If there’s a silver lining in this year’s drought, it might be the desiccation of this year’s crop of mosquitoes, which health officials said Thursday was one of the lowest in recorded history.
Cases of the sometimes fatal West Nile virus, often used as a measuring stick for mosquito populations, were the lowest recorded since the virus came West, with only 62 cases and no deaths this year. For comparison, the benchmark year of 2003 had 3,000 cases and 63 deaths. Last year was the second-worst year for West Nile virus in Colorado, with 576 cases and seven deaths, according to the Colorado Department of Public Health.
“The numbers of mosquitoes are driven mainly by precipitation and temperature,” said John Pape, an epidemiologist for the department. “If you have a puddle, you’ll get mosquitoes breeding. If the puddle warms up, you’ll get more mosquitoes. But if you don’t have a puddle . . .
“We had a (population) spike early in the season, which runs from May through September. But then it got hot and very dry so the population crashed. We had some rain in August, so we had another spike, but the population is on its way down now.”
The number of mosquitoes this year was dramatically different from previous years. The state measures mosquito populations using traps in various counties.
For example, health workers in Adams County in 2007 trapped roughly 1,200 mosquitoes on July 12 and a peak of 1,450 on July 16. This year, the same traps yielded only 590 mosquitoes on July 14 and 280 on July 28.
In Arapahoe County, they trapped 375 mosquitoes on Aug. 6, 2007, and only 62 on Aug. 4, 2008. Jefferson County trapped 380 mosquitoes in early July last year and only 60 mosquitoes that same week this year.
Colorado has about 45 species of mosquitoes, but health officials track only two of them — the Culex group, which carries the West Nile virus, and the Aedes group, which Pape calls “the nuisance biters that ruin barbecues.” Culex bite only at night while Aedes bite day and night.
Only female mosquitoes bite because they require protein from blood to produce eggs. They inject saliva into the bite, which prohibits the blood from clotting and which causes the itch in humans.
The best control of mosquitoes is during the larval stage by eliminating standing water or by changing the water weekly in birdbaths, wading pools and other containers. Bug spray containing the chemical DEET in concentrations of 10 percent to 35 percent is the most effective in preventing bites.
Mike McPhee: 303-954-1409 or mmcphee@denverpost.com
