Reactivity of human sera to different strains of granulocytic ehrlichiae in immunodiagnostic assays.

Sera from 35 patients diagnosed with human granulocytic ehrlichiosis in Connecticut were tested by indirect IFA staining methods with 5 strains of Ehrlichia equi or the human granulocytic ehrlichiosis agent to assess the suitability of different strains in laboratory analyses. Antigens included horse-derived infected neutrophils (MRK and BDS strains) and human isolates cultured in human promyelocytic leukemia cells (NCH-1, RCH, and Webster). Of 35 sera, 23 (65.7%) reacted to all 5 strains. Seropositivity was highest (97.1%) in assays that contained the MRK strain from California and lowest (71. 4%) in tests with the NCH-1 strain from Nantucket, Massachusetts. In parallel testing of 32 sera with the NCH-1 strain by indirect IFA and Western blot analyses, results were concordant for 30 samples (93.8%). All strains of ehrlichiae can be used in IFA analyses for antibody detection, but assay sensitivity varied with the strain used.