Researchers have yet to confirm the exact route and timeline for Zika's entry into Latin America.
The mosquito-borne Zika virus behind last year's outbreak in Rio de Janeiro closely resembles another strain from Asia and may have been introduced by Pacific Island athletes, researchers said Tuesday.
Their report in the journal PLOS Neglected Tropical Diseases offers the first detailed analysis of the outbreak in a Latin American city, and gives further weight to the hypothesis that Zika may have been brought to Brazil by foreign visitors who came to compete in a canoe championship in late 2014.
The researchers also raised new concerns about the potential for an even worse Zika outbreak during this summer's Olympic Games in Rio, saying many of those studied did not remember being bitten by mosquitoes and that transmission of Zika infection in clusters of people appeared to be commonplace.
"At this point, health services must be alerted to the potential for an even larger epidemic during the summer of 2015-2016 spreading to additional locations and affecting the susceptible proportion of the population that was not exposed during the last transmission season," the study warned.
Zika virus is strongly suspected to be behind a surge in microcephaly, a birth defect in which babies are born with unusually small heads, although a definitive link between microcephaly and Zika has not yet been proven.
Brazil has seen thousands of cases.
While still "speculative," 10 randomly selected samples were "clustered within the Asian genotype," suggesting the virus "was possibly introduced to Rio de Janeiro during the VI World Sprint Championship canoe race in August 2014, which included teams from four Pacific countries (French Polynesia, New Caledonia, Cook Islands, and Easter Island) where the virus circulated during 2014," the study said.
Researchers have yet to confirm the exact route and timeline for Zika's entry into Latin America.
A separate study published last month in the journal Science sequenced the genomes of seven Brazilian Zika samples, taken from March to November 2015, and suggested that Zika may have arrived even earlier, perhaps between May and December 2013, on an inbound flight from French Polynesia or Southeast Asia.
Challenge to 'northeast' theoryAnother key finding from Tuesday's study in PLOS was evidence of cases in Rio de Janeiro -- the first city with a high proportion of cases confirmed by molecular diagnosis -- as early as January 2015.
Brazilian officials first notified the public of the Zika virus in May 2015.
"Our findings have demonstrated that Zika virus was circulating in Rio de Janeiro at least five months before its detection was announced by the health authorities," said the study, led by Patricia Brasil of the Oswald Cruz Foundation in Rio de Janeiro.
The study is based on a total of 364 patients who suffered an acute skin rash from January through July 2015.
Since Zika is related to viruses that cause dengue fever and chikungunya, and some symptoms can be similar, researchers relied on blood samples available from 262 of the patients to test for the virus.
They confirmed Zika in 119 of the samples. None of those tested had traveled recently, confirming that the infections had been locally acquired.
Eleven percent of the cases were diagnosed prior to May 2015, when the virus was first reported in northeast Brazil.
That challenges the theory that the virus entered Brazil from the northeast before spreading to the rest of the country, researchers said.
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