Ehrlichiosis Rickketsial Diseases
Diagnostic Tests
Ehrlichiosis can be diagnosed by serology or detection of the organism. Diagnosis is complicated, particularly in dogs, by the possibility of co-infection with more than one organism, and by cross-reactions in serologic tests.
The indirect immunofluorescent antibody (IFA) test is the most commonly used serologic test for equine granulocytic ehrlichiosis, canine ehrlichiosis and tick-borne fever. ELISA tests have also been developed for canine ehrlichiosis.
Immunoblotting (Western blotting) is mainly used in research. A rising titer is diagnostic. In dogs, a single positive titer is also considered to be evidence of exposure. Antibodies may not be found in the terminal stages of canine ehrlichiosis if the bone marrow is exhausted.
Ehrlichiosis can sometimes be confirmed by finding morulae in peripheral blood smears or impression smears from fresh tissues. Depending on the form of the disease, the morulae are mainly seen in monocytes or granulocytes. The organisms can be stained with Giemsa or by immunofluorescence.
This technique is useful for equine granulocytic ehrlichiosis, but is relatively insensitive for the diagnosis of canine ehrlichiosis, as the organisms are found in only a minority of cases.
Polymerase chain reaction (PCR) assays can detect antigens in the blood; these tests are useful for equine ehrlichiosis and may be available for canine ehrlichiosis. Culture is generally impractical, as Ehrlichia can be difficult to culture and also take 14 to 33 days to grow.
Hematologic abnormalities and changes in serum chemistry can help support the diagnosis. Thrombocytopenia is the most common abnormality in dogs. Many dogs also have anemia and leukopenia, although it may be mild in the acute phase.
A response to treatment also supports the diagnosis.
Treatment
Ehrlichiosis is usually treated with the tetracycline antibiotics. In dogs, chloramphenicol and other drugs are also used occasionally. In cases of equine granulocytic ehrlichiosis or tickborne fever, treatment is usually very effective.
Early treatment is critical for canine ehrlichiosis; uncomplicated cases in the acute or subclinical stage usually respond promptly, but antibiotics may be less effective in dogs with neurologic signs, and treatment of the chronic severe form is difficult.
One report described the successful treatment of a dog with severe chronic canine monocytic ehrlichiosis, using a combination of hematopoietic growth factors, low dose vincristine, doxycycline and glucocorticoids.
There is little information on post-treatment follow-up of dogs. Some authors suggest that persistent infections are common, particularly in dogs infected with E. canis or E. chaffeensis. Others note that recent studies suggest many infected dogs clear E. canis infections after treatment.
Hematologic values remain abnormal in some dogs for several months, despite clinical improvement. It is uncertain whether persistent post-treatment antibody titers and/or PCR amplicons correlates with a persistent infection.