The California encephalitis virus is a rare cause of disease in the far west, while its close relative, the LaCrosse virus, is a more frequent cause of encephalitis in the east and midwest. The virus is injected subcutaneously and replicates initially in the skeletal muscles. Plasma viremia carries the virus to the CNS, where it replicates in the neurons ands glial cells.
Damage is due to both inflammation and necrosis.
The disease occurs most frequently in July, August, and September. It starts with non-specific symptoms, which are followed by the CNS signs and symptoms of stiff neck, lethargy, and seizures. Lasting objective cognitive deficits are rare.