Officials have placed more than 400 traps across the county and will continue to set traps out through the season.
"We are generally covering the whole county on a routine basis," Hennessey said.
So far, five other counties - Adams, Beaver, Centre, Dauphin, and Northumberland - have positive results in mosquitoes, according to the state's website.
Last year's hot spots in Delaware County included Thornbury, Chadds Ford, and Concord Townships, she said.
In 2013, there were 11 cases reported in humans across the state. In Montgomery County, three people tested positive. There were two in Delaware County, one in Bucks County, and none in Chester County.
In 2012 - billed as the worst year ever for West Nile - 60 cases were reported across the state. Delaware and Montgomery Counties each had seven cases. Bucks County had two and Chester County had one, according to the state Department of Health.
Nationwide that year, 286 people died - four in Pennsylvania - and 3,491 were hospitalized from the disease, according to the state and the Centers for Disease Control and Prevention.
"If they don't have a chance to breed, there is less chance we will have he virus in the county," Hennessey said of mosquitos.
The best way to prevent the spread of the virus is to control standing water, Hennessey said. She suggested making sure rain gutters are clean and tires, tin cans, plastic containers, and pots are kept dry. Aerate ornamental pools and turn over wading pools, wheelbarrows, and birdbaths when not in use.
"Every mosquito bite is not a cause for concern," Hennessey said. Wearing pants and long-sleeved shirts at dawn and dusk and using repellent will help protect against bites, she said.
The virus was first detected in the United States in 1999 and has since spread to every state but Alaska and Hawaii.
About one in five people who become infected will have a fever with other symptoms, such as headache, body aches, joint pains, vomiting, diarrhea, and a rash. Most will recover. Less than 1 percent will develop encephalitis or meningitis, according to the CDC.