Abstract. Rhesus monkeys were inoculated intravenously or intraperitoneally with various numbers of Rickettsia rickettsii. Monkeys that were given more than lo4 organisms intravenously died on days 3-6 after inoculation, whereas those given less than los intravenously or more than lo4 intraperitoneally died on day 7 or later. There was no significant difference in incidence of lesions between the early-and late-death groups. Vasculitis and thrombosis occurred most frequently in the nares, pinna of the ear, scrota1 skin, and testicle. Adrenal cortical necrosis was caused by capillary thrombosis. Rickettsiae, estimated by plaque formation in culture, were most numerous in the lung and spleen.The morphologic features of naturally occurring Rocky Mountain spotted fever in man, and the experimentally induced disease in monkeys and guinea pigs have been described [2-4,8, 141. The fundamental lesion is inflammation of the small blood vessels. The present study was undertaken to determine the incidence and type of lesions in a group of Rhesus monkeys dying at various intervals after intravenous or intraperitoneal inoculation of Rickettsia rickettsii. Immunofluorescence studies and direct plaquing of organ homogenates were done to correlate lesions with presence of rickettsiae. Materials and MethodsMethods for propagation of R. rickettsii in yolk sacs of embryonated chicken eggs are described elsewhere [I I]. Seed stock was stored at -70°C as 50% yolk sac suspension in sucrose-phosphate-glutamate buffer [6]. Methods for propagation in duck embryo cells have been described previously [7]. Live rickettsiae were quantitated by plaque assay as described [12].
Rickettsia rickettsii was isolated from experimentally infected guinea pigs by culture of blood monocytes and bone marrow cells, and from experimentally infected rhesus monkeys by blood monocyte culture. Rickettsiae were identified in monocyte-macrophage monolayers stained by Giménez or flourescent antibody techniques. A total of 78 culture attempts were made from 20 guinea pigs and 16 monkeys. The success of isolation of R. rickettsii in culture was positively correlated with the numbers of rickettsiae present in the blood and bone marrow. in cultures derived from infected guinea pigs, rickettsiae were usually observed after 5 to 7 days of culture, and in monkeys monocyte cultures they were usually observed within 3 to 5 days. Positive cultures were derived from guinea pigs and monkeys as early as the first day of fever and 1 to 3 days before the appearance of other clinical signs. Monocyte cultures became negative with the resolution of rickettsemia and concomitantly with the appearance of serum antibody. Monocyte culture isolation of R. rickettsii may be as sensitive for the detection of rickettsiae in blood and marrow as the intraperitoneal inoculation of guinea pigs or the plaque assay technique. Because of the simplicity of the method and because rickettsiae were often identified within 3 to 5 days after initiation, the monocyte culture technique may be useful in the early diagnois of human rickettsial disease.
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