May 13, 2016

Yes, Zika will soon spread in the United States. But it won’t be a disaster

Jon Cohen
Science | AAAS
White House Press Secretary Josh Earnest presented maps showing the danger of Zika. Photo: Carolyn Kaster, AP
White House Press Secretary Josh Earnest presented maps showing the danger of Zika. Photo: Carolyn Kaster, AP

[R]esearchers who have studied Zika and the mosquitoes that transmit it say that the country is currently in the calm before the calm. Damaging as Zika is to fetuses, they predict that autochthonous transmission will only affect a small swath of the country that stretches from Florida along the Gulf Coast to Texas. And the dynamics of mosquito-borne disease in the United States are so different from those in Latin America that the number of confirmed cases probably will be in the hundreds, if that, before autochthonous spread sputters out.

There are many mysteries about Zika and how, in particular, it behaves in pregnant women, triggering some to miscarry and others to give birth to babies with brain disorders like microcephaly. There also are questions about how much spread occurs without a mosquito vector: Zika can persist in semen and be transmitted sexually, and there’s an outside chance that viral RNA in saliva, which never has been linked to an infection, might pose a risk. But when it comes to the mosquito species that harbor the virus and the transmission cycle with humans, a great deal is known. Experience with two other diseases spread by the same mosquitoes, dengue and chikungunya, offer insights as well.

The United States simply doesn’t have the ingredients for the type of explosive, autochthonous transmission seen in Latin America, says Thomas Scott, an entomologist and epidemiologist at the University of California, Davis. “I don’t want to blow this off and leave people with the impression that you don’t need to worry about it—who knows where this is going,” he says. “But I don’t think we’re going to have sustained transmission in the U.S., primarily because of our lifestyle. We also don’t have enough mosquitoes.”

The temperature-sensitive Aedes aegypti, the main mosquito vector, only lives in high numbers in a small portion of the United States, and mainly thrives in summer months when the temperature is between 25°C and 32°C. There are many more A. aegypti to spread Zika in Brazil—which saw up to an estimated 1.3 million infections in 2015 alone—than in the United States. There’s also a feast of human skin available, as people in warmer climes often wear tank tops, flip-flops, and shorts