Acute experimental canine ehrlichiosis. I. Sequential reaction of the hemic and lymphoreticular systems.

Dogs with acute Ehrlichia canis infection showed the established clinical features of acute ehrlichiosis and had thrombocytopenia, anemia, increased serum glutamic pyruvic transaminase activity, and decreased total serum protein and albumin concentrations during the first weeks after infection, and increasing gamma globulin concentrations after the third week. Gross lesions in hemic and lymphoreticular tissues were transient lymphadenomegaly, splenomegaly, and increased red long-bone marrow. Lymphoreticular hyperplasia in the paracortical area of lymph nodes and in the splenic red pulp occurred during the second week of infection. Later, small lymphocytes were replaced by medium-sized lymphocytes and plasma cells. Activity of germinal centers increased initially, as shown by numerous mitotic figures and macrophages, but diminished later, and the follicles blended with interfollicular and paracortical tissue because of a decrease in small lymphocytes in the mantle layer. We saw splenic hemorrhages near the perifollicular sinus, and vasculitis, most often phlebitis, in the kidney between weeks 2 and 4. Multifocal reticuloendothelial hyperplasia occurred in the liver during the early stage and injured adjacent hepatocytes by compression. Lesions typical of ehrlichiosis in these dogs were interstitial pneumonia, subendothelial aggregates of mononuclear cells in pulmonary blood vessels, renal periglomerular and perivenular plasmacytosis, hemopoietic hyperplasia, and perivascular cuffs of lymphocytes and plasma cells in many organs.