Zika virus not causing outbreaks in continental U.S.

Although a number of returning U.S. travelers have been infected with the Zika virus while visiting Latin America, the mosquito-borne virus is not causing outbreaks in the continental U.S., health officials said Thursday.

Thirty-one Americans in 11 states and Washington, D.C., have been diagnosed with a Zika infection contracted while traveling abroad, said Anne Schuchat, principal deputy director at the Centers for Disease Control and Prevention. One of the Zika cases is a pregnant woman in New York City, which has two other cases.

Those are isolated cases, however, and very different from the Zika epidemic in Brazil, which had an estimated 1 million Zika infections by the end of last year. Right now, people on the U.S. mainland can only contract Zika if they travel to an area with a Zika epidemic.

In countries where Zika is spreading routinely, people can contract Zika in their homes, said Gonzalo Vazquez-Prokopec, an assistant professor in the department of environmental sciences at Emory College in Atlanta.

The virus, spread by Aedes mosquitoes, has been associated with a sharp jump in the birth of babies with abnormally small heads in Brazil. It is also linked to increases in Guillain-Barre syndrome, in which the immune system attacks the nervous system, causing paralysis.

Travelers frequently contract diseases, from malaria to measles, while abroad. The CDC diagnosed 14 returning travelers with Zika from 2007 to 2014. None of these cases sparked Zika outbreaks.

"We're expecting a lot of travel-associated cases," Schuchat said, when asked how many Americans had contracted the virus at a Thursday press conference. "I know the numbers are important to people, but they aren't that critical" in terms of spreading the disease.

Zika spreads best in tropical areas because warmth helps the virus reproduce, Vazquez-Prokopec said .

That explains why the mosquitoes that spread Zika virus have gained a foothold in two U.S. territories — Puerto Rico and the Virgin Islands. The CDC has confirmed 19 Zika cases in Puerto Rico and one case in the U.S. Virgin Islands among people who had not traveled to outbreak countries. The WHO now includes these territories in its list of places with outbreaks.

Aedes mosquitoes do live in warmer areas of the U.S. But it's still relatively difficult for individual travelers infected abroad to spread the disease here, Vazquez-Prokopec said. That's because most people infected with Zika clear the virus from their blood in less than a week. Mosquitoes can only become infected with the virus if they bite someone during that short window of time. So many travelers have cleared the virus before they return to the U.S., he said.

Although WHO has said the virus is moving at an "explosive" speed across the Americas, Schuchat said the continental U.S. presents a number of barriers to Zika.

Cities in Latin America tend to be much more densely populated than in the continental U.S., Schuchat said. Many people live without air conditioning, so they leave open windows, which often lack screens, to cool their homes. Cities in Latin America also have much more trash, which can collect standing water where mosquitoes lay eggs.

Aedes mosquitoes have adapted to live near humans, Vazquez-Prokopec said, and often rest inside homes during the day, sleeping in closets or under beds.

However, Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, said conditions in many impoverished communities along the U.S. Gulf Coast mirror those in Latin America, making this region more vulnerable to Zika.