Western and St. Louis encephalitis viruses (sylvatic cycles)

The natural inapparent cycle is between Culex tarsalis and nestling and juvenile birds, but this cycle may be amplified by infection of wild and domestic birds and domestic mammals. Western encephalitis virus can replicate in the mosquito at cooler temperatures, so epidemic disease in horses and humans may occur earlier in the summer and farther north into Canada. St. Louis encephalitis virus in the eastern United States involves Culex pipiens and other urban mosquitoes and causes urban epidemics. (Used with permission: Johnson RT. Viral infections of the nervous system. 2nd ed. Philadelphia: Lippincott-Raven, 1998:87-132.)