Reisolation of Ehrlichia canis from blood and tissues of dogs after doxycycline treatment

Abstract

We present evidence that supports the carrier status of dogs experimentally infected with Ehrlichia canis after treatment with doxycycline. Canine ehrlichiosis was induced in five dogs by intravenous inoculation with E. canis-infected DH82 cells. All animals developed mild clinical signs of transient fever, body weight loss, thrombocytopenia, and increased gamma globulin levels in plasma. An indirect fluorescent-antibody test (IFA) revealed that all dogs had seroconverted (titer, 5,120) by day 10 postinoculation (p.i.). E. canis was reisolated from blood samples collected at intervals throughout the 2-month period p.i. Doxycycline was administered orally once daily at 10 mg/kg of body weight per day for 1 week starting at 2 months p.i. Following treatment, gamma globulin levels in plasma were decreased. At necropsy on days 54 to 59 after the start of treatment, spleen, liver, kidney, and lymph nodes were collected for E. canis culture and histopathologic examination. Although the dogs did not show significant clinical signs during or after treatment with the antibiotic, E. canis was reisolated from the blood and tissue samples of three of five dogs. A 16-fold reduction in IFA titer was noted in two dogs which were negative for E. canis reisolation at day 49 after the start of treatment, whereas a zero-to fourfold reduction in IFA titer was seen in the remaining three dogs. Western immunoblot reactions to higher-molecular-size E. canis antigens in the sera of two dogs which were negative for E. canis on blood culture decreased, whereas they remained continuously high or only transiently decreased for the duration of the study for antigens in the sera of three dogs from which E. canis was reisolated. Histopathologically, prominent plasmacytosis in the kidney cortex was present in three dogs from which E. canis was reisolated, whereas the kidney cortices of two dogs had moderate to minor plasmacytosis. These findings pose questions regarding the efficacy, dosage, and duration of doxycycline treatment in dogs with E. canis infection. In addition, it was shown that IFA and Western immunoblotting may aid in assessing the efficacy of antibiotic therapy when definitive reisolation procedures are not readily available.

A Retrospective Study of Ehrlichiosis in 62 Dogs from North Carolina and Virginia

“…E. canis was isolated from experimentally inoculated dogs after therapy with tetracycline at 10 mg/kg PO once daily for 1 week. 21 Reductions in IFA titers of 16-fold magnitude occurred by 49 days after treatment in 2 dogs for whom the organism was cleared. Titers did not decline appreciably in the 3 dogs for whom the organism was not cleared.…”

“…This particular study implied that current drug dosages may be insufficient and that treatment periods should be extended to longer than 1 week. 21 Current therapeutic protocols already suggest treatment periods of 3 to 4 weeks, since shorter treatment intervals may result in ineffective therapy and persistent infection. 21 The role of chronic E. canis infection in the development of concurrent disease processes remains unclear.…”

“…21 Current therapeutic protocols already suggest treatment periods of 3 to 4 weeks, since shorter treatment intervals may result in ineffective therapy and persistent infection. 21 The role of chronic E. canis infection in the development of concurrent disease processes remains unclear. Controlled studies are needed to evaluate the influence of E. canis infection on CMI.…”

Burden of Tick-borne Infections on American Companion Animals

“…Indeed, for human granulocytic ehrlichiosis ("human anaplasmosis") diagnostic antibody titers appear only in convalescence. Although sheep can be chronically infected by A. phagocytophilum (leading to immunosuppression), and E. canis infection may relapse even after appropriate treatment, 14,15 it is more likely that these infections spontaneously resolve. Re-exposure to infected ticks needs to be ruled out before concluding that infection may persist "silently" and unpredictably cause patent disease.…”

Failure of Imidocarb Dipropionate to Clear Experimentally Induced Ehrlichia canis Infection in Dogs

“…Doxycycline, an analog of tetracycline, is the most commonly used treatment. 1,2 Doxycycline has minimal side effects and offers the convenience of administering doses q12h versus q8h with tetracycline. Imidocarb dipropionate, an anticholinergic anthelmintic, has been available in the United States since the mid l990s but has been used for the treatment of canine ehrlichiosis in other parts of the world for many years.…”