Regression of feline sarcoma virus-induced sarcomas in dogs. I. Morphologic investigations

In a study of morphologic changes in the development and regression of feline sarcoma virus (FeSV)-induced tumors in dogs, 27 weaned and newborn beagle and mongrel puppies were inoculated with FeSV in doses from 1.0 to 3.0 gEq; 2 beagle and 2 mongrel puppies were used as uninoculated contact controls. All animals were examined daily, and crude tumor volume was calculated from length, width, and depth measurements of the neoplasms. Biopsies were done at various stages of tumor development and regression. When tumors were no longer palpable, all puppies were necropsied. Two of 8 (25%) weaned beagle puppies, 10 of 12 (83%) newborn beagles, and 1 of 7 (14%) newborn mongrels developed tumors, all histologically confirmed fibrosarcomas. No metastatic tumor foci were detected. The tumor life-span was divided into approximately equal periods of growth and regression. The initial regression period was characterized by focal necrosis and accompanying neutrophil infiltration. The later stages of regression were characterized by lymphocytic or mixed mononuclear infiltrates. Thus the regression histopathology was not uniform and suggested that different immunologic mediation systems effect regression. Nonneoplastic morphologic changes consisted largely of lymphoid depletion and necrosis in lymph nodes and thymus after inoculation.