Local health officials have concluded that the West Nile virus is present in mosquitoes in town after insects trapped in neighboring communities tested positive for the virus. The town’s Department of Public Health issued a notice Monday raising the risk level from low to moderate, and cautioned residents to take steps to avoid mosquito bites. The state Department of Public Health has found mosquitoes that tested positive for West Nile virus in several area communities in recent weeks, including Belmont, Boston, and Watertown. The mosquito-borne virus can cause encephalitis, or swelling of the brain, in humans, and residents are at the highest risk of getting the disease in late July through the fall. Brookline has applied larvicide to catch basins, and an information line has been set up at 617-730-2295. Information can also be found at www.brooklinema.gov/health . Last year a Brookline woman in her 20s was infected with the virus, but recovered.