West Nile virus was detected in New York City's mosquitoes, the city Department of Health said.
Infected mosquitoes were collected from the Glen Oaks neighborhood in Queens and along New Dorp Beach on Staten Island, the health department says.
Officials say that no cases of the virus have been reported in humans this year.
The health department said it is trapping the bugs and treating water-catching basins in the affected areas. Officials are also using larvicide to prevent the growth of mosquito populations.
At least five people in four of the city's boroughs contracted the virus last summer, the city said. Three of the patients had to be hospitalized.
Most people infected with the mosquito-borne virus either show no symptoms or suffer a flu-like illness. A small number develop a serious illness that can lead to neurological damage.
Protective measures against mosquitoes include eliminating standing water, using repellent, wearing long-sleeve shirts and long pants, and repairing doors and screens for tight fits.