Functional and morphologic changes during experimental Rocky Mountain spotted fever in guinea pigs.

Experimental Rocky Mountain spotted fever was studied in guinea pigs following intraperitoneal inoculation of 10(7) Rickettsia rickettsii. After a 2-day incubation period, animals developed fever, progressive emaciation, and scrotal swelling with necrosis. Vasculitis, with increased small vessel permeability for colloidal carbon, was evident in cremaster muscles as early as 1 day after inoculation. Inflammatory changes in vessels became progressively more severe as numbers of circulating rickettsiae increased. Thrombosis and vascular occlusion were first evident on day 4. Mild thrombocytopenia developed, coinciding with the development of vasculitis, and preceding the appearance of either fibrin-split products in blood or thrombi in vessels. Rickettsiae were first detected in blood on day 2; peak rickettsemia occurred on days 5 to 8. Rickettsiae were demonstrated in inflamed vessels on day 5 and later, but not at earlier stages. Serum lysozyme concentration was moderately elevated and hemolytic complement was moderately depressed throughout the illness. Agglutinating antibody was present in low titers on days 3 to 10. Antibody titers increased on days 12 to 16 after the rickettsiae were cleared from blood. These studies indicate that vasculitis seen early in the course of Rocky Mountain spotted fever is the result of rickettsial infection, but is not dependent on the presence of rickettsiae in endothelial cells or other blood vessel components.