Gamma-irradiated scrub typhus immunogens: broad-spectrum immunity with combinations of rickettsial strains

Abstract

Scrub typhus immunogens were prepared from Rickettsia tsutsugamushi strains Karp, Kato, Gilliam, Kostival and Buie by exposing frozen infected yolk sac suspensions o 300 krad of .gamma.-radiation. Mouse protection tests showed that each irradiated immunogens protected C3H/HeDub mice against high challenge levels of Karp and Gilliam, but that none of these single-strain immunogens could protect against all 5 challenge strains. Broad-spectrum protection was achieved by using co combinations of irradiated rickettsiae strains in a vaccination regimen of 3 injections at 5-day intervals. A comparison of vaccination efficacy employing 3 such combinations (Karp-Gilliam-Kato, Karp-Kostival-Kato and Buie-Kostival-Kato) indicated that sequential administration of strains on successive vaccination days and multiple injections of trivalent mixtures produced protective responses superior to those obtained with single-strain immunogens. Trivalent mixtures of rickettsiae exhibited a striking synergistic effect on the immune response of C3H/HeDub mice and elicited a protective response against Kato challenge that could not be obtained with any single-strain immunogen. Mice vaccinated with the trivalent Karp-Gilliam-Kato immunogen resisted challenge with more than 103 mouse LD50 of Karp and Gilliam for 12 mo. and were resistant to similar levels of challenge with Kato and Buie for 6 mo.