The West Nile virus has been detected in mosquitoes in Rockland, the state Department of Public Health said Friday. Though no human cases have been reported in the state this summer, mosquitoes carrying the virus have been detected in 19 Massachusetts communities, including a recent discovery in Jamaica Plain, the first within Boston city limits this summer. The virus is typically transmitted through the bite of an infected mosquito. Symptoms range from headaches to severe fevers, though most do not exhibit symptoms of the virus initially. People over age 50 are said to be more susceptible to severe effects.