Disease manifestations caused by members of the E. canis genogroup (E. canis, E. chaffeensis, E. ewingii) infecting dogs can be indistinguishable, and there can be strain variation in pathogenicity. Based upon experimental infection studies, canine ehrlichiosis has been divided into three phases: an acute, a sub-clinical and a chronic disease phase. Nevertheless, onset and duration of infection is rarely known in the clinical setting.
The acute phase follows an incubation period of 1 to 3 weeks and is characterized by fever, anorexia, weight loss, oculonasal discharge, lymphadenopathy as well as thrombocytopenia, leukopenia, low anemia and hypergammaglobulinemia. During this phase, the organisms are dividing and spreading throughout the body via mononuclear cells. Other clinical signs include: depression, severe loss of stamina, dyspnea and edema of the limbs and the scrotum. A variety of central nervous system signs, including hyperesthesia, muscle twitching and cranial nerve deficits may occur due to inflammation and bleeding into the meninges. Clinical findings in the acute phase of canine ehrlichiosis can be identical to anaplasmosis, canine Rocky Mountain spotted fever or canine distemper. The acute phase usually resolves spontaneously within one or two weeks, even without appropriate treatment.
After 2 to 4 weeks, the subclinical phase develops without clinical symptoms and with a low thrombocytopenia, lasting for 40 to 120 days or even years, during which the animals are persistently infected. Immunocompetent animals may successfully eliminate the parasite during this period. Other animals may develop the so-called chronic stage of infection with severe and life-threatening clinical manifestations.
The chronic phase is characterised by varying degrees of depression, fever and weight loss. A combination of bleeding tendencies, pallor due to anaemia, abdominal tenderness, anterior uveitis, retinal hemorrhages and neurological signs consistent with meningoencephalitis, typify dogs that are chronically affected. Irreversible bone marrow destruction and polysystemic immune complex disease mark the later stages of the chronic phase. Additionally the situation is often complicated by secondary bacterial infection.
Immunosuppression has been historically attributed to infection with E. canis in dogs; however, a recent experimental infection study did not induce detectable immunosuppression. Furthermore, epistaxis, once considered a hallmark of the disease, occurs infrequently in dogs (in the USA) and may be attributable to concurrent infection with a Bartonella species or other infectious agents.
Ehrlichiosis is more severe in certain breeds (e.g. German shepherd) and in younger animals. However, co-infection, immune status and strain variation could all play a role in determining the types of disease manifestations and the severity of pathology in an individual patient.