Doxycycline hyclate treatment of experimental canine ehrlichiosis followed by challenge inoculation with two Ehrlichia canis strains

Abstract

Dogs were experimentally inoculated with Ehrlichia canis Florida to assess the efficacy of doxycycline hyclate for the treatment of acute ehrlichiosis. Treatment with doxycycline eliminated infection in eight of eight dogs. Untreated infected control dogs appeared to eliminate the infection or, alternatively, suppress the degree of ehrlichiemia to a level not detectable by tissue culture isolation or PCR or by transfusion of blood into recipient dogs. Prior infection did not infer protection against homologous (strain Florida) or heterologous (strain NCSU Jake) strains of E. canis. We conclude that doxycycline hyclate is an effective treatment for acute E. canis infection; however, these results may not be applicable to chronic infections in nature. Spontaneous resolution of infection, induced by the dog's innate immune response, provides evidence that an E. canis vaccine, once developed, might potentially confer protective immunity against the organism.

FIG. 1

Mean platelet counts for dogs in the uninfected control, infected control, homologous challenge, and heterologous challenge groups after transfusion of E. canis-infected blood (day 0), after initiation of doxycycline treatment (day 31), and after challenge inoculation (day 90).

FIG. 2

Mean microimmunofluorescence antibody (IFA) titers for dogs in the uninfected control (no seroconversion), infected control, homologous challenge, and heterologous challenge groups after transfusion of E. canis-infected blood (day 0), after the initiation of doxycycline treatment (day 30), and after challenge inoculation (day 90). The equivalent reciprocal IFA titers are given in parentheses.

FIG. 3

Results of PCR for E. canis detection demonstrating enhanced sensitivity of the seminested method. (A) By using first-step primers, a 780-bp PCR product can be visualized in lane 1 (positive control, cultured E. canis) but not in lanes 2 to 11 (DNA extracted from EDTA-anticoagulated blood samples from dogs 2 to 11, respectively). (B) A seminested PCR product yielding a 220-bp band can be identified for the positive control (lane 1) and blood samples from dogs 2, 4, 6, 8, 9, and 10 (the same samples tested in panel A). Lane M, 100-bp DNA ladder (Promega). Numbers on the left are in base pairs.

FIG. 4

Mean PCV, neutrophil and platelet counts, and IFA titers for dogs in the homologous challenge and heterologous challenge groups following challenge inoculation (day 90). The equivalent reciprocal IFA titers are given in parentheses.