The La Crosse Virus from the same genus is also a common cause of encephalitis in the United States.

An early symptoms phase of 1–4 days commonly precedes the onset of encephalitis, manifesting as fever, chills, nausea, vomiting, headache, lethargy and abdominal pain.

[2] The encephalitis is characterized by fever, drowsiness, and lack of mental alertness and orientation.

Secondary spread and the multiplication of the virus in the CNS (central nervous system) causes symptoms such as stiff neck, lethargy and seizures.

A primary spread of virus occurs, with seeding of the reticuloendothelial system, mainly in the liver, spleen, and lymph nodes.

The California encephalitis virus invades the CNS through either the cerebral capillary endothelial cells or the choroid plexus.

Factors influencing the transmission and control of arboviral encephalitis, in general, include: the season, geographical location, patient age, and the regional climate.

[2] In the US the highest occurrence is in the Midwestern states, with most cases occurring in the late summer to early fall.

[4] In all three cases there was strong laboratory evidence confirming infection, due to the presence of neutralizing antibodies linked to California encephalitis.