IV. Encephalitis

C. Pathological Changes (cont) :

2. Examples:  

a. Arthropod-borne viral (arboviral) encephalitis is responsible for most outbreaks of epidemic viral encephalitis.  Most of these viruses have animal hosts and mosquito vectors. The pathology due to different viruses is similar, although the severity and area affected (cortex or basal ganglia) varies.

b. Herpes Simplex Virus Type 1 (HSV-1) produces the most common non-epidemic form with high mortality levels. Acyclovir is an effective treatment, leading to reduction in the mortality rate. The most common presenting symptoms are alterations in mood, memory and behavior, due in part to lesions in the limbic system. Lesions involve the inferior and medial regions of the temporal lobes, and the orbital gyri of the frontal lobes.  Pathological changes include necrosis and hemorrhagic regions. Perivascular inflammatory infiltrates and inclusion bodies are usually present.

c. Rabies is transmitted by the bite of a rabid animal. The virus enters the CNS by ascent along peripheral nerves. Microscopic changes include Negri bodies, pathognomonic findings that consist of inclusions in pyramidal neurons of the hippocampus.