Isolation of St. Louis Encephalitis Virus During Inter and Epidemic Periods

Abstract

In the interval between the 1933 encephalitis epidemic and the present outbreak of the disease, neutralization tests have been made to determine the presence of virucidal antibodies in serum, obtained during convalescence, from 10 patients whose illness had been diagnosed as acute encephalitis. In none of these instances was the test positive. Efforts have also been made to isolate the virus from human brain material obtained at autopsy from 6 fatal cases in which encephalitis was regarded as the cause of death. Intracerebral injections in mice∗ 1 2 which proved susceptible in 1933, failed to demonstrate the virus in any of these 6 attempts. Pathological lesions consistent with encephalitis were observed in 3 of these fatal cases, 2 of them being in children.

The clinical diagnosis in the cases without brain lesions might be regarded as incorrect. Concerning those which showed pathological lesions, however, another possibility is that these sporadic cases of the disease were produced by a virus of altered virulence which was not infective for mice in the dilutions employed.

Recalling this interepidemic experience, it was surprising to note the ease with which the virus was isolated from the fatal cases occurring this summer. Using again the same method of intracerebral injection of human brain emulsion into mice, 7 different strains of the virus have already been obtained from 19 different fatal cases. This incidence of the infectivity of brain tissue is almost the same as that observed in 1933 when the virus was first isolated. At that time 7 out of 15 brains were shown to contain active virus by producing the disease when injected into Macacus rhesus monkeys. 1 Webster 2 obtained 5 strains out of 11 trials in mice.