The Zika virus that spread through South Florida last year likely started two months before it was confirmed, was introduced by at least four infected people — and as many as 42 — and originated in the Caribbean, according to a new study published Wednesday in the journal Nature.
Using gene sequencing, a team of scientists looked at infected patients and mosquitoes in Florida to figure out how Zika was transmitted in the state. Was it here but invisible before the first transmission zone was confirmed in July? Did mosquitoes spread the virus linked to severe birth defects, or was it infected people? Was there anything unique about the four counties — Miami-Dade, Broward, Palm Beach and Pinellas — that ultimately began spreading the disease?
It turns out that an abundance of the Aedes aegypti mosquitoes, not infected travelers, played a bigger role.
“It sounds obvious, but most locally acquired cases were occurring when Aedes aegypti were high,” said University of Florida epidemiologist and co-author Derek Cummings. “That makes sense intuitively: When you have more cases, you have more Aedes aegypti mosquitoes.”
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In looking at the genes, the team — scientists from 32 organizations including the Florida Department of Health, the University of Miami and Miami-Dade County’s Mosquito Control Division — found that at least four infected people introduced the virus, although the number of separate introductions could be as high as 42. They found more lines of the virus from the Dominican Republic and Guadeloupe, suggesting that while the outbreak occurred throughout the Americas, the Caribbean was the more likely source for the Florida transmission. They also found that mosquitoes carrying the virus were not confined to a single transmission zone, suggesting that the virus moved around in the mosquito population.
“Each one of these introductions on average doesn’t give rise to new cases, but every once in a while you get these little flare-ups and the flare-ups last year were concentrated in areas where Aedes aegypti numbers were really high,” Cummings said. “So this year that’s an area we really need to focus on. Increases in surveillance will help us know when we’re having high numbers of Aedes aegypti.”
To corroborate their findings, scientists looked at infection rates and travel patterns and found that the Caribbean had the highest rate of incidence even though Brazil and Colombia — both more populated countries — recorded the highest absolute number of cases. Also, about 67 percent of patients with travel-related infections reported visiting the Caribbean, including Puerto Rico, where about 40,000 cases have been confirmed, the Dominican Republic, Jamaica and Haiti.
The team also found some good news. When they modeled the reproduction rate of the Florida virus, they found it was unable to sustain itself. Even at the high end, it was not likely to survive over a year. Worldwide, the Pan American Health Organization has reported a general decline in numbers with a few exceptions, including Guatemala, Bolivia, Brazil, Ecuador and Peru.
Two other studies were also published Wednesday in Nature that used gene sequencing to reconstruct the spread of Zika, confirming again that the virus in Brazil began spreading by late 2013 or early 2014 in the northeast part of the country and spread from there. They also determined that the virus spread undetected in many areas for months before the first locally transmitted case was confirmed, highlighting the need for better surveillance and the ability to track viruses through gene sequencing in close to real time.
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