Detailed serologic characterization of 79 isolates of Rickettsia tsutsugamushi from Pakistan was performed by using complement fixation and direct immunofluorescence techniques for comparison with three prototype strains. Seventy-one were of the Karp serotype and one was of a Gilliam serotype. Seven isolates were multiply-reactive: one with Karp and Gilliam, 3 with Karp and Kato, and 3 with Karp, Gilliam and Kato. No strain failed to react with at least 1 of the 3 prototype antisera. No correlation of antigenic types with local geographic factors, animal hosts, vector mites, or specific strain virulence was observed. These findings suggest the possibility that antigenic heterogeneity may be more restricted in areas peripheral to the geographic epicenter of scrub typhus endemicity, that the serotypes are relatively stable genetically and that new geographic variants are probably infrequent. The importance of these findings to the practical problems of immunoprophylaxis and serodiagnosis are discussed.
Present address: Rickettsial Disease Section, Department of Hazardous Microorganisms, Walter Reed Army Institute of Research, Washington, D. C. 20012.