By Chris Williams on December 4, 2017.

Many people, especially those of us who deal with pest mosquitoes, are wondering what happened to the Zika virus this year. Even experts aren’t sure just how the U.S. has managed to get off so easy…or have we?

Zika virus is a serious disease spread by the bite of an infected Aedes aegypti, or yellowfever, mosquito that occurs in the southern and coastal U.S. There is speculation that the related Asian tiger mosquito, which occurs in the Northeast, could also spread the disease. Zika virus is an especially scary mosquito-transmitted disease because it can result in severe birth defects in newborns and chronic neurological and other problems in infected adults.


We’re almost at the end of 2017 and there have been very few Zika virus cases this year, so far just 349 in the U.S. according to the Centers for Disease Control. And all but two of those were brought into the country by travelers who picked up the disease in countries where Zika was prevalent. Compare this with the summer of 2016 when there were almost 5,000 travel-related cases plus 224 cases that were presumed to be transmitted by local mosquitoes. Local transmission means the virus is circulating among mosquitoes and people in the region, and could signal the beginning of a disease epidemic.

Much credit goes to local government mosquito prevention and control efforts in 2016, especially in the two states with local transmission, Florida and Texas. We made it through 2016 without a major outbreak but we worried about what 2017 would bring. Fortunately, it brought very little. What happened?


Zika virus made its way into the U.S. in early 2016 from Latin America and the Caribbean. These countries were exposed to Zika only a year or two earlier. Yet, Zika cases have dropped off dramatically in these countries as well. Puerto Rico, for example, had almost 35,000 documented Zika cases in 2016 and only about 500 so far this year.
Experts believe that we can credit this reprieve from Zika virus in the Americas to the concept of “herd immunity.” After people are exposed to the virus, they develop a resistance to the disease. At some point, when enough people in a region have become immune, the virus loses its steam. When there are not enough susceptible people left to infect, the virus can no longer keep up its momentum, and gradually dies out or at least becomes inactive.

It’s thought that herd immunity developed rapidly in the countries south of us because Zika virus hit those countries fast and hard. Without adequate disease protection, some people paid a heavy price. But the fact that immunity developed quickly apparently helped to save the U.S. from a disastrous disease outbreak of its own. It’s thought that Zika will always be with us, somewhere in the world, and periodic outbreaks may occur. It’s also not known how long immunity to Zika will last. People in a region may become vulnerable again.

That’s not to say that health officials have become complacent. This disease is too serious to take lightly. As long as people travel to and from countries where Zika is present, isolated cases will show up in the U.S. and may require intervention. Research on development of a vaccine will continue while genetically-modified mosquitoes have already been approved and are being produced.



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