Local health officials announced Monday that the town has been upgraded from low to moderate risk for the West Nile virus, based on the presence of mosquitoes carrying the disease in the area. No human cases had been reported in Massachusetts this year as of Wednesday, said state Department of Public Health spokeswoman Anne Roach. Also elevated to moderate risk were Arlington, Medford, Malden, and Melrose, bringing the number of communities with the designation statewide to 24; the list includes Boston, Newton, and Cambridge, according to the state. Belmont’s health director, Stefan Russakow, in a statement on the new risk level, said that August is frequently associated with West Nile findings, and warned that people over 50 years old are at higher risk for severe complications from an infection. The announcement also urged residents to take precautions against being bitten by mosquitoes, with the recommended measures including using insect repellent and wearing long sleeves and pants when outside after dusk, and eliminating areas of standing water.