Koret School of Veterinary Medicine, The Hebrew University of Jerusalem, Rehovot, Israel
Canine monocytic ehrlichiosis (CME), caused by the obligate intracellular bacterium Ehrlichia canis is an important disease of dogs and other canids worldwide. It is transmitted by the brown dog-tick, Rhipicephalus sanguineus, a worldwide distributed tick. Most CME cases occur during the warm season when the vector ticks are abundant. Dog owners living in or traveling to endemic regions should be aware of this disease as it may be fatal. Common clinical signs of CME include depression, lethargy, anorexia, weight loss, lymphadenomegaly, splenomegaly and bleeding (mainly subcutaneous and mucosal petechiae and ecchymoses as well as epistaxis).
The most common hematological sign of CME is thrombocytopenia occurring in more than 90% of infected dogs. The relationship between the degree of thrombocytopenia and infection with E. canis in an endemic area was investigated. It has been shown that while only one of 71 (1.4%) non- thrombocytopenic dogs was found positive for E. canis DNA (16S rRNA), 13 of 62 (21%) dogs with platelet counts of 100,000–200,000 per µL, and 53 of 84 (63.1%) dogs with platelet counts of less than 100,000 per µL were found positive. The authors concluded that platelet counts and their magnitude may be a good screening test for CME (Bulla et al. 2004). Pancytopenia is a common hematological finding in dogs suffering from the chronic severe form of the disease. Nineteen dogs with chronic ehrlichiosis exhibiting bicytopenia or pancytopenia (packed cell volume < 36%; white blood cell count < 6.0 x 103/µL; platelet count < 175 x 103/µL) were included in one study. All these animals eventually died, irrespective of the treatment applied indicating the poor prognosis of this disease phase (Mylonakis et al. 2004).
Common biochemical abnormalities in CME are hypoalbuminaemia, hyperglobulinaemia and hypergammaglobulinaemia. Serum protein electrophoresis reveals a polyclonal gammopathy in most dogs infected with E. canis. However, some dogs may develop monoclonal gammopathy (Harrus et al. 1996). The latter dogs may have high serum protein concentrations and may suffer from hyperviscosity which may lead to subretinal bleeding, retinal detachment and acute blindness (Harrus et al. 1998). The blindness in most of these dogs is irreversible despite intensive antirickettsial and antiinflammatory treatment.
Diagnosis of the disease is challenging due to its different phases and multiple manifestations. The suspicion of CME should be considered when a compatible history (tick infestation, travel to or living in endemic region), typical clinical signs (lymphadenomegaly, splenomegaly, dermal and mucosal petechiae and ecchymoses, epistaxis), typical hematological signs (thrombocytopenia, pancytopenia) and biochemical abnormalities (hypoalbuminaemia, hyperglobulinaemia) are present.
Classical diagnostic techniques (hematology, cytology, serology, isolation) are useful tools in the diagnosis of CME. Demonstration of a typical morula within the cytoplasm of a monocyte in blood smear indicates a monocytotropic ehrlichiosis. Evaluation of blood smears is usually unrewarding as the sensitivity of morulae detection is low. Multiple buffy coat smears have been shown to have a higher sensitivity for detection of E. canis morulae (Mylonakis et al. 2003). Blood smears of infected dogs may present reactive monocytes, erythrophagocytosis, platelet phagocytosis and granular lymphocytosis (Harrus, Waner 2011).
The indirect immunofluorescence antibody (IFA) test is considered the serological "gold standard" test, indicating exposure to E. canis. It is considered as a valuable screening test for CME. The appearance of IgM antibodies after experimental infection was shown to be inconsistent and therefore IgM-serology is not in use. In contrast, IgG titers of 1:40–1:80 or greater are considered positive. Two consecutive IFA tests, 7–14 days apart, are recommended, and a 4-fold increase in antibody titers is suggestive of an active infection. Point of care enzyme linked immunosorbent assay (ELISA) kits for the detection of E. canis are also available (the Snap 4Dx® assay by IDEXX Laboratories Inc., USA and the Immunocomb® by Biogal, Israel). They are sensitive and specific and are in common use in clinics.
A definitive diagnosis of E. canis infection should be done by polymerase chain reaction (PCR) and sequencing. Polymerase chain reaction and sequencing are sensitive methods for detecting and characterizing E. canis-DNA, respectively. Detection of E. canis DNA can be achieved as early as 4 to 10 days post-inoculation. Several conventional or real-time PCR assays, based on different target genes, are commonly used (Harrus, Waner 2011).
Tetracyclines in general and doxycycline in particular are the therapeutic agents of choice for the treatment of CME. Doxycycline should be administered at a dose of 5 mg/kg q 12 h (or 10 mg/kg q 24 h) for the duration of 3 weeks for dogs at the acute phase. Dogs in the subclinical phase may require prolonged treatment. One study suggests that some dogs may require prolonged treatment courses (McClure et al. 2010). In cases of immune mediated complications, glucocorticosteroids may be indicated.
The prognosis of the acute and the subclinical phases of the disease is good, however grave for the chronic phase. Dogs in the latter phase will eventually die due to bone marrow hypoplasia and its outcomes: peripheral pancytopenia, sepsis and/or bleeding. In a retrospective study investigating prognostic indicators for survival or death in CME, pronounced pancytopenia (WBC < 4 x 103/µL; HCT < 25%; PLT < 50 x 103/µL) was found as a risk factor for mortality. In this study, severe leucopenia (WBC < 0.93 x 103/µL), severe anemia (PCV < 11.5%), prolonged activated partial thromboplastin time (APTT > 18.25 s) and hypokalemia (K < 3.65 mmol/L) were each found to predict mortality with a probability of 100%. In contrast, WBC counts above 5.18 x 103/µL, platelet counts above 89.5 x 103/µL, PCV > 33.5%, APTT < 14.5 s and serum potassium concentration above 4.75 mmol/L, each provided 100% prediction for survival. These prognostic indicators can be easily obtained at presentation, are inexpensive, and may be useful aids when treatment and prognosis are being considered (Shipov et al. 2008). To date, there is no commercial vaccine and tick control is the most effective preventive measure.
References
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