Proliferating sites of Rickettsia tsutsugamushi in mice by different routes of inoculation evidenced with immunofluorescence.

: The sites of multiplication of Rickettsia tsutsugamushi in mice by different routes of inoculation were investigated by means of immunofluorescence. R. tsutsugamushi (Kato, Karp, Gilliam and Miyake strains) inoculated by intraperitoneal route multiplied significantly in mesothelial and inflammatory cells of the peritoneal cavity of DDD and BALB/c (nu/nu) mice. The rickettsial growth was not detected in parenchymal cells of the liver, spleen, kidneys and lymph nodes of DDD mice, but slightly observed in those cells of the nude mice. When inoculated by subcutaneous route, the rickettsial antigens were detected in small amount in the peritoneal mesothelial and inflammatory cells, whereas a marked proliferation of R. tsutsugamushi was demonstrated in liver cells, Kupffer cells, and endothelial cells of the sinus of spleen and lymph nodes. A specific antibody to Karp strain, administered intraperitoneally, prevented effectively rickettsial growth in the peritoneal cavity of BALB/c (nu/nu) mice challenged intraperitoneally, 6 hours later, with Miyake strain (Karp type) and the animals were infected after a long-term incubation period showing the findings of rickettsial localization similar to that in subcutaneously inoculated animals. This fact suggests a partial involvement of humoral immunity in resistance of mice against R. tsutsugamushi infection. The potential importance of these findings in experimental infection with R. tsutsugamushi was discussed.