Judge Clay Jenkins declared the emergency Friday in his capacity as director of the county's Homeland Security and Emergency Management agency and instructed the department to file a local disaster declaration with the state.
Ground and aerial spraying will happen as weather allows.
"The insecticide is safe," Jenkins said. "The planes are quite sophisticated, and they get the spray to where it needs to go."
The Indiana State Department of Health announced Wednesday that state's first West Nile virus death this year. The death occurred in Vanderburgh County, said spokeswoman Amy Reel. She said she was not permitted to provide further details.
Six other cases have been reported across Indiana.
Louisiana health officials said the more serious form of the virus has seen its highest level since 2006. So far, six people have died from the virus in Louisiana this year.
In the United States, most infections occur between June and September, and peak in August, according to the CDC.
"It is not clear why we are seeing more activity than in recent years," CDC medical epidemiologist Marc Fischer said Friday. "Regardless of the reasons for the increase, people should be aware of the West Nile virus activity in their area and take action to protect themselves and their family."
Symptoms include fever, headache, body aches, joint pains, vomiting, diarrhea or rash.
"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues)," the CDC said.
Those at greater risk are people older than 50 and those with conditions such as cancer, diabetes and kidney disease, or with organ transplants.
There are no medications to treat the West Nile virus or vaccines to prevent infection. People with milder illnesses typically recover on their own, but those more seriously affected may need hospital care.
Health experts say prevention measures include avoiding mosquito bites, using insect repellant and getting rid of insect breeding sites.