Tissue diagnosis of Ehrlichia chaffeensis in patients with fatal ehrlichiosis by use of immunohistochemistry, in situ hybridization, and polymerase chain reaction

In the United States, human ehrlichiosis is a complex of emerging tick-born e diseases caused by 3 distinct Ehrlichia species: Ehrlichia chaffeensis, E hrlichia ewingii, and the human granulocytotropic ehrlichiosis agent. Ehrli chioses are characterized by a mild to severe illness, and similar to4% of cases are fatal. Because these obligate intracellular bacteria are difficul t to resolve with routine histologic techniques, their distribution in tiss ues has not been well described. To facilitate the visualization and detect ion of ehrlichiae, immunohistochemistry (IHC), in situ hybridization (ISH), and polymerase chain reaction (PCR) assays were developed by use of tissue s from 4 fatal cases of E. chaffeensis infection. Evidence of E. chaffeensi s via IHC, ISH, and PCR was documented in all 4 cases. Abundant immunostain ing and in situ nucleic acid hybridization were observed in spleen and lymp h node from all 4 patients. Significantly, in 2 of these patients, serologi c evidence of infection was absent. Use of IHC, ISH, and PCR to visualize a nd detect Ehrlichia in tissues can facilitate diagnosis of ehrlichial infec tions.