Will the Zika virus spread into the United States?

South Texas, Florida seen as vulnerable

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Climate and demographic factors could make parts of the southern U.S. vulnerable to the spread of the Zika virus.

Staff Report

Combing climate data with travel patterns, researchers with the Center for Disease Control and the National Center for Atmospheric Research say Zika virus outbreaks could occur as soon as this summer in parts of south Texas and Florida.

The study shows that the Aedes aegypti mosquito, which is spreading the virus in much of Latin America and the Caribbean, probably will become more abundant across much of the southern and eastern United States as the weather warms.

Summer weather conditions are favorable for the disease-carrying mosquito as far north as New York City and across the southern tier of the country as far west as Phoenix and Los Angeles, the models show.

“This research can help us anticipate the timing and location of possible Zika virus outbreaks in certain U.S. cities,” said NCAR scientist Andrew Monaghan, lead author of the study. “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.”

The study make specific predictions for this year, butt climate forecasts for the summer point to a 40-45 percent chance of warmer-than-average temperatures over most of the continental United States.

Monaghan said this could lead to increased suitability for Aedes aegypti in much of the South and East, although above-normal temperatures would be less favorable for the species in the hottest regions of Texas, Arizona and California.

Even if Zika gains a foothold in the U.S., it probably won’t spread a widely or quickly because more Americans live and work in air-conditioned and largely sealed homes and offices, Monaghan said.

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 then be transmitted if the mosquito bites 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 in some Arizona cities. Weather conditions in southern and western cities remain suitable as late as November.

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.

Along with environmental factors, the scientists studied travel demographics and economics to assess the risks, counting, for example, the number of passengers arriving in U.S. cities on direct flights from airports in 22 Latin American countries and territories listed on the Centers for Disease Control and Prevention’s Zika travel advisory as of Jan. 29, 2016.

Cities that had both high potential numbers of Aedes aegypti and a large volume of air travelers included Miami, Houston and Orlando. The study explains that the summertime peak in air travel coincides with the peak season in mosquito abundance.

The study also estimated that nearly five times as many people cross the U.S.-Mexico border per month than arrive by air in all 50 cities. This could indicate a high potential for transmission in border areas from Texas to California, although the Zika virus has not been widely reported in northern Mexico.

Those border areas, as well as other parts of the South where the mosquitoes are expected to be abundant, have a high percentage of households living below the poverty line, according to 2014 U.S. Census data analyzed by the research team.

Lower-income residents can be more exposed to mosquito bites if they live in non-air conditioned houses or have torn or missing screens, enabling mosquitoes to enter homes more easily. However, Aedes aegypti populations tend to thrive in densely populated urban areas, and some of the most impoverished areas are rural.

 

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