Leprosy in its indeterminate form (I) is a clinical presentation of the disease preceding the forms described in the Ridley and Jopling (R & J) classification and any other special forms of leprosy or the reactions. In this chapter, the histopathological and bacilloscopic characteristics of the I form of leprosy are described, and the main differential diagnoses are discussed. The histopathological criteria that distinguish the I form from the other forms of leprosy and the reaction processes that may occur during the disease course are also discussed. The identification of the histopathological characteristics of I leprosy is of great importance with respect to the selection of the treatment. I leprosy should not be confused with other forms of leprosy, especially the multibacillary forms, which require more prolonged treatment and which can develop reaction phenomena, causing permanent sequelae.
Keywords: Bacilloscopy, Hansen’s disease, Histopathology, Indeterminate leprosy, Leprosy, Mycobacterium leprae.