tions of the adjacent lymph nodes from cases of the disease ; also observed by Ogata (Zent. f. Bakt., I Abt., Orig., 163, 1955, 150) as early as 1927 in preparations of infected rabbit testicles and by Nagayo et al. (op. cit., 1930) in the endothelial cells overlj-ing Descemet's membrane in rabbits inoculated intraocularly with infectious material. Habitat: Found in trombiculid mites (particularly Tromhicula akamushi and T. (or var.?) deliensis). Passes through the mite ova to the next generation. Only the larvae are parasitic on vertebrates. Reser- voir animal hosts are probably wild rodents, including house and field rats, mice and voles, and probably some birds in which infection may be persistent. The etiological agent of tsutsugamushi disease and of scrub typhus (for numerous other designations of the disease, see Farner and Katsampes, U. S. Naval Med. Bull., ^S, 1944, 800). Many human cases have recently been discovered well south of the classic foci in Japan, and Sasa (Jap. Jour. Exp. Med., 2^, 1954, 335) discusses four epidemiologically distinct "types." Subgenus C. Dermacentroxenus (Wolbach, 1919) Philip, 1943. (Wolbach, Jour. Med. Res., 41, 1919-20, 87; subgenus Dermacentroxenus Philip, Amer. Jour. Hyg., 37, 1943, 304; Acaroxenus Zhdanov and Korenblit, Jour. Microbiol., Epidemiol. and Immunobiol. (Russian), No. 9, 1950, 42; Ixodoxenus Zhdanov, Opredelitel Virusov Celovska i Zivotmych, Izd. Akad. Med. Nauk, U.S.S.R., Moskau, 1953, 51 and 155; Garnasoxenus Zhdanov, ibid., 159; see Philip, Canad. Jour. Microbiol., 2, 1956, 264.) Der.ma.cen.tro'xe.nus. M.L. noun Dermacentor a genus of ticks; Gr. noun xenus host, guest; M.L. mas.n. Dermacentroxenus tick dweller. Organisms capable of intranuclear parasitism. Produce a typhus-like disease. Trans- mitted by acarid vectors. The type species of the subgenus is Rickettsia rickettsii (Wolbach) Brumpt. 4. Rickettsia rickettsii (Wolbach, 1919) Brumpt, 1922. {Dermacentroxenus rickettsi (sic) Wolbach, Jour. Med. Res., 41, 1919-20, 87; Rickettsia rickettsi (sic) Brumpt, Precis de Parasitologic, 3rd ed., 1922, 757; Rickett- sia brasiliensis Monteiro, Mem. Inst. Butantan, 6, 1931, 3; Rickettsia typhi Franco do Amaral and Monteiro, Rev. Sud. Amer. de Med. et Chirurg., 4, 1933, 806; Derma- centroxenus rickettsi VEiT. brasiliensis Finker- ton. Parasitology, 28, 1936, 186; Rickettsia {Dermacentroxenus) rickettsi Philip, Amer. Jour. Hyg., 37, 1943, 304; Rickettsia colombi- ensis Veintemillas, Tratado sobre rickett- siasis, etc., Bolivia, 1944, 102; Ixodoxenus rickettsi Zhdanov, Opredelitel Virusov Celovska i Zivotmych, Izd. Akad. Med. Nauk, U.S.S.R., Moskau, 1953, 51 and 155.) ri.ckett'si.i. M.L. gen. noun rickettsii of Ricketts; named for Howard Taylor Rick- etts, who first saw and described the organ- isms causing Rockj' Mountain spotted fever. Minute, paired organisms surrounded by a narrow clear zone or halo; often lanceo- late, resembling in appearance a minute pair of pneumococci. Average 0.6 by 1.2 microns under the electron microscope. Non-motile. In smears of mammalian tissues there occur, in addition to the lanceolate forms, slender rod-shaped forms stained blue with Giemsa stain, sometimes exhibiting polar granules stained purplish or reddish. There are also minute, pale blue-staining, rounded forms. In the tick there are three forms: (1) pale blue bacillary forms curved and club- shaped, (2) smaller, bluish rods with deeply staining chromatoid granules, and (3) more deeply staining, purplish, lanceolate forms. A very minute form may appear in tightly packed masses in the nuclei of the cells. Occurs in the cytoplasm and nucleus in all types of cells in the tick including sperm cells; also occurs in mammals in the vascular endothelium, in macrophages, in the serosal cells of the peritoneal cavity and in smooth- muscle cells of arteriolar walls. In yolk-sac cultures and in the Maitland media cultures, bacillary forms often occur in pairs. In single smears from infected yolk sacs, the cells are rather uniform in size and mor- phology and are definitely larger than those of Rickettsia prowazekii and R. typhi. They
