Infections caused by Actinomyces viscosus.

Two cases of Actinomyces viscosus infection of the lungs were seen in nonimmunosuppressed patients. One patient had a peripheral actinomycotic lung mass resembling a tumor. Both patients responded to a long course of penicillin therapy. Reports of A. viscosus infections are rare, although the organism colonizes the mouths of most adult humans. Only ten cases have previously been described. There is no characteristic of A. viscosus infection that can distinguish it from Actinomyces israelii or Actinomyces bovis infections. The illness usually manifests as a chronic disease weeks to months before the diagnosis, which can only be made by identification of the organism from a clinical specimen uncontaminated by sputum or mouth flora. Ignorance of the biochemical reactions and growth characteristics of this organism have in the past hindered its isolation and identification. At least three weeks of antibiotic therapy using agents to which A. viscosus is sensitive in vitro are required for cure.