A mosquito capable of carrying yellow fever virus and dengue was detected in Fresno last week, the third time this year the bug has shown up in California.

Aedes aegypti, a small black-and-white mosquito, is not native to the state but is found in certain urban areas of the Southeast, Texas and Arizona. Its presence does not pose an immediate threat—the mosquito has been found, not yellow fever—but public health officials are concerned that the bug could be problematic if it gains a foothold in the state.

Yellow fever, endemic to South America and Africa, is a serious disease which causes chills, nausea, muscle pain and headaches. In some cases, it can cause liver damage. Yellow fever can only be passed via a mosquito bite and is not contagious, human to human. Dengue is an infectious virus that affects 50-100 million people annually and can be life-threatening.

The mosquito was first detected in the Fresno-area cities of Madera and Clovis in June, prompting public service notices to residents and door-to-door inspections for possible breeding grounds. It was found again in August in San Mateo County. The Associated Press reported that the mosquitoes have been trapped at more than 100 locations in Clovis, while public health officials are directing insecticide spraying in and around thousands of homes.

No illnesses have yet been associated with the bugs. A public health official with the Fresno Mosquito and Vector Control District told the AP that the nightmare scenario is that dengue is turned loose in California.

The AP said scientists at University of California, Davis, and Yale University are performing genetic testing on the mosquitoes to determine their country of origin. Preliminary results indicate they might be from Central America and may have come from eggs on containers imported into the country.

The mosquito was found in San Mateo in 1979, but was eradicated before it could become established. It lays its eggs in water, just above the water line in small containers and vessels that hold water.

Vector control agents statewide are already battling the mosquito-borne West Nile Virus. So far 275 cases have been reported this year.   

–Ken Broder

To Learn More:

Yellow Fever Mosquito Detected in California (by Tracie Cone, Associated Press)

Yellow Fever Mosquito Found in San Mateo County (by Laura Dudnick, San Mateo Patch)

A Short History of Yellow Fever in the US (by Bob Arnebeck)