Mycobacteria in prurigo nodularis: The cause or a consequence?

Background: Prurigo nodularis (PN) is a chronic skin disorder; its cause remains unknown. Objective: We evaluated mycobacteria as a possible cause of PN. Methods: Forty-three patients with PN were examined. Skin biopsy specimens were obtained for microbiologic and histopathologic studies. The patients were tested for intracutaneous reactivity to 12 mycobacterial antigens with the Mantoux technique. Results: Six specimens (14%) grew mycobacteria in culture: M. avium-intracellulare (3), M. malmoense (1), and Mycobacterium sp. (2). Histopathologically, 12 samples (28%) were positive for acid-fast bacilli, and granulomatous changes were present in one sample. Patients whose cultures were positive for mycobacteria had significantly larger skin reactions to mycobacterial antigens. Two patients underwent 2 years of antituberculous chemotherapy; one had an excellent response and the other a partial response. Conclusion: Detection of mycobacteria by culture or staining, combined with elevated skin reactivity to mycobacteria in a high proportion of patients with PN, suggests a mycobacterial cause.