The Maryland Department of Agriculture, which helps conducts the tests, estimates that fewer than one in 1,000 mosquitoes is infected, entomologist Michael Cantwell said.
Now that cool weather has moved into the area, mosquitoes are dying off quickly, reducing the threat to people, Bucher said.
C&O Canal National Historical Park staff will treat the infected area to contain the spread of the virus, acting superintendent Kevin Brandt said.
No one in Washington County has tested positive for the virus, but one case each has been reported this year in neighboring Berkeley County, W.Va., and Franklin and Fulton counties in Pennsylvania.
West Nile virus produces no symptoms in about 80 percent of people who are infected, according to the Centers for Disease Control.
About 20 percent of infected people experience mild flu-like symptoms and the disease is serious and perhaps fatal in less than 1 percent of cases, the CDC says.
The C&O Canal has been monitoring its 184.5-mile towpath for evidence of West Nile for the past three years.
Testing over the last two years has been more comprehensive, with more than 400 mosquito samples tested, Bucher said.
You can protect yourself from the threat of West Nile by eliminating standing water, which is mosquito breeding grounds, health officials say.
Health officials also suggest limiting exposure to mosquitoes during their most active times at dawn and dusk and wearing long sleeves and pants and insect repellents containing DEET.