FAMILY I. RICKETTSIACEAE 



937 



Subgenus A. Rickettsia Philip, 1943. 



(Amer. Jour. Hyg., 37, 1943, 307.) 



Ri.ckett'si.a. M.L. fem.n. Rickettsia named for H. T. Ricketts. 



Organisms intracytoplasmic only; insect vectors, no transovarial transmission; OX19 

 agglutinins stimulated, but no eschar in human host. 

 The type species of the subgenus is Rickettsia prowazekii da Rocha-Lima. 



1. Rickettsia prowazekii da Rocha- 

 Lima, 1916. (Da Rocha-Lima, Berl. klin. 

 Wochnschr., 53, May 22, 1916, 5G7 ; Rickettsia 

 kairo da Rocha-Lima, in Kolle and Wasser- 

 mann, Handb. d. path. Mikroorg., Fischer, 

 Jena, 8, 1930, 1350; Rickettsia exanthemato- 

 typhi Kodama, Kitasato Arch. Exp. Med., 

 9, 1932, 360; Rickettsia prowazeki var. pro- 

 wazeki Pinkerton, Parasitology, 28, 1936, 

 186; Rickettsia prowazeki subspecies pro- 

 wazeki Philip, Amer. Jour. Hyg., 37, 1943, 

 307.) 



pro.wa.ze'ki.i. M.L. gen. noun prowazekii 

 of Prowazek; named for S. von Prowazek, 

 who lost his life studying typhus fever. 



Minute, cocco-bacillary, sometimes el- 

 lipsoidal or long, rod-shaped cells which are 

 occasionally filamentous. Often occur in 

 pairs and occasionally in chains. In infected 

 lice the minute coccoid and paired coccoid 

 forms predominate over the short and long 

 rods and over the filamentous forms, which 

 are up to 40 microns in length.* Single ele- 

 ments from yolk sacs under the electron 

 microscope average 0.5 by 1.1 microns with 

 maxima of 0.3 to 0.7 by 0.5 to 2.0 microns. 

 In resistant hosts, clumps resembling 

 morulae have been reported in infected 

 cells resembling those seen in some of the 

 other genera. Within the same smear of 

 infected mammalian cells, the organisms are 

 quite uniform in size and morphology. Occur 

 intracytoplasmically in vascular endothelial 

 cells and in serosal cells. Non-motile. Char- 

 acteristically colored purplish with Giemsa 

 stain; the two individuals of a pair are con- 

 nected by a zone of faintly blue-stained 

 material. Colored blue with Castaneda stain 

 and bright red against a blue background 

 with Macchiavello stain. Gram-negative. 



Cultivation: Growth occurs in plasma 

 tissue cultures of mammalian cells, in the 

 louse intestine, in modified Maitland media 

 with and without agar and in chorio-allan- 

 toic membrane and yolk sac of chick em- 

 bryo, the latter being currently the medium 

 of choice. Mouse and rat lungs have yielded 

 rich harvests following intranasal inocula- 

 tion. 



Optimum temperature, 32° C. in plasma 

 tissue culture and 35° C. in chick embryo 

 cells. 



Resistance to chemical and physical 

 agents: Readily inactivated by heat and 

 chemical agents. Death occurs at 50° C. 

 in 15 to 30 minutes; 0.5 per cent phenol and 

 0.1 per cent formalin also kill the organism. 



Immunology: Immunity prolonged but 

 may not be permanent in man. Indistin- 

 guishable from endemic (murine) typhus in 

 cross-immunity tests in guinea pigs, but 

 distinguishable from Rocky Mountain 

 spotted fever and other rickettsial diseases 

 in such tests though there is variable 

 reciprocal or partial cross-immunity. Neu- 

 tralizing antibodies are found in the serum 

 of recovered guinea pigs and convalescent 

 persons two to three weeks after deferves- 

 cence. Recrudescence of infection ("Brill's 

 disease") many years after an initial typhus 

 episode without intervention of lice has 

 recently been confirmed. Killed vaccines 

 produced from infected lice, rat lungs and 

 yolk sacs afford a high degree of protection 

 against the disease. Hyperimmune antisera 

 for therapeutic use have been produced in 

 rabbits by injection with infected yolk-sac 

 suspensions and in horses and donkeys with 

 infected mouse- lung suspensions. Atten- 

 uated living strains have also been used 

 successfully in human vaccination. 



* Gonnert (Zent. f. Bakt., I Abt., Orig., 162, 1947, 203) describes "atypische Ri. pr.- 

 Formen" with exaggerated pleomorphism and so-called "R-Formen" in lice infected by 

 injection. 



