Photo by Faith Hark
Nathan D. Grubaugh (left) and Karthik Gangavarapu mapping the spread of Zika
A study published in Nature appears to explain how Zika virus entered the US via Florida in 2016 and how the virus might re-enter the country this year.
By sequencing the Zika virus genome at different points in the outbreak, researchers created a family tree showing where cases originated and how quickly they spread.
The team discovered that transmission of Zika virus began in Florida at least 4—and potentially up to 40—times last year.
The researchers also traced most of the Zika lineages back to strains of the virus in the Caribbean.
“Without these genomes, we wouldn’t be able to reconstruct the history of how the virus moved around,” said study author Kristian G. Andersen, PhD, of The Scripps Research Institute in La Jolla, California.
“Rapid viral genome sequencing during ongoing outbreaks is a new development that has only been made possible over the last couple of years.”
Why Miami?
By sequencing Zika virus genomes from humans and mosquitoes—and analyzing travel and mosquito abundance data—the researchers found that several factors created a “perfect storm” for the spread of Zika virus in Miami.
“This study shows why Miami is special,” said study author Nathan D. Grubaugh, PhD, of The Scripps Research Institute.
First, Dr Grubaugh explained, Miami is home to year-round populations of Aedes aegypti mosquitoes, the main species that transmits Zika virus.
The area is also a significant travel hub, bringing in more international air and sea traffic than any other city in the continental US in 2016. Finally, Miami is an especially popular destination for travelers who have visited Zika-afflicted areas.
The researchers found that travel from the Caribbean islands may have significantly contributed to cases of Zika reaching Miami.
Of the 5.7 million international travelers entering Miami by flights and cruise ships between January and June of 2016, more than half arrived from the Caribbean.
Killing mosquitos produces results
The researchers believe Zika virus may have started transmission in Miami up to 40 times, but most travel-related cases did not lead to any secondary infections locally. The virus was more likely to reach a dead end than keep spreading.
The researchers found that one reason for the dead-ends was a direct connection between mosquito control efforts and disease prevention.
“We show that if you decrease the mosquito population in an area, the number of Zika infections goes down proportionally,” Dr Andersen said. “This means we can significantly limit the risk of Zika virus by focusing on mosquito control. This is not too surprising, but it’s important to show that there is an almost perfect correlation between the number of mosquitos and the number of human infections.”
Based on data from the outbreak, the researchers see potential in stopping the virus through mosquito control efforts in the US and other infected countries, instead of, for example, through travel restrictions.
“Given how many times the introductions happened, trying to restrict traffic or movement of people obviously isn’t a solution,” Dr Andersen said. “Focusing on disease prevention and mosquito control in endemic areas is likely to be a much more successful strategy.”
When the virus did spread, the researchers found that splitting Miami into designated Zika zones—often done by neighborhood or city block—didn’t accurately represent how the virus was moving.
Within each Zika zone, the researchers discovered a mixing of multiple Zika lineages, suggesting the virus wasn’t well-confined, likely moving around with infected people.
Drs Andersen and Grubaugh hope these lessons from the 2016 epidemic will help researchers and health officials respond even faster to prevent Zika’s spread in 2017.
Behind the data
Understanding Zika’s timeline required an international team of researchers and partnerships with several health agencies.
The researchers also designed a new method of genomic sequencing just to study the Zika virus. Because Zika is hard to collect in the blood of those infected, it was a challenge for the researchers to isolate enough of its genetic material for sequencing.
To solve this problem, the researchers developed 2 different protocols to break apart the genetic material they could find and reassemble it in a useful way for analysis.
With these new protocols, the researchers sequenced the virus from 28 of the reported 256 Zika cases in Florida, as well as 7 mosquito pools, to model what happened in the larger patient group.
As they worked, the researchers released their data immediately publicly to help other researchers. They hope to release more data—and analysis—in real time as cases mount in 2017.
The research was supported by the National Institutes of Health, The Pew Charitable Trusts, The Ray Thomas Foundation, a Mahan Postdoctoral Fellowship from the Computational Biology Program at Fred Hutchinson Cancer Research Center, the US Centers for Disease Control and Prevention, the European Union Seventh Framework Programme, the United States Agency for International Development Emerging Pandemic Threats Program-2 PREDICT-2, and the Defense Advanced Research Projects Agency.
