DALLAS - Suffering from the nation's deadliest outbreak of West Nile virus this year, Dallas County authorized aerial spraying of insecticide on Friday for the first time in nearly five decades to help fight the mosquito-born illness.

Texas' second most populous county announced the decision after its leaders met with the state's top health official and experts from the Centers for Disease Control and Prevention. Nearly 90 cases of the most severe form of West Nile have been confirmed in the county so far, nine residents have died, and the virus' peak season is just beginning.

"This is a matter of extreme concern, and we're going to follow the science and do what's best for our people," said Dallas County Judge Clay Jenkins, the county's top elected official.

Five planes have been requested for spraying the heavily populated northern part of Dallas as well as the nearby enclaves of Highland Park and University Park — the most affected areas — but they won't be used until leaders in those jurisdictions approve, said Jenkins, who urged the cities to allow the planes in.

Dallas Mayor Mike Rawlings and officials from the other cities said they plan to meet with other city, county and state officials and industry experts early next week to decide how to proceed.

Jenkins on Thursday declared a public health emergency. Three nearby counties have reported one death each.

There is no vaccine for the virus, which has been in the U.S. since about 1999, according to the CDC. The virus, which most often affects people over 50, can cause high fevers, headaches and disorientation.

Public health officials typically advise residents of mosquito-prone areas to drain standing water, apply insect repellent containing the ingredient DEET and wear long-sleeve shirts and long pants. But officials at Friday's meeting said it's clear that those warnings and ongoing ground-based spraying aren't enough.

"It seems like the avoidance strategy is not working, so now you have to kill the bug," said Dr. Rick Snyder, president of the Dallas County Medical Society.

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The Texas Department of State Health Services this year has tracked 214 cases of neuroinvasive West Nile, the most serious form of the illness, including 89 in Dallas County. Noting that the peak season for the illness is just beginning, agency officials said they fear the state will break the record number for such cases — 438 — reported in 2003.

"This is a major outbreak," said Dr. David Lakey, the department's commissioner. "People need to do all they can to protect themselves."

Aerial spraying is controversial. Some fear health effects from chemicals falling on them from the sky, and others have questioned whether the approach was scientifically proven to stop West Nile cases.

But at least one study in California has concluded that the odds of infection are about six times lower in treated areas than those that are untreated.

The American Mosquito Control Association doesn't keep exact statistics, but an association spokesman said spraying is common in Florida, and ongoing programs exist in other states including Louisiana, New Jersey, Georgia, North Carolina, Michigan and Minnesota.

The Texas health agency said the intensity of the disease fluctuates from year to year in the state depending on factors such as weather and the number of birds and mosquitoes that spread it.

The large number of West Nile cases this year is due to the extremely hot weather and recent rains, according to the agency.

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Associated Press writer Mike Stobbe in Atlanta contributed to this report.