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CORVALLIS, Ore. - Storm drains on city streets have long had a useful function, taking away the runoff from rain, watered lawns and summer car washes, but a new study has found that they also can act as a haven for unwanted pests - mosquitoes.

And as West Nile Virus continues to claim human lives - with mosquitoes responsible for transmission - such a finding could have health implications.

Jill Townzen, a doctoral student at Oregon State University, sampled 40 catch basins in storm drains in Corvallis, Ore., during 42 days of sampling and counted more than 33,000 mosquitoes. Corvallis, a city of about 50,000 residents, has about 7,500 of these basins, and Townzen said if she extrapolated her findings to include all the storm drains, the sampling could have yielded more than 3 million mosquitoes.

"Not all of the catch basins had mosquitoes when the project started," Townzen said, "The goal was to get a general sense of the size of their population, what species they are, and what they have been feeding on."

Emilio DeBess, DVM, the Oregon public health veterinarian and a member of Townzen's graduate committee, said the finding was surprising and should be incorporated into municipal planning efforts for combating the spread of West Nile Virus.

"We don't know yet if there is an impact," he said, "but clearly the sheer number of mosquitoes in one place is troubling."

Catch basins are frequently used in many cities in the United States as part of municipal stormwater management programs. These basins are designed to hold water equal to the first 10 minutes of a storm, Townzen said, and through detention can filter out some solids and pollutants before the water goes further into the stormwater system.

"Many people thought the oil from cars and the chemicals from lawn fertilizers would have killed any mosquitoes," Townzen said, "or that many of the basins would go dry in the summer. But 80 percent of the basins had water, and most of them had mosquitoes."

Townzen worked for Multnomah County near Portland, Ore., draining mosquito habitat before beginning her doctoral studies at Oregon State. She said some mosquito species are found near natural water sources, including river floodplains, ponds and sloughs, while other species find abandoned swimming pools, buckets or old tires filled with water and other human-caused habitats.

"It doesn't take much water to encourage mosquitoes," Townzen said. "I've seen them in small jars with as little as an inch of water in them. But those are all localized habitats. Catch basins are systemized throughout a city and present a larger, more pervasive problem."

Phil Rossignol, a professor of fisheries and wildlife at Oregon State and a well-known expert on mosquito habitat, biology and disease transmission, served as one of Townzen's major professor, along with Darlene Judd, for her doctoral study.

"Her research adds more evidence to the idea that mosquitoes are in part an anthropogenic problem," Rossignol said. "Humans continue to influence the environment in a number of ways that can encourage their population and distribution - and the possible spread of disease."

Western Oregon has not had a severe problem with West Nile Virus, Townzen said, although all three species of mosquitoes found in the Corvallis catch basins are known to be "very competent vectors" for transmission. The Centers for Disease Control lists 62 different mosquito species that are known to carry West Nile Virus.

"The major lesson here is that when we try to manipulate the environment, even for highly beneficial practices such as improving water quality, there may be unintended consequences," she said.