Even in peak mosquito season, Zika risk is low in California

As summer approaches, some worry that warmer weather could attract mosquitoes and bring the fast-spreading Zika virus stateside. But new research finds that, in the West at least, that probably won’t be the case.

In a study published Wednesday in the journal Public Library of Science Currents: Outbreaks, researchers from the National Center for Atmospheric Research analyzed travel and weather patterns to estimate the potential size of mosquito populations from month to month in 50 major U.S. cities. They focused on Aedes aegypti mosquitoes, which have been carrying the Zika virus across more than 30 countries in the Americas.

The researchers found that even in the warmest months, the dry climate probably will keep mosquito populations at low levels in cities in the West, including Albuquerque, Bakersfield, Las Vegas, Los Angeles, San Diego, Sacramento and Tucson.

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Other parts of the country were not so lucky.

Weather conditions in Miami and Orlando, Fla., and New Orleans create summer conditions suitable for large infestations of mosquitoes, the study found. New York, Washington and Philadelphia could have moderate infestations.

The study, however, looks only at the factors that could bring mosquitoes to the U.S., not that stop them from spreading.

Lead author Andrew Monaghan said that even if weather conditions are suitable for severe mosquito infestations, health and insect control agencies in the United States are effective at limiting the insects’ spread. They’ve prevented outbreaks of dengue or chikungunya, two diseases also carried by the insects, he said.

“You can count the number of outbreaks on your fingers and toes,� said Monaghan, a meteorologist at the National Center for Atmospheric Research.

The Zika virus, suspected of causing birth defects, was once found only in Africa and Southeast Asia, but has been spreading rapidly across the Americas this year. So far, no one in the United States with Zika virus has been infected by a mosquito here.

For the virus to be transmitted, an infected person needs to be bitten by an Aedes mosquito, which then must go bite another person. Experts say that stopping the mosquitoes from multiplying is vital to preventing outbreaks of Zika, which has no vaccine.

They found that Zika risk is low in many parts of the country for much of the year. In March, only parts of Texas and Florida have weather conditions suitable for any kind of mosquito infestation, the study found.

The Aedes mosquitoes aren’t native to California, but were introduced to the state five years ago. Experts think an infestation in L.A. County began when a shipment of bamboo from Southeast Asia arrived in El Monte.

Kelly Middleton, director of community affairs for the Greater Los Angeles County Vector Control District -- a public entity charged with managing mosquitoes -- agreed that a local outbreak is unlikely.

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“However, I can definitely tell you that in some communities, the populations of these mosquitoes is certainly not ‘low’ -- especially by late summer,� she said in an email.

The National Center for Atmospheric Research scientists said that unseasonably warm weather this year due to El Nino could make weather conditions in most of the country more favorable for mosquitoes.

But in the hottest parts of California, Texas and Arizona, increasing temperatures could have the opposite effect, pushing temperatures past mosquitoes’ “sweet spot of development and survival,� Monaghan said.

“Once we get beyond about 90 degrees Fahrenheit it starts going the other way,� he said.

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