Identification of Rickettsia rickettsii in a guinea pig model by immunofluorescent and electron microscopic techniques.

Moribund guinea pigs infected with Richettsia rickettsii were examined by necropsy, histology, immunofluorescence, electron microscopy, and serology. Untreated animals died at 9 and 10 days after inoculation. Animals given saline subcutaneously survived from 1 to 4 days longer. Prolonged survival was accompanied by more severe lesions: scrotal necrosis; infarction of ears; and swollen, hemorrhagic footpads, epididymis, and cremaster muscle. Histopathologic examination demonstrated that acute, necrotizing vasculitis, perivascular hemorrhage, and focal necrosis were more extensive. Direct immunofluorescence indicated many more rickettsiae in endothelium and vascular wall of saline recipients. Ultrastructurally, typical rickettsiae were present focally in the cytoplasm of endothelial and vascular smooth muscle cells. Cytopathology in infected and adjacent cells included swelling, mitochondrial enlargement with decrease in matrix density and loss of cristae, and increased pinocytosis. In addition, treated animals had more cytonecrosis, thrombosis, extravascular fibrin deposition, prominent inflammatory cells with polymorphonuclear phagocytosis of rickettsiae, and antibody production.