Spotted Fever Vaccine; Potency Assay by Direct Challenge of Vaccinated Mice with Toxin of Rickettsia Rickettsii

A mouse protection test, devised for the assay of Rocky Mountain spotted fever vaccine, determined the minimal quantity of vaccine capable of protecting mice against 2 LD50 toxic doses of Rickettsia rickettsii. This technique, which has several advantages over the standard guinea pig potency test, gave relatively consistent results. Thus, potency values were usually reproducible within 0.5 log dilution of vaccine, but variations as great as 0.8 log were observed. Several samples of yolk-sac products and tick-tissue (Dermacentor andersoni) vaccines which varied from 1 to 14 years in age exhibited mouse protection titers of from 1.0 to 2.2 logs. The minimal doses of vaccine required to protect guinea pigs and mice against challenge with R. rickettsii were similar. An antitoxin test similar to the standard method for potency assay of typhus vaccine was also studied. Minimal doses of spotted fever vaccine capable of stimulating a demonstrable amount of antitoxin in the serum of vaccinated guinea pigs were also capable of inducing resistance in these animals. Marked resistance was found to develop in both mice and guinea pigs by the 3rd day following vaccination. Serum antitoxin also developed rapidly in guinea pigs, reaching maximal titer by the 5th day after vaccination. When immunized guinea pigs were challenged with doses of R. rickettsii varying from 50 to 5000 ID50, the resulting vaccine protection titers were similar. However, a challenge dose of 5 ID50 resulted in a 4-fold increase in protective titer.