Emergency insecticide spraying last month by the city reduced the number of mosquitoes that can carry West Nile by more than 40 percent, but officials caution the risk of West Nile virus remains high and a new round of spraying is possible. The number of Culex mosquitoes, the type that carries West Nile, caught in four primary monitoring traps around the city two days before the spraying was 1,152, according to Colorado Mosquito Control. That number rose to 1,353 following the first round of spraying and then dropped to 655 following the second round, CMC reported.

That's a 43.1 percent drop. "We want to clearly send the message: We have gone from extremely high risk to high risk. You still need to take all the precautions," said Dr. Adrienne LeBailly, director of the Larimer County Department of Health and Environment. "The risk hasn't gone away," she added.

LeBailly said she may recommend additional spraying to city officials next week, and predicted the number of human cases will rise significantly in the coming weeks.

Larimer County has four confirmed West Nile human cases this year, with at least another three under investigation, LeBailly said. Three of the four confirmed cases are in Fort Collins; the fourth is in Loveland. "Despite the spraying, we are going to see the human cases increase exponentially," she said. "That doesn't mean the spraying didn't help. Those people were infected before the spraying.

"There are probably 100 more people who already have been infected. We just won't know about them for some weeks. And they were infected before the spraying."

The four traps used by CMC to create its initial count estimate are located south of Drake and Timberline roads; at the Fort Collins visitor center at Prospect Road and I-25; Rookery Road on the north side of Fossil Creek Reservoir; and along the Poudre River trail, near Hoffman Hill Road.  The city agreed to the July 19-20 and July 23-24 sprayings at the prompting of health officials who warned that high numbers of mosquitoes could increase the risk of West Nile virus.

While spraying is generally far less effective than killing mosquitoes before they hatch, officials believe spraying can make a major short-term different under the right conditions. Weather conditions play a major role in the growth of mosquitoes: Rain followed by warm temperatures can boost hatches, while cooler weather can delay or minimize them.

CMC's Larimer County operations director, Jessica Schurich, said in this case, the weather after the spraying was relatively consistent, which means the number of mosquitoes might have stayed the same or increased without the spraying.

"It was ideal conditions for the ideal spray application," Schurich said. "It's very hard to say what could have been (but) we got a substantial population of mosquitoes knocked down ... we took one step ahead in the game."

Schurich said adult females that have just eaten blood can be resistant to the spray, making the second application necessary. She also cautioned residents that not every area of the city will see the same reductions. Overall mosquito counts dropped by nearly 70 percent, CMC reported, but health officials worry primarily about the Culex species because those are the mosquitoes that carry the virus.

State and county health officials this year recommended that spraying be done earlier in the mosquito season, arguing that the time to spray is when large numbers of mosquitoes are being found, rather than waiting for human cases to arise. "Hopefully we've cut off that peak so we don't have 550 confirmed cases this year," LeBailly said. "Maybe we'll only have 100 confirmed cases this year in the long run."

Colorado state epidemiologist John Pape said he's expecting similar increases on the state level and predicted a large jump in cases and urged residents to take precautions even though spraying may continue.

"Don't let the small number of cases fool you into complacency," he said.

Database online

http://www.coloradoan.com/news/data/mosquitos.html

On the Internet

*www.fightthebitecolorado.com

*www.co.larimer.co.us/health/cd/westnile.asp