The gold standard for anaplasmosis diagnosis is PCR on whole blood in the first week. IgG antibody titers by IFA in paired serum samples (first week and 2–4 weeks later) are positive also.
Identification of morulae in granulocytic leukocytes and a single positive IFA titer are considered diagnostic. The morulae are deep blue to grey-blue coccobacilli arranged in spherical groups within the cytoplasm of granulocytes, as seen in the image to the right. A retrospective study of 14 confirmed cases identified morulae in 11 (79%) cases before counting 100 granulocytes. The remaining three cases (21%) had morulae identified when 200 granulocytes were examined. More morulae are seen in anaplasmosis than in ehrlichiosis.
Immunohistochemical (IHC) staining in biopsy or autopsy samples is diagnostic. A culture of A. phagocytophilum from a clinical specimen can be performed in HL60 cell culture.
Clinicians should consider coinfections with Babesia or B. burgdorferi.
