WHO: Zika Virus 'Spreading Explosively'

There are currently 31 people in the U.S. who have been diagnosed with the Zika virus, including three pregnant women -- two in Illinois and one in New York.



Those infected are spread across 11 states and Washington, D.C., according to the U.S. Centers for Disease Control and Prevention. All of those infected contracted the virus outside of the U.S. before returning, according to health officials.



Another woman in Hawaii is believed to have had the Zika virus after her infant was born with the associated birth defect called microcephaly -- characterized by an abnormally small head and brain, which can lead to developmental delays. The birth defect has been associated with the virus in Brazil, where more than 4,000 children have been diagnosed with the condition.

The Zika virus currently sweeping through the Americas looks to have hitched a ride on a plane into Brazil in 2013 and begun its invasion of the continent from there, scientists said on Thursday. In the first genome analysis of the current Zika epidemic, which has been linked in Brazil to cases of birth defects known as microcephaly, researchers said the virus' introduction to the Americas almost three years ago coincided with a 50 percent rise in air passengers from Zika-affected areas.



The strain of the virus circulating in the current outbreak is most closely related to one from French Polynesia, the scientists said, although it is also possible that Zika was introduced separately to the Americas and French Polynesia from South East Asia. Oliver Pybus, a biologist in Britain's Oxford University who co-led the research with a team from Brazil's Evandro Chagas Institute, said the findings suggested increased international travel helped the virus extend its reach.



Using next-generation genetic sequencing, the researchers mapped the samples' gene codes and found there was little genetic variability among them. This suggests there was a single introduction of Zika into the Americas, probably between May and December 2013 – more than a year before virus was first reported in Brazil.

Nuno Faria, a researcher at Oxford University and at the Evandro Chagas Institute who worked on this study, said these first genomic data from the Brazil outbreak provided "a good baseline for future research". He said the team had also looked for links between Zika and microcephaly - and had found some spatial and temporal correlations. To test that link conclusively, however, scientists need to see results of full case-control epidemiological studies.