also grow more sparsely. Stain blue with the Castaneda stain and bright red against a blue background of tissue with the Mac- chiavello stain. Gram-negative. Cultivation: May be cultivated in plasma tissue culture of mammalian cells, in Mait- land media with and without agar, on the chorio-allantoic membrane, in the yolk sac of the chick embryo and in ticks. Growth and toxin production are enhanced in killed embryo continued in incubation 24 hours. Growth in intrarectally injected human- body lice destroys intestinal epithelium and may destroy these insects (Weyer, Acta Tropica, 11, 1954, 193). Optimum temperature, 32° C. in plasma tissue culture and 35° C. in chick embrj'o cells. Killed in 10 minutes at 50° C. Resistance to chemical and physical agents: Readily inactivated bj^ heat and chemical agents. Destroyed by 0.5 per cent phenol and 0.1 per cent formalin. Destroyed by ordinary desiccation in about 10 hours. Immunology: Prolonged immunity in man and animals after recovery from infection. Killed vaccines produced from infected ticks and from infected yolk sacs afford considerable protection against the disease. Therapeutic antisera have been produced by the injection of rabbits with infected tick and yolk-sac suspensions. No cross im- munity between spotted fever in guinea pigs recovered from infections with Rickettsia rickettsii and typhus in guinea pigs re- covered from infections with R. prowazekii and R. typhi. Cross immunity between spotted fever in guinea pigs recovered from infections with 72. rickettsii and boutonneuse fever in guinea pigs recovered from infec- tions with R. conorii. Spotted-fever vaccine does not experimentally protect against the boutonneuse-fever group of infections in the Mediterranean and other Eastern Hemi- sphere areas. Serology : Distinguishable from Rickettsia proivazekii and R. typhi by complement fixa- tion and by agglutination with specific anti- gens. Because of confusing cross fixation, the complement-fixation test is inadequate to distinguish between agents of the sub- genus Dermacentroxenus . Pathogenic for man, monkeys and guinea pigs. Rabbits and white rats are moderately susceptible. Animals susceptible in varj'ing degrees include species of ground squirrels, tree squirrels, chipmunks, cottontail rab- bits, jack rabbits, snowshoe hares, marmots, sheep, dogs, wood rats, weasels, meadow mice and deer mice. In Brazil, the opossum, rabbit, dog and cavy have been found naturally infected, and the Brazilian plains dog, capybara, coati and certain bats are also susceptible. Does not persist in brains of rats and ground squirrels but has been recovered from node tissues of man conva- lescent one year (Parker et al.. Jour. Im- munol., 73, 1954, 383). A febrile reaction occurs in guinea pigs with typical scrotal lesions, involving petechial hemorrhages in the skin, which may become necrotic. Virulent strains kill 80 to 90 per cent of the animals, milder strains kill 20 to 25 per cent. Passage in guinea pigs is accomplished by transfer of blood, spleen or tunica from infected ani- mals. A febrile reaction accompanied by exanthema occurs in man. Mortality is consistently high in some localities, low in others. Comments: In 1906 Ricketts (Jour. Amer. Med. Assoc, 47, 1906, 33) infected monkeys and guinea pigs with blood from patients suffering from Rocky Mountain spotted fever. Later in the same year it was dem- onstrated independently by Ricketts (ibid., 358) and by King (U. S. Public Health Re- ports, 21, 1906, 863) that the wood tick Dermacentor andersoni was the primary vec- tor in the Rocky Mountain area. (See How- ard Taylor Ricketts, 1870-1910, Chicago, Univ. of Chicago Press, 1911, 333.) Source: Observed by Ricketts (Jour. Amer. Med. Assoc, 52, 1909, 379) in the blood of guinea pigs and monkeys experi- mentally infected with Rockj^ Mountain spotted fever and in the salivary glands, ali- mentary sacs and ovaries of infected ticks as well as in their ova. Habitat: Found in the infected wood tick {Dermacentor andersoni) , the dog ticks (D. variabilis and Rhipicephahis sanguineus) , the rabbit ticks {Haemaphy salts leporis-pahis- tris, D. parumapertus and Otobius lagophilus) and in Amblyomma brasiliense, A. cajen- nense, A. striatum, A. americanum and Ixodes dentatus. A number of ticks belonging
