Laboratory diagnosis of Rocky Mountain spotted fever by immunofluorescent demonstration of Rickettsia rickettsii in cutaneous lesions

Direct immunofluorescent staining for Rickettsia rickettsii was performed on cryostat sections of skin biopsies from 27 patients suspected of having Rocky Mountain spotted fever. In nine of the 17 patients whose final diagnosis was Rocky Mountain spotted fever, coccobacillary forms of R. rickettsii were identified in endothelium and vascular walls within the dermis. Factors recognized as contributing to false-negative results were prior treatment with tetracycline or chloramphenicol for 24-48 hours or longer and failure to obtain a section through the focus of vasculitis. No false-positive result was obtained in the ten patients whose final diagnoses were not Rocky Mountain spotted fever. The laboratory test offers an immediate, positive laboratory diagnosis for this treatable, life-threatening disease.