Day-biting mosquitoes in Clovis and Madera that can spread deadly dengue and yellow fever diseases most likely came from the southeastern United States and not Mexico, researchers from Yale University say.
The mosquito -- Aedes aegypti -- probably hopped rides on planes or trains to get to California, the Yale researchers suspect.
"At the border with Mexico, California has some of the best defenses against introduction of foreign insects," Jeffrey Powell, professor of ecology and evolutionary biology, said in a Yale news release.
"This study suggests we need also to be looking at air and rail transportation from other regions as well," said Powell, senior author of the paper on the mosquito that is in the journal PLoS Neglected Tropical Diseases.
Powell and colleagues did a comparative analysis of genomes of A. aegypti in the southeastern United States and those found in Mexico and Arizona. The genetic analysis showed that the mosquitoes found in Clovis and Madera most likely came from the southeastern United States. The work was funded by the National Institutes of Health.
Typically, the mosquito likes hot, humid tropical areas, like the Southeast, Mexico, and Central and South America. Valley mosquito control officials were troubled when they discovered the A. aegypti last year in Clovis and Madera.
That the mosquito survived the winter in the Valley and is biting people this summer is more troubling, said Steve Mulligan, manager of the Consolidated Mosquito Abatement District that includes Clovis. "These mosquitoes got here somehow and they are surviving in our type of climate."
The mosquito continues to be found in Madera and is spreading. But fewer of the mosquitoes are being trapped, said Leonard Irby, manager of the city's mosquito control district.
The drought has kept the mosquito numbers down, Irby said. "People are watering less."
The mosquito is a carrier of yellow fever, dengue and chikungunya -- tropical diseases whose symptoms include fever, severe headaches and body aches. In severe cases, the diseases can lead to death.
So far, there have been no cases of these mosquitoes carrying the diseases in the Valley, Mulligan said.
But people traveling outside of the United States to infected areas can be bitten, become infected and return to the area, he said. Mosquitoes here can then become infected and spread the disease, Mulligan said.
In 2013, there were 124 imported cases of dengue reported in California, according to Yale.
Mulligan said Valley residents should take measures to protect themselves from bites by using repellent and eliminating sources of standing water in yards. The A. aegypti bites from sunrise to sunset. The mosquito that carries West Nile virus typically bites in the evening.
"We are trying to do control efforts to eliminate sources of the A. aegypti production," Mulligan said. "But we really need the participation of the public to help us control this mosquito."