Rickettsial infection of the pulmonary microcirculation: the basis for interstitial pneumonitis in Rocky Mountain spotted fever.

Pulmonary tissue from 10 patients with fatal Rocky Mountain spotted fever was examined by brightfield microscopy for histopathologic lesions and by immunofluorescence for Rickettsia rickettsii. The distribution of rickettsiae and the vasculitis of the pulmonary microcirculation coincided. The lungs demonstrated the consequent interstitial pneumonia--alveolar septal congestion and interstitial edema; alveolar edema, fibrin, macrophages, and hemorrhage; and interlobular septal edema. The effects of the rickettsial damage to the pulmonary microcirculation are an important component of the pathophysiology of severe Rocky Mountain spotted fever. The distribution of rickettsial organisms within the lung indicates that person to person aerosol transmission is extremely unlikely.