Isolation of Ehrlichia canis from dogs following subcutaneous inoculation

Subcutaneous inoculation of dogs with Ehrlichia canis was investigated as a more appropriate model of canine ehrlichiosis, which is naturally transmitted by arthropod vectors. A dose-dependent response occurred following subcutaneous inoculation of seven groups of dogs with log concentrations of E. canis-infected canine-origin cells. Ehrlichial infection in dogs was defined as concurrence of an increased titer of anti-E. canis immunoglobulin G (IgG) antibody in serum, a decreased platelet concentration, and isolation of E. canis by blood culture. In dogs administered the two lowest doses, no changes were detected. In seven of nine dogs administered three intermediate doses, the only change detected was a transient and mild increase in the anti-E. canis IgG antibody titer in serum. Only two of nine dogs inoculated with the intermediate doses developed an ehrlichial infection. Five of six dogs administered the two highest dose of E. canis developed an ehrlichial infection. These dogs had the highest IgG antibody titers in serum and the earliest isolation of E. canis from blood. In dogs that developed an ehrlichial infection, thrombocytopenia occurred by 28 days after inoculation, while increased IgG antibody titers in serum and blood cultures positive for E. canis occurred as early as 14 days postinoculation. Thrombocytopenia and seroconversion occurred later in the course of infection than previously reported for ehrlichial infections induced by intravenous inoculation. The route of administration and E. canis inoculum size can influence the course of ehrlichial infection and should be regarded as important variables in experimentally induced canine ehrlichiosis.