Lymph node abscess due to Actinomyces viscosus in a cat.

In a four-year-old male cat, a subcutaneous phyma about 3.5 cm in diameter was surgically removed from the left inframandibular region. Histopathologically, the phyma was found to be the swollen medial retropharyngeal lymph node containing an actinomycotic abscess. The filamentous organisms in the abscess stained positively by the Gram's, Grocott's and periodic acid-Schiff methods, and were negative by the Ziehl-Neelsen method. By the immunoperoxidase method, the organisms were specifically identified as Actinomyces viscosus serotype 2 by its antiserum absorbed with A. viscosus serotype 1 antigen.