No pre-existing immunity to Zika, a large concentration of Aedes aegypti mosquitoes and travelers moving back and forth from Zika-infested countries were all likely contributors to the outbreak of the virus in Miami last year, according to a new series of studies published in Nature.
Scientists believe that the latest outbreak of Zika likely began in Brazil sometime in early 2014, even though it wasn't detected until over a year later in May 2015.
By then, the virus had begun to spread to other Latin American countries and eventually to the Caribbean. After that, it made its way to Florida in March 2016, but the outbreak wasn't recognized until July of that year.
Since then there have been 256 documented cases of Zika in Miami.
In their papers, scientists from the United States and around the world, describe how they're able to track the path of the virus after successfully sequencing it last year, or in other words, after mapping its genetic blueprint.
Sequencing Zika's genome wasn't easy, said Kristian Andersen, professor at the Scripps Research Institute in La Jolla, and one of the papers' authors. He compared sequencing Zika to reverse engineering a fully cooked turkey dinner. You know what the final result is (or what the virus looks like) when it's whole, but you have to figure out all of the little parts that it took to make it.
To sequence the virus, Andersen's lab and others had to get a hold of human and mosquito samples positive for Zika, which was easier said than done. Symptoms of the virus often don't manifest in a carrier, so it can be tough to identify them to collect samples. And even if someone notices symptoms and does provide a sample, there's often little of the virus to study.
When they got a hold of viable samples they isolated RNA of the virus, sequenced it and then started to identify mutations in different versions of Zika.
"Basically the blueprint of the virus is slightly different in each infected individual," said Andersen.
This allowed scientists to match genetic mutations in different viruses from samples in Florida to mutations in samples taken from the Caribbean, and other parts of Latin America.
It wasn't just one mosquito and one bite that was responsible for the outbreak, said Andersen. He estimates that different versions of the virus made their way to Florida between 30 to 40 times over the course of 2016.
"It's not just a one off event where the virus comes over once and then it causes the entire outbreak. We actually think that this, the size of the outbreak in Florida, was caused by multiple introductions into the country," he said. "Most of these probably only lead to a single or maybe two cases and then it stops. And then there's no more cases of that version of the virus."
The good news?
"The number of mosquitoes and the number of human cases follow each other almost perfectly," said Andersen. "So this means if the number of mosquitoes go down, the number of human cases go down the same way."
As of December 2016, Miami was declared free of locally transmitted Zika.