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West Nile Virus Season Subsiding, But It’s Not Over ‘Til It’s Over, UC Davis Entomologist Warns

October 13, 2006

WestDAVIS--The 2006 West Nile virus season is subsiding, but transmission will continue until the older female mosquitoes die off and the younger females enter reproductive diapause, warns research entomologist William Reisen of the Center for Vectorborne Diseases at the University of California, Davis.

Shorter days, colder weather and cooler water temperatures determine when Culex mosquitoes go into diapause, a hibernation-like state when growth and development pauses and physiological activity diminishes.

Reisen predicts that most of California’s mosquitoes entered diapause on Oct. 15, "the day when all the conditions come together after the autumnal equinox."

"Diapause induction requires a day length shorter than 12 hours light and 12 hours dark, and cool water temperature, below 50 degrees Fahrenheit," he explained.

"But it’s not over until it’s over."

The three main species of Culex mosquitoes in California—Cx. tarsalis, Cx. pipiens and Cx. quinquefasciatus—are still active, he said.

"The old ladies in the Culex tarsalis and Culex pipiens populations will be blood-feeding until the first frost, which should kill them," he said. "The newly emerged females will be entering reproductive diapause and survive until next year." Mosquitoes enter diapause in response to adverse environmental conditions. Culex quinquefasciatus, however, "continues blood feeding although they slow down and may go into quiescence (a resting or inactive period) during cooler periods in the Central Valley and especially southern California," Reisen said.

Diapause ends after the winter solstice as the weather warms. In some parts of California, such as Coachella Valley, this can be as early as late December, Reisen said.

If the mosquitoes are infected with the West Nile virus (WNV) when they enter diapause, it overwinters with them and will be transmitted to birds when they emerge the following spring, the UC Davis entomologist said. Previous research has shown that they blood feed predominantly on birds at that time.

Temperature is a crucial factor in the amplification of the virus. "Temperatures in the United States during the epidemic summers of 2002–2006 indicated that WNV dispersal and resulting epicenters were linked closely to above-average summer temperatures, such as they are experiencing in Idaho this summer," said Reisen.

In 2004, WNV dispersed to all 58 counties in California; however, Los Angeles was the epicenter. The following year, Sacramento became the epicenter. This year the virus has subsided at these locations and the hot spots are Yolo and Kern counties.

Statewide, 248 human cases have been confirmed in California so far this year, as of Oct. 10, state health officials said. Epicenters are Kern County with 47 human cases, and Yolo County with 26. Fifty three of the state’s 58 counties have reported WNV activity. In addition to the 248 human cases, the statistics include 52 horses, 1,212 dead birds, 814 mosquito samples, 568 sentinel chickens and 30 squirrels.

Vigilance continues to be crucial in the battle against WNV, entomologists and mosquito abatement experts agree. They urge these precautions:

Avoid being outside at dawn and dusk when mosquitoes blood-feed.

Wear long-sleeved shirts and pants outdoors.

Mosquito-proof your home.

Spray DEET repellant on your clothing.

Empty standing water in your environment.

Report dead birds at 1-877-WNV-BIRD or online at

UC Davis Center for Vectorborne Diseases (CVEC)

Media contact:
Kathy Keatley Garvey,
University of California Statewide Mosquito Research Program
Department of Entomology
Phone: (530) 754-6894 Fax: (530) 752-1537