Papillary squamous cell carcinoma in three young dogs.

Papillary squamous cell carcinomas were located on the gingiva of 3 young dogs. The tumors locally invaded the soft tissues of each dog, and invaded bone in 2 dogs. Surgical excision was unsuccessful in eliminating 2 of the tumors. Surgery and radiotherapy were effective, and recurrence has not been observed in 39 months in 1 dog, 32 months in a second, and 10 months in a third. Superficially, the oral masses resembled papillomas, which are known to be caused by viruses. Cytopathologic indication of productive infection was not evident, and papillomavirus antigens could not be detected by immunohistochemical methods. Electron microscopy failed to identify viral particles in 2 of the tumors. High and low molecular weight DNA extracts from 2 of the tumors contained no detectable papillomavirus genome when probed under conditions of either high or low stringency by Southern blot hybridization with a cloned canine oral papillomavirus genome