Causes
Tumour cells passed by direct contact between dogs during mating, and sometimes through licking or sniffing. Common in stray and unneutered dogs.
Diagnosis
Cytology or biopsy of the mass, which has a very characteristic cellular appearance.
Treatment
It responds very well to chemotherapy, with high cure rates. It is one of the tumours with the best prognosis when treated.
Prevention
Neutering, and avoiding contact with stray dogs that have no health monitoring.

