In vitro proliferation of a canine granulocytic Ehrlichia

Canine granulocytic Ehrlichia sp., an agent which parasitizes the neutrophilic leukocytes in dogs, was transiently propagated in vitro. Dogs were experimentally inoculated with blood containing canine granulocytic Ehrlichia. Bacteremias in experimentally infected dogs varied from 1.2 to 9.3% granulocytes infected. Granulocytes from experimentally infected dogs were harvested and cultured in the presence of RPMI 1640 medium supplemented with fetal bovine serum, conditioned medium, and HEPES buffer. The percentages of granulocytes containing ehrlichial morulae increased significantly with time for 2 to 4 days, with at least one culture from each dog achieving 20% of granulocytes infected. Granulocytes taken from infected dogs early in bacteremia yielded cultures with the greatest percentage of infected cells. By 5 days post-infection the percentage of infected granulocytes decreased as did leukocyte viability. Attempts to maintain the in vitro cultures for prolonged periods by addition of uninfected granulocytes failed to increase the number of infected host cells, suggesting that no new infections were initiated and that observed increases in the percentage of infected cells in in vitro cultures were due to growth of the organism in granulocytes that were infected in vivo.