Pseudospirochetes, a cause of erroneous diagnoses of leptospirosis.

Motile filaments of varying lengths and thicknesses were observed by darkfield microscopy in a blood culture of a specimen from a patient who had fever of unknown origin. Similar structures were also seen in cultures inoculated with donor blood from healthy controls. Since the movement and configuration of the structures were not characteristic for spirochetes, the morphologic features were further examined by electron microscopy. The filaments observed were identical in appearance to pseudospirochetes described more than 50 years ago. The authors' observations indicate that they are derived from erythrocytes. Because of possible confusion of these pseudospirochetes with living organisms, the diagnosis of leptospirosis by darkfield microscopy should be confirmed by cultural or serologic tests.