Zika transmission in U.S. linked to travel from Caribbean

Travel between the continental U.S. and the Caribbean led to locally acquired Zika virus infections in Florida, according to new research published this week.

By sequencing the virus’s genome at different points in the outbreak, scientists from institutions that include the University of Florida created a family tree showing where cases originated and how quickly they spread. They discovered that transmission of Zika virus began in Florida at least four -- and potentially up to 42 -- times last year. The researchers also traced most of the Zika lineages back to strains of the virus in the Caribbean.

The researchers noted that most of the local Zika transmission in Florida occurred in Miami-Dade County. Other counties had A. aegypti mosquitoes, but Miami received more travelers from regions with Zika transmission than any other city in the country. In fact, 72 percent of the traffic from regions with Zika transmission to Florida arrived in either Miami or Ft. Lauderdale.

The researchers found a correlation between Zika virus transmission and mosquito populations, showing that periods with high numbers of A. aegypti mosquitoes – which scientists have shown are capable of transmitting Zika virus and several related viruses – resulted in an increase in local transmission in humans.

The paper was published Wednesday in the journal Nature.

Zika transmission in the Caribbean and other parts of the Americas remains the key indicator for both locally acquired and travel-associated Zika infections in the United States.

“If the large outbreaks that have occurred throughout the Caribbean and Central and South America have created enough immunity to Zika that it can’t effectively spread in those regions, then introductions to Florida will be minimal,” said Derek Cummings, a professor of biology at UF’s Emerging Pathogens Institute..

Cummings developed mathematical models that analyzed the transmission of Zika virus in Florida for the manuscript.

The tendency for cases to decline sharply after big increases, Cummings said, is typical in outbreaks where a new pathogen causes an epidemic in a region and gives people infected in the initial outbreak immunity to subsequent infection.

Most people who came to Florida last year with a Zika virus infection did not transmit the virus to others, Cummings said. But some introductions resulted in sustained chains of transmission of tens of people.  An increase in “mosquito abundances” was associated with these local Zika cases.

“When mosquito abundances went down, local cases went down,” Cummings said. “This isn’t surprising for a mosquito-transmitted infection, but it suggests that control measures to reduce mosquitoes worked last year, and we should keep those up when additional cases are seen.”

The study was a collaboration of more than 60 researchers from nearly 20 institutions, including the Scripps Research Institute, the U.S. Army Medical Research Institute of Infectious Diseases, Florida Gulf Coast University, the University of Oxford, the Fred Hutchinson Cancer Research Center, the Florida Department of Health, and the Broad Institute of MIT and Harvard.