Dallas County Judge Clay Jenkins, the county's director of homeland security and emergency management, declared the emergency Friday, CNN reported.
"This declaration will expand our avenues for assistance in our ongoing battle with West Nile virus," Jenkins said in a statement. "While we are busy doing everything we can to keep residents well informed and as protected as possible, we need your help."
Small planes will be spraying insecticide over certain areas that have been hit the worst by the virus, which is spread by mosquitoes, Jenkins said.
"The insecticide is safe," Jenkins said. "The planes are quite sophisticated, and they get the spray to where it needs to go."
Tarrant County, which has received 146 reported cases of West Nile virus in the past few weeks, has not yet declared an emergency, the Fort Worth Star-Telegram reported.
"We're just, again, trying to coordinate with different cities and have not felt we need to be looking at aerial spraying at this point in time," said Tarrant County Judge Glen Whitley.
Officials in Houston Friday warned residents of an increased threat of West Nile-infected mosquitoes, the Star-Telegram said.
"Houston can definitely expect an increase in West Nile disease," said Kristy Murray, an infectious disease specialist in Baylor College of Medicine's National School of Tropical Medicine. "From mid-August through September is the big season here."