DIAGNOSING A COINFECTION
A diagnosis of infection by a single tick-borne pathogen is typically made by using a combination of physical examination
findings, clinicopathologic abnormalities, and results from pathogen-specific tests such as direct microscopic visualization,
serologic testing, immunoblotting, immunodetection of organisms in blood or infected tissue, microbial culture, or PCR testing.29 The same strategies apply to diagnosing coinfections.
One of the most common hematologic abnormalities associated with tick-transmitted diseases in dogs is thrombocytopenia.28 In fact, because of the high incidence of thrombocytopenia in dogs infected with E. canis, platelet counts have been suggested as a screening test for this disease in endemic regions. One study demonstrated that
only one of 71 nonthrombocytopenic dogs has positive results for E. canis,30
suggesting that a platelet count as a screening test has a high positive predictive value but low negative predictive value.
However, because other tick-transmitted diseases were not excluded, the contribution of coinfection to the thrombocytopenia
observed cannot be assessed.
Cytologic examination of blood smears and joint fluid is a simple diagnostic tool that is generally underused and may help
raise the suspicion for a coinfection. Cytologic examination has moderate sensitivity for diagnosing acute, but not chronic,
E. canis infection. A study in dogs comparing the sensitivity of buffy coat, peripheral blood, lymph node, and bone marrow evaluation in acute
E. canis infection determined that the combination of a buffy coat smear and lymph node evaluation had a sensitivity of 74%.31 In contrast, only about 10% of chronically infected dogs had cytologic evidence in bone marrow.32
In a study of experimental Hepatozoon canis infection, four of five puppies inoculated by ingestion of Rhipicephalus sanguineus had gametocytes evident on a blood smear, and the fifth dog had gametocytes in aspirates of the spleen and bone marrow.33 Dogs naturally and experimentally infected with A. phagocytophilum have exhibited inclusion bodies within neutrophils.34,35
Babesia species parasitemia is often low in peripheral blood smears; however, blood collected from peripheral capillary beds of the
ears or nail beds or smears made from cells near the buffy coat often yield higher numbers of organisms.36 In addition, cells examined in the periphery of the smear contain more intracellular organisms.36 Observing inclusions within different cell types should alert practitioners to the existence of concurrent infections.
The obvious disadvantages of a cytologic examination are that it is labor-intensive and time-consuming and it requires thorough
examination of many fields and the use of appropriate stains (Giemsa). Nonetheless, it is accessible to most clinicians. With
the increasing use of in-house automated hematology analyzers in many small-animal practices, blood smear examination is regrettably
too frequently omitted in the hematologic assessment of ill patients. Cell tropism is summarized in Table 2.
Table 2. Intracellular Inclusions