(CNN) -- A West Nile virus epidemic has prompted a public health emergency in Dallas County, Texas, where the disease has killed nine people, a judge declared Friday.
The virus there infected 175 people, said Patricia Huston of Dallas County Health and Human Services.
Dallas County Judge Clay Jenkins declared the emergency in his capacity as director of the county's Homeland Security and Emergency Management and instructed the department to file a local disaster declaration with the state.
"This declaration will expand our avenues for assistance in our ongoing battle with West Nile virus," Jenkins said in a statement.
Insecticide spraying by planes will be offered to certain communities hit hard by the virus as long as those local governments request it, Jenkins told reporters.
The aerial spraying would occur from 10 p.m. to 6 a.m., when children are inside, Jenkins said.
"The insecticide is safe," Jenkins said. "The planes are quite sophisticated, and they get the spray to where it needs to go."
The judge organized an invitation-only work session Friday with county, state and federal officials to discuss a response to the epidemic.
The United States is experiencing its biggest spike in West Nile virus since 2004, with 241 cases of the disease reported nationwide this year so far, including four deaths, health officials said last weekend, before the latest totals.
Of the 42 states that have reported infections in people, birds or mosquitoes, 80% of them have been in Texas, Mississippi and Oklahoma, the U.S. Centers for Disease Control and Prevention said in a statement. The CDC listed a breakdown of infections by state.
"It is not clear why we are seeing more activity than in recent years," said Marc Fischer, a CDC medical epidemiologist. "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."
The virus is transmitted through infected mosquitoes.
In the United States, most infections occur between June and September, and peak in August, according to the CDC.
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 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.
CNN's Carma Hassan contributed to this report.