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Potential Zika Virus Risk Estimated for 50 U.S. Cities

Mosquitoes flourish in summer months

Key factors that can combine to produce a Zika virus outbreak are expected to be present in a number of U.S. cities during peak summer months, new research shows.

The Aedes aegypti mosquito, which is spreading the virus in much of Latin America and the Caribbean, will likely be increasingly abundant across much of the southern and eastern United States as the weather warms, according to a study led by mosquito and disease experts at the National Center for Atmospheric Research (NCAR). The study was published in PLOS Current Outbreaks.

Summertime weather conditions are favorable for populations of the mosquito along the East Coast as far north as New York City and across the southern tier of the country as far west as Phoenix and Los Angeles, according to computer simulations conceived and run by researchers at NCAR and at the National Aeronautics and Space Administration (NASA) Marshall Space Flight Center.

Spring and fall conditions can support low-to-moderate populations of the Aedes aegypti mosquito in more southern regions of its U.S. range. Wintertime weather is too cold for the species outside southern Florida and southern Texas, the study found.

By analyzing travel patterns from countries and territories with Zika outbreaks, the research team further concluded that cities in southern Florida and impoverished areas in southern Texas may be particularly vulnerable to local virus transmission.

“This research can help us anticipate the timing and location of possible Zika virus outbreaks in certain U.S. cities,” said lead author Dr. Andrew Monaghan, an NCAR scientist. “While there is much we still don’t know about the dynamics of Zika virus transmission, understanding where the Aedes aegypti mosquito can survive in the U.S. and how its abundance fluctuates seasonally may help guide mosquito control efforts and public health preparedness.”

Monaghan stressed that, even if Zika establishes a toehold in the mainland U.S., it is unlikely to spread as widely as in Latin America and the Caribbean. This is partly because a higher percentage of Americans live and work in air-conditioned and largely sealed homes and offices.

To determine the potential risk in the mainland U.S., the research team ran two computer models that simulated the effect of meteorological conditions on a mosquito’s lifecycle (egg, larval, pupal, and adult stages) in 50 cities in or near the known range of the species. Monaghan and several team members have studied Aedes aegypti for years because it also carries the viruses that cause dengue and chikungunya.

Generally, the mosquitoes need warm and relatively stable temperatures as well as water-filled containers, such as buckets, barrels, or tires, for their eggs to hatch. Once a mosquito bites an infected person, it also needs to live long enough––probably a week or more, depending on ambient temperatures––for the virus to travel from the mosquito’s mid-gut to its salivary glands. Once in the saliva, the virus can be transmitted by the mosquito biting another person.

The study results show that, as springtime weather warms, the potential abundance of the mosquito begins to increase in April in the Southeast and some Arizona cities. By June, nearly all of the 50 cities studied have the potential for at least low-to-moderate abundance, and most eastern cities are suitable for moderate-to-high abundance. Conditions become most suitable for mosquito populations in July, August, and September, although the peak times vary by city. Weather conditions in southern and western cities remain suitable as late as November.

Even some cities where the Aedes aegypti mosquito has not been detected, such as St. Louis and Denver, have suitable midsummer weather conditions for the species if it were introduced via the transport of used tires or by other human activities, according to the computer models.

The researchers stressed that additional factors outside the scope of the study could affect populations of the species, such as mosquito control efforts, competition with other mosquito species, and the extent to which eggs can survive in borderline temperatures.

The study noted that northern cities could become vulnerable if a related species of mosquito that is more tolerant of cold temperatures, Aedes albopictus, begins to carry the virus.

Source: NCAR; March 16, 2016.

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