Characteristic peripheral blood findings in human ehrlichiosis

Human ehrlichiosis is a potentially fatal tick-borne illness if not treated promptly. Ehrlichia infection is difficult to diagnose as the organism does not grow in standard blood culture medium and serological confirmation of infection takes several days to weeks. The most timely way of confirming Ehrlichia infection is identification of characteristic cytoplasmic morulae in peripheral blood leukocytes. A total of 23 patients with clinical and laboratory findings suggesting a rickettsial infection were tested for Ehrlichia using polymerase chain reaction and culture: 16 cases contained Ehrlichia DNA by polymerase chain reaction (15 E. chaffeensis, one E. ewingii), including 14 cases in which the blood culture grew Ehrlichia. The cases that contained Ehrlichia DNA by polymerase chain reaction had lower mean white blood cell and platelet counts and more numerous atypical lymphocytes and pronounced toxic change than cases in which Ehrlichia DNA was not detected. Cytoplasmic morulae were identified on peripheral blood smears in six (five E. chaffeensis, one E. ewingii) of 16 (38%) of the cases that contained Ehrlichia DNA, including 4/4 (100%) immunocompromised and 2/12 (17%) immunocompetent patients. Morulae were present in monocytes in E. chaffeensis-infected cases and granulocytes in the E. ewingii-infected case. In two immunocompromised patients, the number of infected cells was 1-10%, but in four patients it was <0.2%. In conclusion, peripheral blood film examination is diagnostic in a substantial number of Ehrlichia infections, particularly in immunocompromised patients. The number of infected white blood cells may be less than 0.2%, requiring examination of more than 500 white blood cells. Associated changes prompting careful film review include prominent toxic granulation and atypical large granular lymphocytes.