Should Americans Worry About Zika Virus?

The Arkansas Department of Health (DOH) today announced that a resident had tested positive for Zika virus, joining at least a dozen other cases of Zika found thus far in the U.S.

This announcement is certain to scare many. Zika has all the elements of yet another runaway epidemic, including staggering case counts and devastating complications.

However, in the alarming news from Little Rock and a related update from the CDC, there is an incredibly important fact: Thus far, all U.S. cases have occurred in travelers returning from already endemic areas. No cases in the U.S. have been due to local acquisition or transmission, called autochthonous. That means that if you stay in the U.S., you are safe.

For now.

The problem, as detailed in an update from PAHO (the Pan American Health Organization, a partner of WHO), is that the mosquitos that carry Zika are known to be in the U.S. There are two types of mosquitos known to move Zika virus from here to there: Aedes aegypti and Aedes albopictus. As this map from the CDC demonstrates, Aedes mosquitos are thriving south of the Mason-Dixon line.

To ramp up the Zika epidemic in the U.S., one of two distinct events needs to occur. One is a hostile takeover: Infected mosquitos could swarm north and join their confederates already buzzing over Alabama and Georgia. Given the way that mosquitos move along, this is an inevitability sooner or later—but probably not this year.

The chikungunya virus is carried by the same mosquitos and has caused major public health issues in the Caribbean—yet the infected mosquitos thus far have failed to make geographic inroads into the U.S. (though a little autochthonous transmission has occurred).

The other way to kick-start an epidemic is also unlikely, at least any time soon. If a critical mass of infected people hang around together, they can give uninfected mosquitos a chance to catch Zika locally. Here’s how it works: Let’s say several airplanes full of people from Charlotte, North Carolina, return next week from Carnival in Brazil, newly infected. Since it is February and relatively few mosquitos are circulating, probably little will occur.

But say the same large group decides to go to the Summer Olympics in Brazil and there, become infected. On their return, they will have lots of Zika in their blood streams and lots of thirsty Carolinian Aedes mosquitos looking for a meal. Bingo—a perfect mess.

The key is that mosquitos are basically inefficient and rely on big numbers to spread disease. If only a few people in a town have Zika, the likelihood that a mosquito will find an infected person, take a blood meal, not die, and go bite an uninfected person is extremely low. But if hundreds of infected people are milling around, the work for the Aedes mosquito is simple and inevitable—as the Brazilian outbreak attests.

In the weeks ahead one thing is certain: Many, many more people in the U.S. will be diagnosed with Zika virus. This is because doctors will be testing more and Zika, which is asymptomatic in 80 percent of those infected, surely will be found. But as long as all the cases are imported, as is currently the situation, the fear factor in the U.S. should remain quite low.

However, if and when we enter into autochthonous transmission, things could get very ugly, very fast. There appears to be almost no native immunity—meaning that epidemics affecting millions, as is occurring in Brazil, would happen here. Therefore our main protection right now, as we wait once again for a vaccine to come along, is not shots or antivirals or magic pills—but mosquito control. This is the same challenge that the U.S. met successfully after World War II when malaria was driven out of the U.S. South.

Hopefully we will coordinate efforts against Zika virus and the Aedes mosquitos in the same purposeful, non-politically distorted manner.