Studies on the E strain of Rickettsia prowazeki

Cultivation of E strain of Rickettsia prowazeki in mouse lung resulted in a marked increase in virulence for guinea pigs following 12 passages (but not 6 passages) in both immunosuppressed and normal mice. Attempts at passaging E strain in normal guinea pigs were unsuccessful. Isolates of E strain in the chicken embryo yolk sac either from testes and peritoneal washings or from the brain of immunosuppressed guinea pigs did not show enhanced virulence for guinea pigs.