Effect of X-Ray on Experimental Encephalitis in Mice Inoculated with the St. Louis Strain. ∗

Abstract

Since perivascular lymphocytic infiltration is the essential lesion in encephalitis and since lymphocytes are highly sensitive to irradiation, it seemed logical that small doses of X-ray should be effective as a therapeutic measure. The treatment by X-ray of 3 cases of encephalitis in human patients has been reported. 1 Since then 4 other cases have been treated. One was an early case and recovered completely in a short time. Two others were more protracted, having had the disease for 3 months before treatment was instituted. In these 2 latter cases various groups of muscles were showing progressive paralysis. The patients showed marked improvement after a series of X-ray treatments and are symptom-free to date. The fourth case, one of 6 months duration with early Parkinsonian syndrome, is still under treatment but shows improvement.

Although the clinical evidence is very suggestive the findings are empirical and since the clinical studies can not be controlled, it was decided to carry out the treatments under experimental conditions, Mice are known to be highly susceptible to the St. Louis strain of virus. 2 The infective dose has been standardized and the incubation period is known. 3 If mice that were treated with X-ray after inoculation with the virus recovered or had the period of incubation or the duration of the disease prolonged, it seemed certain that the X-ray had a definite effect upon the disease.

A series of normal mice exposed to varying amounts of X-ray to determine how much they could tolerate, showed no apparent ill effects after a series of 10-minute treatments (each equivalent to 1/2 a human erythema dose) given daily for 10 days. Preliminary experiments showed that the small X-ray doses given 48 to 72 hours apart that proved so effective in human beings were not effective in mice because of the fulminating character of the disease. Therefore more prolonged treatments at 24-hour intervals were given. Each treatment was 135 K.V., 5 ma., distance 15 inches, filters 3 mm. aluminum, time 5 minutes, equivalent to one-quarter of a human skin erythema dose.