Western equine and St. Louis encephalomyelitis.

In a clinical review of 50 cases of western equine and 16 cases of St. Louis encephalomyelitis in humans it was noted that fever, headache, lethargy, drowsiness, tremor and stiffness of the neck were the most frequent signs or symptoms initiating the illness. The great majority of patients recovered without residual effect. These two diseases of the central nervous system can only be differentiated on an immunological basis but may be suspected during seasonal periods in geographical areas where these virus infections are known to exist. Neuropathological studies were done in four cases of human western equine and two cases of St. Louis encephalomyelitis. The primary point of attack by the virus is the cell body, the lesions being concentrated in the striate body, diencephalon, the brain stem and cerebellum. All histo-anatomical findings (nerve cell destruction, microglial nests, small isolated and confluent areas of necrosis and perivascular round cell infiltration) are secondary to the injury of the nerve cell body caused by the neurotropic virus.