PNEUMONIC PLAGUE IN MONKEYS: AN ELECTRON MICROSCOPE STUDY

The infection of monkey lungs by Pasteurella pestis was studied with the electron microscope. The disease was first apparent clinically at 60 to 70 hours and pathologically at 72 hours. At this time, the initial response seen with the light microscope was alveolar edema with massive proliferation of organisms. With the electron microscope, interstitial edema around veins and bronchi was the earliest observed response to the presence of numerous intra- alveolar bacilli. Early in the disease, before the onset of a leukocytic exudate, interstitial fluid also accumulated in the form of subepithelial blebs. No evidence of significant vascular damage was found to account for the marked increase in permeability, even with the aid of intravenous injections of colloidal carbon prior to sacrifice. Leukocytic and mast cell factors in the pathogenesis of the edema appear unlikely. Once the cellular component of the reaction developed, P. pestis resisted both phagocytosis by polymorphonuclear neutrophils and intracellular digestion by mononuclear phagocytes. Invasion of the tissues by the organism was observed only in the advanced stages of the pneumonia, when all components of the alveolar septa were undergoing necrosis.