Gamma-Irradiated Scrub Typhus Immunogens: Development and Duration of Immunity

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The development and duration of immunity to lethal scrub typhus infection was studied in BALB/c mice vaccinated with gamma-irradiated Rickettsia tsutsugamushi, strain Karp. One intraperitoneal injection containing approximately 108 50% mouse lethal doses (MLD50) of irradiated organisms elicited an immune response protective against challenge with 105 MLD50 of viable Karp. The same mass of immunogen given in three injections at 5-day intervals increased homologous (Karp strain) protection 25-fold and heterologous (Kato strain) protection 60-fold. Further temporal expansion of the immunization regimen did not increase protection. Subcutaneous vaccination provided significant, but lower, levels of protection than were achieved by intraperitoneal immunization, but the levels of cell-transferable immunity elicited by the two routes were approximately the same. Immunologically specific protection after intraperitoneal vaccination developed rapidly enough to provide resistance against simultaneous challenge with 200 MLD50 of Karp. Homologous immunity was protective against a 106-MLD50 challenge 7 days after completion of the three-injection regimen, remained at that level for 3 months, dropped to 104 MLD50 by 9 months, and was effective against a 50-MLD50 Karp challenge at 12 months. Protection against heterologous challenge was first observed on day 17 and peaked on day 38, when the mice resisted a 105-MLD50 Kato challenge. Thereafter, heterologous protection waned rapidly and was not significant at 6 months.

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