Infection with Saint Louis Encephalitis virus rarely results in severe symptoms. It is closely related to West Nile virus and distinguishing between the viruses requires advanced diagnostic tests.
Infected mosquitoes pass the virus to birds when they take a blood meal.
Mosquitoes can transmit St. Louis encephalitis virus to humans too. Few infected people develop symptoms, but serious disease can occur. The health risk is higher for elderly and immune compromised persons. Saint Louis Encephalitis was last detected in mosquito samples within the District in 1993, but is currently re-emerging in various areas of California.