CHICAGO (CBS) — Now that mosquitoes in portions of the northwest and southwest sides have tested positive for the West Nile virus spraying will take place overnight.

Mosquito abatement technicians hired by the city sprayed in portions of the Auburn-Gresham, Forest Glen and Washington Heights neighborhoods at dusk, and will continue spraying until about 1 a.m. Thursday, weather permitting.

Although the Zenivex spray is said not to be harmful to pets or people, and degrades rapidly, The Chicago Department of Public Health is urging residents of all three neighborhoods to stay inside and close windows when the trucks pass.

The department’s Environmental Health Medical Director, Dr. Cort Lohff, said he considers the spraying “sensible and effective.”

Lohff said Chicagoans can do their part by regularly ridding property of standing water from flowerpots, gutters, pool covers, pet water dishes and birdbaths.

The department continues to monitor traps citywide for infected mosquitoes.