Preparation of Vaccines for Rocky Mountain Spotted Fever from Rickettsiae Propagated in Cell Culture

Abstract

Rickettsia rickettsii was propagated in several established and primary cell cultures. Highest infectivity was obtained with primary cultures of cells from duck embryos (DEC), which developed a mean infectivity of 7.3 log yolk-sac LD₅₀/ml. Virulence for guinea pigs and toxicity for mice were lost during the initial passage in cell culture, but after 13 passages in DEC these characters were regained immediately on propagation in the chicken yolk sac. Rickettsiae propagated in DEC were inactivated by exposure to 0.1 % formaldehyde or to 5 × 10⁵ r of gamma radiation. The resulting vaccines were more than 900 times as active as yolk-sac vaccine, as estimated by the dilution giving equivalent inhibition of febrile response in guinea pigs after challenge. Treatment with formaldehyde yielded vaccines that were slightly more effective than did irradiation. The formaldehyde-treated vaccine retained its antigenicity after 14 days at 37 C, and guinea pigs maintained immunity for at least six weeks after immunization.