Ehrlichiosis mimicking thrombotic thrombocytopenic purpura. Case report and pathological correlation

Human ehrlichiosis is a tick-borne zoonosis caused by the newly described human hematotropic rickettsiae, Ehrlichia chaffeensis. The pathology and pathogenesis of human ehrlichiosis have not been adequately studied. Even with immunoperoxidase, the only previously known method to detect these organisms in tissue, ehrlichae are difficult or impossible to identify. This led many investigators to speculate that the pathogenesis of ehrlichiosis was not caused directly by the organism but could be caused by host-mediated injury. In this case study, a patient presented with rapidly progressive central nervous system symptoms and severe thrombocytopenia, prompting a presumptive diagnosis of thrombotic thrombocytopenic purpura (TTP). Despite corticosteroids, and later, antibiotics, the patient rapidly deteriorated and died. Postmortem examination showed hemorrhages in multiple organs and mononuclear inclusions of infection with a monocytic ehrlichia. Other findings included widespread lymphohistiocytic perivascular infiltrates, focal hepatic necroses, interstitial pneumonitis, interstitial nephritis, mononuclear phagocyte invasion and proliferation in splenic, liver, and bone marrow, and hemophagocytosis. The diagnosis was proven by serology, immunohistology with both polyclonal and monoclonal anti E chaffeensis, and polymerase chain reaction on paraffin-embedded tissues using E chaffeensis-specific oligonucleotide primers. The presence of numerous ehrlichia with notable tissue and cellular injury but without a marked host response indicate that unlike other cases of documented human ehrlichiosis, this patient died after significant direct ehrlichia-mediated injury, and that immune mechanisms initiated after ehrlichiosis played little if any role in the pathogenesis.