread mainly by mosquitoes that bite infected people and then spread it to others. Most infected people suffer no symptoms. Others get only a mild, week-long illness.
For decades it was considered exotic and non-threatening. The virus began to pop up in other parts of the world in the last decade, but it was not detected in the Americas until last year, when it was reported in Brazil.
The new research is based on blood and brain tissue samples taken last year from seven Brazilians. One was a blood donor from the southeast part of the country The other six were from northern Brazil, where Zika virus disease outbreaks — and Zika-associated birth defects — have been most pronounced.
The researchers extracted genetic and found the virus specimens were similar to each other, and most closely related to Zika virus found in French Polynesia in 2013.
Some scientists have speculated the virus first landed in Brazil in 2014. But viruses mutate over time and from place to place. The small amount of genetic change suggests it arrived in 2013, and at a single location, the researchers concluded.
A good bet is the virus landed in June 2013 in the city of Recife, around the time of the 2013 Confederations Cup soccer tournament in Brazil, said one of the study's lead authors, Dr. Pedro Fernando da Costa Vasconcelos of Brazil's Health Ministry.
The tournament included a team from Tahiti, part of French Polynesia. Tahiti played in Recife in northeast Brazil and in two cities in southern Brazil.
It could have been carried by one or more infected travelers from French Polynesia. Or perhaps the virus was introduced to Brazil and French Polynesia at about the same time — carried by people from some third location, said the University of Oxford's Oliver Pybus, another study co-author, in an email.
Also on Thursday, U.S. health officials said 273 lab-confirmed Zika infections have been reported in the 50 states in the last year — a count that includes 19 pregnant women. Almost all of them were travelers, but six caught it from sex with recent travelers. Officials blame mosquito transmission for nearly all of the 341 cases reported in Puerto Rico and two other U.S. territories.
The kind of mosquito that spreads Zika is found on the southern United States, so experts think it's likely the pests will spread the virus there, too.