Pathology of Experimental Rocky Mountain Spotted Fever in Rhesus Monkeys

Abstract

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].

Orchitis in Two Dogs With Rocky Mountain Spotted Fever

“…In another experimentally induced RMSF infection of dogs, the testicles were one of the most notable sites of microscopic lesions 4 . Experimental infection of rhesus monkeys led to coagulation necrosis of the seminiferous tubules and parenchyma of the testicle and epididymis with necrotic areas generally located adjacent to thrombosed blood vessels 5 . In that study, indirect immunofluorescence microscopy identified rickettsiae in the testicles, and relatively high counts of viable rickettsiae were recovered from the testes as well 5 .…”

“…Although orchitis is reported as an experimental sequela of RMSF, 3–5 there are no reports that describe orchitis and epididymitis as a predominant feature of natural disease in dogs. Because many dogs receiving routine veterinary care are neutered at a young age, there is a paucity of information regarding the involvement of the testicles in dogs with RMSF.…”

“…Experimental infection of rhesus monkeys led to coagulation necrosis of the seminiferous tubules and parenchyma of the testicle and epididymis with necrotic areas generally located adjacent to thrombosed blood vessels 5 . In that study, indirect immunofluorescence microscopy identified rickettsiae in the testicles, and relatively high counts of viable rickettsiae were recovered from the testes as well 5 . Vasculitis was observed most frequently in skin, followed by the testicle; it has been postulated that the proximity of these sites to the environment may be associated with increased susceptibility to R. rickettsii ‐induced inflammation, perhaps because of the preference of the organism to select tissues that are lower than core body temperature 5 …”

“…The organism induces a systemic disease in dogs that is accompanied by vasculitis. Although highly vascular organs are most commonly affected, 1,2 orchitis as a result of RMSF is infrequently reported in dogs 3–5 . The marked orchitis that can develop in association with canine RMSF can be diagnosed sonographically, but it can mimic testicular neoplasia and torsion.…”

Evaluation of a killed Rocky Mountain spotted fever vaccine in cynomolgus monkeys

“…Vaccinated guinea pigs challenged by the subcutaneous (s.c.) route or by aerosol were fully protected. RMSF infection has been studied extensively in rhesus monkeys (7,9,12,15), and a suitable nonhuman primate model has been described (10). The vaccine has been tested and found to protect rhesus monkeys challenged s.c. (11).…”