SAUSALITO, CA (ASRN.ORG) -- An outbreak of West Nile virus has engulfed Dallas County, with nearly 200 cases of human infection and 10 deaths, leading the mayor of Dallas to declare a state of emergency and to authorize the first aerial spraying of a pesticide in the city since 1966.

The high number of infections and deaths from the mosquito-borne disease marks the nation’s worst outbreak of West Nile in a year that has already logged a record number of cases across the country. The virus has become endemic in the United States since the first outbreak in 1999.

An official with the federal Centers for Disease Control and Prevention said the Dallas-area outbreak was probably a harbinger of a larger spread of the virus into other parts of the country. In Cook County, Ill., which includes Chicago, human cases of infection rose steadily this week, from 5 on Monday to 8 on Wednesday to 10 on Thursday, though no deaths had been reported, the authorities said.

Texas officials say the statewide death toll so far is 17, the most West Nile-related fatalities of any state.

In a report, the C.D.C. said that as of last Tuesday, 693 cases of infection had been reported nationwide. Louisiana had six deaths, according to the report, and no other state had more than one.

“With this huge outbreak in Texas, the jury is still out on what’s going to happen with the rest of the country,” said the official, Dr. Lyle R. Petersen, director of the C.D.C.’s Division of Vector-borne Infectious Diseases. “But in Chicago, we’ve already observed high numbers of West Nile virus-infected mosquitoes. This is looking like a large regional event. We don’t know if the number of cases is going to drastically increase, but we do expect more cases.”

And yet, as local and state officials have stepped up their efforts to fight West Nile in the Dallas area, there has been a kind of backlash, with many residents growing more concerned about the aerial spraying than the virus itself.

More than 1,700 people signed an online petition on Change.org calling on Dallas officials to stop the spraying, describing it as ineffective, unsafe and harmful to insects like honeybees and ladybugs. A number of other cities in Texas and around the country do aerial spraying to reduce their mosquito populations — including New York, which recently sprayed over uninhabited wetlands on Staten Island — but this is the first time Dallas is doing so in more than 45 years.

Though officials in Dallas describe the procedure as safe and effective, they have added to some residents’ worries by advising those concerned about exposure to avoid being outside, close their windows and keep their pets inside while spraying occurs. “I think residents need to take the precautions that they’re comfortable with,” said Frank Librio, a city spokesman.

The aerial spraying was to begin Thursday at 10 p.m. in a 106,000-acre section of the city and county, including the wealthy areas of University Park and Highland Park. Twin-engine planes flying about 300 feet above the ground will spread a pesticide called Duet to kill the adult mosquito population.

Duet has been approved by the federal Environmental Protection Agency for ground and aerial use in outdoor residential and recreational areas, and it is similar to the pesticide the city has been using as part of a truck-mounted spraying operation it began in June. The last time Dallas conducted aerial spraying was in the summer of 1966, to combat an outbreak of St. Louis encephalitis, a mosquito-borne infection. That outbreak killed 14 people in the county.

The spraying operation this time is being led — and paid for — by the state. More than half of the human cases of infection in the United States this year are in Texas, which has confirmed more than 400 cases statewide.

More planes are scheduled to spray Friday night and possibly over the weekend. “The disease poses an immediate public health threat to Dallas County,” Dr. David Lakey, the commissioner of the Texas Department of State Health Services, said in a statement. “We need to use all possible tools, including aerial spraying, to fight this outbreak.”

Five of the 10 deaths in the county occurred in Dallas, the third-largest city in Texas, with a population of 1.2 million. Mayor Michael S. Rawlings declared the state of emergency on Wednesday, one week after officials issued a similar declaration for Dallas County. Dallas officials have asked the state to spray the entire city.

Dr. Petersen with the C.D.C. said it was difficult to say why the Dallas area has had such a severe outbreak, but he said that the early spring and the hot summer were likely culprits, because heat affects factors like mosquito abundance. Hot weather both increases the mosquito population and causes more of the virus to build up in their salivary glands.

“That summer in New York City when it was discovered in this country — 1999 — was a very hot summer,” Dr. Petersen said. “In 2002, 2003, when it was all over the U.S., it was abnormally hot. We had an early spring and abnormally hot weather this year, so that could be a factor.”

A spokeswoman for the New York City health department, Alexandra Waldhorn, said the city had only 3 cases so far, with only 11 in all of last season.

At 10:16 p.m. in University Park, a low-flying plane with two misty trails streaming behind it flew over James Smith, 41, who stood with his girlfriend on a shop-lined street near Southern Methodist University. He had no reservations about being outside.“I think whatever the risk may be, if there’s any, it’s outweighed by the risk of the mosquitoes that are infecting people,” he said. “There’s a lot of things out there that can kill you. I don’t think this is one of them.”

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