Why Zika Virus Is Unlikely to Become Endemic in the Continental US

Lessons from West Nile virus and how Zika is different.

Health experts have long predicted a small outbreak like this in the continental U.S., but there is less concern that the virus could reach epidemic levels or become a permanent problem in the way West Nile has remained in the U.S. since 1999.

The West Nile virus, which can live in birds, insects and humans, initially appeared in North America in 1999, when it infected many people. In recent years West Nile has continued to be present but no longer has widespread outbreaks.

Experts say that lessons from West Nile suggest that Zika is less easily spread across the U.S.

Dr. Stephen Morse, an infectious-disease expert at the Mailman School of Public Health at Columbia University, explained that since Zika virus is present mainly in mosquitoes and humans, there is less chance it will live on in the wild after steps have been taken to stop an outbreak.

"West Nile has a natural ecology in birds and mosquitoes, so it has a place to maintain itself in a natural reservoir" outside humans, he explained.

Morse said that in some countries with large populations of primates, there's a higher chance the virus could remain endemic but that it was less likely in the U.S. He added that it's less likely that swaths of people in the U.S. will be infected, since many people either have air conditioning or keep screens on their windows, which keep out mosquitoes.

Most important is to have "response plans ready" and to "be flexible, as things may not occur as we expect it to," he said.

He added that he doubted the Zika virus would return year after year as West Nile has, as long as health officials are able to reduce or eliminate affected Aedes aegypti mosquito populations.

Officials remain concerned about the Zika virus and the chance, even if unlikely, that it could become a permanent problem in the U.S.

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