Pre-emptive strikes are being mounted in the Valley against mosquitoes to try to reduce the threat of the often disabling and sometimes deadly West Nile virus.
Environmental and health officials say it’s a critical time to take preventive action against mosquito population growth in the spring and another big outbreak of the disease that first hit Arizona about 18 months ago in its westward spread across the country.
This year, Maricopa County has seen 355 of 390 confirmed West Nile virus infections in humans in the state and 12 of the 14 deaths in Arizona attributed to the infection, according the Arizona Department of Health Services.
The infection rate should slow temporarily as mosquitoes retreat from winter’s cold temperatures and go into diapause — a kind of partial hibernation — waiting for the return of warmer weather to start their breeding season.
County and municipal environmental operations are attempting to keep storm drains, retention basins and wells clear of stagnant water that attracts mosquitoes and are using larvicides and chemical fogging where large numbers of mosquitoes are found.
"We hope that by keeping the pressure on them throughout the winter we can slow down the outbreak (of viral infections) in the spring’’ said John Townsend, vector control manager for Maricopa County Environmental Services.
It won’t be easy.
There are an estimated 4,500 Valley sites that could harbor mosquitoes, and not enough county and municipal resources to frequently monitor all of the sites, Townsend said.
"The best we can do is to reduce the mosquito population so that we won’t have hordes of them out there carrying the virus,’’ he said.
Scottsdale is draining stagnant water from city properties and using larvicides, said Larry Person, coordinator of the city’s environmental programs.
Mesa is doing the same, said Christine Zielonka, environmental management director. City departments have been instructed to put their workers on the lookout for possible mosquito breeding sites, she said.
Tempe utility workers are making immediate inspections after rainy days to ensure drainage systems are not clogging and water isn’t collecting in low-lying areas, said water department spokesman Ron Coleman.
The state health services department is applying for $1 million in federal funding to help fight the spread of the virus, and would use much of it for more mosquito "surveillance,’’ said Craig Levy, state vector control manager.
"It tells you where to target your effort to more effectively protect people," Levy said.
But the threat will be reduced only a little by making public facilities inhospitable to mosquitoes. The virus has been spread in the Valley predominantly by mosquitoes breeding in poorly maintained swimming pools and other standing water in the yards of homes, Levy said.
Unless enough residents clean up potential breeding grounds on their properties, "we could get hit fairly hard again next year’’ by the virus, he said.
For information on West Nile virus and preventive measures, visit