H

eads up, residents of Miami, Charleston, S.C., and New Orleans: You’re living in the hotspot of expected Zika transmission in the United States.

But Memphians, Angelenos, and even New Yorkers: it might be wise for you to remain vigilant, as well.

According to a new modeling study, the mosquitoes that carry Zika may be marching north- and westward in the coming months as the summer heats up.

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The analysis, led by researchers at the National Center for Atmospheric Research, backs up assumptions that such cities are more likely to see some transmission of Zika because they maintain populations of Aedes aegypti mosquitoes, which thrive in dense urban environments that are warm and humid. Some Florida cities and those along the US-Mexico border region in Texas can foster the mosquitoes year round.

For the study, published Wednesday in the journal PLOS Currents: Outbreaks, the researchers crafted estimates for the potential abundance of Aedes aegypti for 50 cities with documented populations of mosquitoes or near the insects’ known territory. They also examined the number of travelers arriving in each city from abroad, the history of dengue and chikungunya transmission, climate, and socioeconomics to explore how likely mosquitoes were in those cities to become infected with Zika, and how likely they were to spread the virus locally.

The high levels of poverty along the Mexican border, for example, put people living in Texas cities like Laredo and Brownsville at greater risk for being exposed to mosquitoes.

Conditions in the summer could support Aedes aegypti mosquitoes in a stretch of territory from Los Angeles and Phoenix through the Gulf Coast and reaching up to New York City, according to the study. The researchers also found that at least for a few months a year, the conditions in cities such as Denver; Louisville, Ky.; and Albuquerque, N.M., which have not documented Aedes aegypti, could harbor the mosquitoes.

Are you at risk of contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

Aedes aegypti, also known as the yellow fever mosquito, tends not to fare well in colder climates, but there is evidence that it may be finding ways to expand its territory northward. According to a study published last fall, a population of Aedes aegypti mosquitoes could be living in Washington, D.C., year-round, surviving in underground infrastructure during the colder months.

Since appearing in Brazil last spring, the Zika virus has spread through much of Latin America and the Caribbean. Almost 200 people in the United States have had confirmed Zika infections, mostly in people who contracted the virus abroad and started showing symptoms once they returned. But as of now, there has been no documented case of mosquito-borne transmission in the continental United States.

Experts say that any local outbreak in the United States, should it happen, would likely be short-lived because of strong housing construction and the presence of screens on windows and air conditioning.

The study’s authors noted numerous limitations with their models, including that they did not factor in mosquito control efforts in the different cities. The exact effect temperature has on mosquito reproduction in colder areas is also not understood. Plus, they could not develop accurate estimates for the number of artificial containers filled with water in the different cities, which are prime Aedes aegypti mosquito breeding sites.

The study also did not examine which cities could see activity of a related mosquito species, Aedes albopictus. Experts suspect these so-called Asian tiger mosquitoes are able to transmit Zika, but are a less effective transmitter because they don’t primarily feed on humans or live close to people like aegypti does.

But the authors of the new study note that it is important to understand exactly which parts of the United States can support albopictus mosquitoes because they appear to cover more territory, are able to survive in colder climates, and could spread the virus more widely.

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