DALLAS — 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.