The Andover Board of Health is advising residents to drain any standing water on their property to prevent their yard from becoming a source of mosquitoes, which thrive in water-holding containers, including birdbaths, children's toys, stored tires, and clogged gutters. Residents should also ensure that air conditioner drip pans are empty and that water features in landscaping are aerated with pumps. Mosquitoes are the primary transmitters of West Nile virus, a virus that can cause fever, meningitis, or encephalitis. Early symptoms include fever, headache, stiff neck, and muscle weakness. A sumithrin-based insecticide with low mammalian toxicity recently was sprayed south of Route 133, from the Tewksbury line to Bellevue Road, along Bellevue Road and Blanchard Street and the nearby side streets, to address the recent discovery of mosquitoes in Tewksbury that tested positive for West Nile virus. Residents also are being advised to take common-sense precautions: Avoid going outdoors between dusk and dawn, wear long sleeves and pants, and use a mosquito repellent with DEET when outside. For more information, visit the Massachusetts Department of Public Health website at www.mass.gov/eohhs/gov/departments/dph/.