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 636; Rickettsia {Rickettsia) orientalis Philip, Amer. Jour. Hyg., 37, 1943, 305; Dermacen- troxenus orientalis Moshkovskiy, Uspekhi Souremennoi Biologii (Advances in Modern Biology), 19, 1945, 13; Rickettsia orientalis var. tropica Hayakawa and Hokari, A com- parative study of Japanese and tropical (scrub typhus) tsutsugamushi diseases (R. orientalis var. tropica), Tokyo, 1947, 35; Zinsser a orientalis Macchiavello, Prim. Reunion Interamer. del Tifo, Mexico, 1947, 416; Rickettsia {Zinsser a) tsutsugamushi Philip, Ann. N. Y. Acad. Sci., 56, 1953, 487; Tromhidoxenus orientalis Zhdanov and Korenblit, Jour. Microbiol., Epidemiol, and Immunobiol. (Russian), No. 9., 1950, 42; also see Zhdanov, Opredelitel Virusov Celovska i Zivotmych, Izd. Akad. Med. Nauk, U. S. S. R., Moskau, 1953, 54 and 160.) tsu.tsu.ga.mu'shi. From two Japanese ideographs transliterated tsutsuga some- thing small and dangerous, and mushi a creature now known to be a mite. Small, pleomorphic bacterium-like micro- organisms, usually wider and less sharply defined than the cells of Rickettsia prowa- zekii, R. typhi, R. rickettsii and Coxiella burnetii. Ellipsoidal or rod-shaped, often appearing as a diplococcus or as a short bacillus with bipolar staining resembling that of the plague bacillus. Diffuselj^ dis- tributed in the cytoplasm of the cell. 0.3 to 0.5 by 0.8 to 2.0 microns. Non-motile. Colored purplish with Giemsa's stain and red against a blue background with Mac- chiavello's stain. Stains well with azur III and methylene blue. Gram-negative. Cultivation: Grows in plasma tissue cul- ture of mammalian cells, on the chorio- allantoic membrane and in the yolk sac of the chick embryo and in rabbit testes and in the endothelial cells overlying Descemet's membrane of the rabbit eye. The only species of Rickettsia known to separate in the interface during ether treatment after harvest of yolk sacs. Wej^er (Acta Tropica, 11, 1954, 194) has reported the unusual dif- ficulty of growth in inoculated arthropods. Resistance to chemical and physical agents: Readily inactivated by heat and chemical agents. Destroyed by 0.1 per cent formalin and 0.5 per cent phenol. Killed in 10 minutes at 50° C. Immunology: Immunity conferred by infection, which is probably influenced by strain differences, appears less complete than that found in typhus and Rocky Mountain spotted fever. Strains from several different areas have been found to cross immunize in guinea pigs, but the true relationship of the disease occurring in dif- ferent localities remains to be determined. Reciprocal cross immunity between mite strains and human strains has been demon- strated in rabbits, hamsters and mice. Serology: Antigens from different strains vary in sensitivity when tested by comple- ment fixation with immune sera. There are probably a number of different types on the basis of complement fixation with immune sera. Possesses a common antigenic factor with Proteus OXK. Pathogenic for man, monkeys, gibbons, guinea pigs, hamsters, rats, voles, mice, gerbilles, rabbits (by intraocular injection) and chick embryo. There is a wide variation in the virulence of different strains for lab- oratory animals: infection with a few is established with difficulty while other strains may cause a high mortality. The white mouse is the laboratory animal of choice. Infection may persist in the brains of white rats for at least 98 days. A febrile reaction may occur in guinea pigs. Passage in guinea pigs and mice is accomplished by inoculation of peritoneal washings, spleen or blood from an infected animal, passage in rabbits by intraocular or intratesticular inoculation of blood, lymph node or organ emulsions of infected native animals. As- cites and enlarged spleen, often with a fibrinous deposit, are characteristic. A specific toxin, lethal for white mice, has been reported in the Gilliam strain. In man an eschar often develops at the site of the mite bite with accompanying adenopathy. A febrile reaction with exanthema occurs, and mortality is variable in different localities but remains about the same in a given focus. In rabbits, infection of Descemet's mem- brane follows intraocular injection of in- fected material. Source: Observed by Hayashi in smears and sections of the lesion (eschar) at the site of the mite bite and in smears and sec-