During the last decade the nomenclature describing the various ehrlichial organisms has undergone substantial revision due to the use of modern molecular techniques for DNA identification. The names of some of these organisms have recently been changed from Ehrlichia to Anaplasma. The Ehrlichial diseases of dogs are caused by organisms of the genera Ehrlichia, Anaplasma and Neorickettsia, all of which are obligatory intra-cellular bacteria. Ehrlichia canis and E. chaffeensis infect monocytes, Anaplasma phagocytophila and E. ewingii infect granulocytes, and A. platys infects platelets. Most ehrlichial organisms are thought to be vector-borne and in particular, tick transmitted diseases. Diagnosis of ehrlichial diseases is based on the demonstration of a typical ehrlichial morula in the target cell, the demonstration of a four-fold rise in antibody titer by the indirect immunofluorescence antibody test, by dot ELISA in-clinic kits, or by species-specific PCR in experimental laboratories.
Canine monocytic ehrlichiosis has a world wide distribution which includes many endemic areas where it is considered an important canine infectious disease. In fact, in many areas it is the most common infectious disease of dogs. Frequently, canine monocytic ehrlichiosis occurs in combination with other tick-borne diseases and this may be one of the reasons for the variation in the severity of the disease and the variation in clinical and clinicopathological manifestations reported in the naturally occurring disease. However, the possibility of strain variation pathogenicity and host immune response must also be considered as potential factors. The etiologic agent of canine monocytic ehrlichiosis is Ehrlichia canis. This rickettsia is transmitted by the brown dog-tick Rhipicephalus sanguineus.
The pathogenesis of canine monocytic ehrlichiosis starts with an initial incubation period of 8-20 days followed by 3 successive phases: an acute phase, an asymptomatic phase and in some cases a chronic phase. The "classical" manifestations of the acute phase of E. canis infection is associated with nonspecific findings, including, depression, lethargy, anorexia, fever, and more typical findings of lymphadenomegaly, splenomegaly and hemorrhages (mainly petechiae, ecchymoses and epistaxis). Thrombocytopenia is the most frequent hematological sign occurring in about 90% of cases, while hypergammaglobulinemia with a monoclonal gammopathy is the most common biochemical finding. The acute phase may last from 1-4 weeks after which the dog recovers completely or enters an asymptomatic phase which has been shown to last from months to years. The only hint to the asymptomatic phase may be a permanent mild thrombocytopenia. The factors that are responsible for the induction of the chronic phase of the disease remain uncharacterized and the proportion of infected dogs entering this phase is not known, since both experimentally and naturally-infected dogs can remain healthy for several years following exposure. However, there may be a breed predilection for the chronic phase of the disease. The "classical" manifestations of the chronic phase of E. canis infection is associated with chronic weight loss, peripheral edema, bleeding abnormalities, including prolonged bleeding from venipuncture sites, petechiae, retinal hemorrhage, or epistaxis. The clinicopathological manifestations are severe pancytopenia due to bone marrow hypocellularity. The prognosis of these dogs is grave.
The treatment of canine monocytic ehrlichiosis remains a therapeutic challenge to the clinician and this is related to the paucity of controlled therapeutic trails and valid documentation of therapeutic elimination of E. canis infection. There is little information in the veterinary literature regarding long-term post-treatment follow-up of dogs with ehrlichiosis. Most studies indicate that some naturally and experimentally infected dogs in the acute phase clear E. canis infection following two weeks of doxycycline (5 mg/kg PO q12h or 10mg/kg PO q24h) therapy. However, we prefer to recommend 3-4 week therapeutic regimens. As an alternative or supplement to doxycycline, imidocarb dipropionate (5mg/kg IM, 2 injections at 14 day intervals) may be used. Most dogs will show rapid recovery within 48-72 hours. Steroids may be considered in those not responding as expected. Despite this rapid clinical improvement, some dogs may remain hematologically abnormal for several months and high antibody titers persist for many months. Therapy for chronic cases is most frequently unrewarding and often extremely costly. We have had limited success using various combinations of blood transfusions, erythropoietin, granulocyte colony stimulating factors, steroids and vincristine.