The pathological changes observed in the organs in Macacus rhesus monkeys which have succumbed to severe infection with Bartonella bacilliformis are similar to those found in human organs in persons dying of Oroya fever.

The characteristic changes in the liver are the zonal necrosis of the cells around the hepatic veins, involving active macrophagocytosis of invading polymorphonuclear leucocytes in the necrotic areas, and a marked endothelial hyperplasia in the sinusoids or around the portal veins. In some instances there is fatty infiltration of hepatic cells.

In the spleen persistent hyperplasia of the endothelial cells of the capillaries leads to the formation of minute foci of infarction, owing to occlusion of the lumina. The follicles are dispersed or reduced, and there is an active macrophagocytosis of cellular débris, polymorphonuclear leucocytes, and erythrocytes. In some specimens an increase in normoblasts is noted. Pigment is sometimes present.

The lymphatic system shows general progressive endothelial hyperplasia, with active invasion of macrophages which contain polymorphonuclear leucocytes, erythrocytes, and greenish or dark pigments.

In the bone marrow there is increased activity of macrophagocytes. Numerous normoblasts are found in some instances.

In one monkey, sacrificed during the course of infection, small, verruga-like nodules were found in the lungs and spleen.

Bartonella bacilliformis has been detected microscopically, though in small numbers, in all tissues showing histological changes. Parallel cultural determinations of the presence of Bartonella bacilliformis in the blood, liver, spleen, lymphatic glands, bone marrow, and local lesions established the relationship between the pathological conditions and the presence of the parasite. The organism seems to persist longest in the lymphatic glands. Cultural methods offer a simple and conclusive means for the determination of the presence or absence of the infecting organism.

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