Coinfection with multiple tick-borne pathogens - Veterinary Medicine
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Coinfection with multiple tick-borne pathogens
It's not always cut-and-dried—a single tick bite delivering a single pathogen. Sometimes ticks carry more than one infective agent, and sometimes a pet has been bitten by multiple ticks carrying different microorganisms.So when should you suspect that your patient is infected with more than one tick-borne pathogen?



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.

Hematologic abnormalities

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.

Table 2. Intracellular Inclusions
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.


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