Epidemiologic and clinical characteristics of persons with serologic evidence of E. canis infection

The results of a serosurvey of Oklahomans for the presence of antibody to Ehrlichia canis is reported. Paired serum specimens, from patients lacking the serologic criteria for diagnosis of Rocky Mountain spotted fever (RMSF), were tested. A four-fold increase in E. canis-IFA antibody was found in 16/144 (11 percent) of these paired serum samples. Patients with serologic evidence of E. canis infection had a mean age of 34 years, 69 percent were male, and 63 percent lived in a town less than 10,000 population. Signs and symptoms included: fever 94 percent, headache 94 percent, fatigue 94 percent, anorexia 81 percent, nausea 60 percent, and rash 44 percent. When compared to control patients, whose sera were submitted for RMSF testing but did not meet serologic criteria for RMSF or E. canis, case-patients were more likely to have had leukopenia (OR = 4.9, 95 percent Cl = 1.2, 19.0) and tick exposure (OR = 9.5, 95 percent Cl = 1.4, 62.7). The results suggest E. canis, or a closely related agent, is a cause of human illness. Ticks are probable vector.