Duration of Infection and Efficacy of Doxycycline Treatment in Dogs Experimentally Co-Infected with Anaplasma platys and Ehrlichia canis
Rhipicephalus sanguineus is a ubiquitous tick responsible for transmitting Ehrlichia canis and most likely Anaplasma platys to dogs, as either single or co-infections. The occurrence of co-infection may be difficult to diagnose since thrombocytopenia and anemia result from either infection. The purpose of this study was to evaluate the clinical significance of A. platys and E. canis co-infection in dogs. Results from the initial phase of the study were reported previously and revealed a more severe thrombocytopenia and anemia in co-infected dogs as compared to single infections with either organism (ACVIM, 2007). Results from the chronic phase of the study, which evaluated the duration of A. platys and E. canis infection and treatment efficacy, are reported here. Six month old, female hound-type dogs were inoculated intravenously with Louisiana isolates of A. platys and/or E. canis. Six groups of six dogs each were evaluated: non-infected controls, A. platys infected, E. canis infected, A. platys and E. canis co-infected, A. platys infected administered E. canis 112 days later, and E. canis infected administered A. platys 112 days later. Doxycycline treatment (10mg/kg PO daily x 28d) was initiated at 211 days post-infection (PI), followed by immunosuppression (dexamethasone 0.3mg/kg IM daily x 5d) beginning 410 days PI. Molecular evidence of infection was assessed by two independent laboratories using either conventional or real-time PCR on whole blood collected during the study, as well as bone marrow and lymph node aspirates collected at the time of immunosuppression. Thrombocyte counts in dogs infected only with A. platys returned to normal by 115 days PI and PCR results were consistently negative after 110 days PI, prior to doxycycline treatment. Most A. platys/E. canis co-infected dogs (8/9) were also PCR negative in blood for A. platys prior to doxycycline treatment. In contrast, all untreated E. canis infected dogs (12/12), whether single or co-infected, remained E. canis PCR positive in blood for more than a year and had reduced platelet concentrations (mean 125,600/uL). Most of these dogs also tested positive by PCR for E. canis DNA in bone marrow (8/12) and lymph node (12/12) following immunosuppression. All E. canis infected dogs, both single and co-infected, receiving doxycycline (12/12) were E. canis PCR negative in blood and lymph node prior to immunosuppression and had normal thrombocyte concentrations (mean 308,600/uL). Despite immunosuppression, neither A. platys nor E. canis DNA was detected in doxycycline-treated dogs. Although co-infection of A. platys and E. canis may result in more severe hematologic abnormalities, these experimental infections can be successfully treated with doxycycline.