Molecular identification of Ehrlichia ewingii in a polyarthritic Texas dog

An 8-year-old, neutered male, Golden Retriever presented for bilateral carpal joint effusion. A complete blood count revealed mild leukopenia and marked thrombocytopenia. Samples were sent to the Texas A&M Veterinary Medical Diagnostic Laboratory for blood smear review and serologic testing for tick-borne diseases. Numerous morulae were observed within neutrophils, and antibodies against Ehrlichia canis were detected at a 1:512 dilution via the indirect fluorescent antibody (IFA) test. As neutrophilic morulae are morphologically indistinguishable between Ehrlichia ewingii and Anaplasma phagocytophilum, and genus-wide cross-reactivity is possible with serologic testing, additional molecular testing was performed. Quantitative real-time polymerase chain reaction (qPCR) followed by conventional PCR and Sanger sequencing were performed on serum identified with E ewingii as the sole disease-causing agent. Three months after diagnosis and treatment, no morulae were found, molecular testing for E ewingii detected no DNA, and convalescent IFA testing demonstrated a continued detection of antibodies for E canis at a 1:512 dilution. To the authors' knowledge, this is the first reported case of E ewingii confirmed with molecular diagnostics in a Texas dog. The zoonotic transmission potential of E ewingii should be noted as Texas supports competent tick vectors, and dogs represent effective sentinels for human ehrlichiosis. This report also highlights the utility of molecular diagnostics when serologic and microscopic evaluations are not sufficient in providing the species-level identity of a causative agent.