The female mosquito certainly gets around. She can fly up to two miles while seeking a blood meal from birds, her favorite hosts.

When she encounters people, instead, they become her victims, as well. She leaves them with itchy reminders about the power of her proboscis. If she draws blood from a West Nile Virus (WNV) infected bird, she can also infect humans.

Although eight out of 10 people bitten by a WNV-infected mosquito won't develop symptoms, about 60 percent of the remaining 20 percent will think they have a really nasty flu, according to Russ Parman, acting district manager of the Santa Clara County Vector Control District. About 40 percent of those people will develop a neuro-invasive form of the virus and can come down with encephalitis. 

"WNV has become a part of nature because it's so efficiently transmitted, even when the mosquito numbers aren't high, and that's the way it's been across the country," Parman says. "It's not going to go away." 

In 1999, WNV was first discovered in the U.S. in College Point, NY, part of New York City. Prior to that, it was found only in the Nile River region of Africa. 

"It was probably brought over in infected birds—that's the theory," Parman says. "It might have been associated with the pet trade. A mosquito bit the bird, and New York City had no mosquito control program at the time. The rest is history." 

Since the virus arrived in California in 2003, approximately 2,900 human cases and 110 deaths have been reported statewide. In 2004, WNV arrived in Santa Clara County where 15 non-fatal cases have been reported, none since 2008. So far this year, 14 WNV bird cases have been confirmed in Santa Clara County, with three in San Jose during the past week. 

Male mosquitoes eat nectar, not blood, so it's the females that spread WNV. After biting a bird infected with WNV, a female becomes infected and passes the virus to other birds as she bites them. Those birds, in turn, get bitten by other mosquitoes, who become infected, and spread the virus to more birds. Fatalities are massive in crow, jay and magpie populations while other birds, squirrels and mammals succumb in fewer numbers. 

Chickens aren't affected by WNV. 

"We use chickens as sentinel flocks—we have seven in the county—since they are immune to WNV, but they develop antibodies [to it]," Parman says. "We take blood tests every couple weeks, so we know if the chicken has antibodies, it was exposed to an infected mosquito."

Since 2007, Parman says about 4,500 neglected swimming pools—each filled with at least 200,000 mosquitoes—were discovered in Santa Clara County. Parman says the district uses mosquito-killing measures in such cases. 

Dr. Nayer Zahiri, a vector ecologist at the district, explains that the county sets out traps around the county to monitor mosquito concentrations. Trapped mosquitoes are checked for WNV in the district's San Jose lab, as are dead birds the lab receives.

When positive test results become greatly amplified in an area, fogging is ordered, Parman says. The latest one took place in Sunnyvale last month. 

"Opponents of our fogging operation often like to quote the stats and say it's not significant, but it is," Parman says. "We don't portray it [WNV] as an emergency, we simply portray it as something we can educate the public about and something that is avoidable. If we didn't have an aggressive control program, there would be a lot more cases." 

To hatch and become adults, mosquito eggs need warmth and standing water for just five days. Storm gutters, flower pot saucers and unchanged bird baths are perfect breeding sites, so removing or replacing water every couple days will kill larvae. 

"Each female mosquito in her lifetime can produce up to four batches of eggs," Parman says. "Each of those egg rafts can contain up to 200 eggs." 

A female usually lives up to a month, but survives winter if she finds a shelter where she—and the WNV if it's inside her—can enter dormancy until the spring. 

Visit www.westnile.ca.gov for regularly updated WNV information. 

About this column: Each week, Susan Wiedmann will write about nature or outdoor activities enjoyed by local residents.  Susan is a longtime freelance writer and photographer with a passion for capturing wildlife through her camera's lens. Please leave any comments about this article at the bottom of this page. You can contact Susan about possible topics at Susan@UpCloseWithMotherNature.com or at UpCloseWithMotherNature.com.