Pathogenesis of myocarditis in Rocky Mountain spotted fever.

Because myocarditis has been noted frequently as present at necropsy in fatal Rocky Mountain spotted fever (RMSF) and ECG alterations may also accompany the disease, we studied the pathogenesis of myocarditis in RMSF. Increased heart weight in eight of nine cases and increased interstitial volume in six of nine cases suggest that myocardial edema is present. Specific immunofluorescent demonstration of Rickettsia rickettsii in myocardial capillaries, venules, and arterioles in eight of nine cases correlates well with the patchy distribution of interstitial mononuclear myocarditis and provides the pathogenic mechanism for vascular injury-induced myocardial edema. Absence of left ventricular dilation suggests that severe myocardial failure was not the apparent cause of hypotension, shock, and death in RMSF.