Small, rod-shaped, ellipsoidal, coccoid and diplococcus-shaped, often pleomorphic organisms which are often intimately associated with arthropod tissues, usually in an intra- cellular position. Gram-negative. The species pathogenic for vertebrates have not been cultivated to date in cell-free media. May be parasitic in man and other animals causing disease (typhus and related ills) that may be transmitted by invertebrate vectors (chiefly lice, fleas, ticks and mites). Information is still inadequate for the systematic assignment of many of the species which inhabit arthropod hosts and which were originally described in this family. Key to the tribes of family Rickettsiaceae. I. Adapted to existence in arthropods; vertebrate hosts include man; cells rod-shaped, ellipsoidal, coccoid and diplococcoid; rarely filamentous. Tribe I. Rickettsieae, p. 935. II. Only a few species adapted to invertebrate existence; pathogenic for certain mammals but not for man; cells spherical, occasionally pleomorphic. Tribe II. Ehrlichieae, p. 948. III. Adapted to existence in arthropods as symbiotes but not in vertebrates as highly pathogenic parasites; cells pleomorphic, coccoid to short or long and curved rods, or even filamentous. Tribe III. Wolbachieae, p. 952. TRIBE I. RICKETTSIEAE PHILIP, TrIB. NoV. {Rickettsiaceae (sic) Philip, Ann. N. Y. Acad. Sci., 56, 1953, 486; Rickettsieae Philip (nomen nudum), Canad. Jour. Microbiol., 2, 1956, 262.) Ri.ckett.si'e.ae. M.L. fem.n. Rickettsia tj^pe genus of the tribe; -eae ending to denote a tribe; M.L. fem.pl.n. Rickettsieae the Rickettsia tribe. Small, pleomorphic, mostly intracellular organisms adapted to existence in arthropods and pathogenic for suitable vertebrate hosts. Key to the genera of tribe Rickettsieae. I. Non-filterable; produce tj^phus-like rash and usually Proteus X (Weil-Felix) agglutinins in man. Genus I. Rickettsia, p. 935. II. Filterable; produce neither rash nor Weil-Felix agglutinins in man. Genus II. Coxiella, p. 947. Genus I. Rickettsia da Rocha-Lima, 1916. (Da Rocha-Lima, Berl. klin. Wochnschr., 53, 1916, 567; Dermacentroxenus Wolbach, Jour. Med. Res., ^7, 1919-20, 87; Rochalimaea Macchiavello, Prim. Reunion Interamer. del Tifo, Mexico, 1947, 410; Zinssera Macchiavello, ibid., 416.) Ri.ckett'si.a. M.L. fem.n. Rickettsia named for H. T. Ricketts, one of the discoverers of the organisms bearing his name, who eventually lost his life while studying typhus infection in Mexico. Small, often pleomorphic, rod-shaped to coccoid organisms which usually occur intra- cytoplasmically in lice, fleas, ticks and mites. Occasionally occur extracellularly in gut lumen. Non-filterable. Gram-negative. Have not been cultivated in cell-free media. Patho- genic species parasitic on man and other animals. Cause mild to severe typhus-like infee-
