Ehrlichia canis-related polyarthritis in a dog.

Ehrlichia canis-related polyarthritis was diagnosed in a 7-month-old Boxer. The diagnosis was based on intraneutrophilic morulae found in synovial fluid specimens, thrombocytopenia, a positive result for indirect fluorescent antibody testing for E canis, the presence on the dog of the known vector of E canis infection (Rhipicephalus sanguineus), and a favorable response to treatment with tetracycline hydrochloride. The dog has had no recurrence of lameness for 18 months after cessation of treatment.