Host defenses in experimental scrub typhus: histopathological correlates

Intraperitoneal (i.p.) infection of BALB/c mice with 1,000 50% mouse lethal doses of the Karp strain of Rickettsia tsutsugamushi was inevitably lethal, and associated pathological alterations were confined to the peritoneal cavity. These included: (i) continuous proliferation of rickettsial organisms in peritoneal macrophages until death; (ii) hepatic granulomas appearing 6 days after infection and increasing in size and number until death; (iii) splenomegaly, resulting principally from proliferation of lymphoid tissue, and (iv) terminal peritonitis. Under two circumstances, i.p. infections with R. tsutsugamushi were not lethal: (i) infection with 100 50% mouse infectious doses of the Gilliam strain, which, in fact, resulted in immune protection against otherwise lethal Karp challenge; and (ii) Karp infection of animals immunized with the Gilliam strain. In both cases, the associated pathological abnormalities were, as with primary Karp infection, restricted to the peritoneal cavity. Also similar was the striking splenomegaly due to lymphoid proliferation, which was particularly prominent in immunized animals. In contrast to primary and lethal Karp infection, however, these infections were characterized by: (i) minimal and transient proliferation of rickettsial organisms in peritoneal macrophages; (ii) disappearance of hepatic granulomas; and (iii) absence of peritonitis. It was concluded that the survival of an animal bearing an i.p. infection of scrub typhus depended on its ability to concentrate a sufficiently vigorous immune response in the peritoneal cavity, resulting in the evolution of rickettsiacidal macrophages capable of suppressing the infection.