Relationship of Ehrlichia canis-infected mononuclear cells to blood vessels of lungs.

The lung tissue of six dogs with ehrlichiosis and two control dogs was examined with the electron microscope. Mononuclear cells containing inclusions (morulae) of Ehrlichia canis were adhered at one or more sites to the luminal surfaces of endothelial cells of arterioles or capillaries by way of interdigitations or areas of adherence, or an endothelial cell-bound mononuclear cell was chained to another parasitized or nonparasitized mononuclear cell in the lumen. The bifurcation of arterioles was the most common site at which mononuclear cells clung to the endothelium. Cell-free bodies (morulae), enclosed by a single membrane, were present in the lumens of arterioles. Such cell-free bodies were swollen and vesiculated as compared with intracytoplasmic inclusions (morulae) in mononuclear cells.