COMPANION ANIMALS

Transmissible venereal tumor in three dogs    

Andrew Brooks, Kristiina Ruotsalo, Margaret Stalker

Three cases of transmissible venereal tumor (TVT) were diagnosed at the Animal Health Laboratory in the spring of 2016. Case 1 was a mixed-breed, female, spayed, 11-year-old dog with a 3-cm nodular mass on the mucosal surface of the vulva. The dog originated from Africa and may have been bred there prior to arriving in Ontario. Case 2 was a 6-year-old, female, mixed-breed dog with a 1.5-cm mass on the vulva. Case 3 was a 6-year-old neutered male Thai Ridgeback with a rapidly growing 5-cm mass on the prepuce and penile sheath. Cases 2 and 3 were from a rescue facility that housed dogs originating from Europe and Africa.

Cytologically the tumors contained a monomorphic population of individual round cells (Fig. 1). These cells exhibited a moderate nuclear-to-cytoplasmic ratio with cen-trally located, round nuclei. The nuclear chromatin was fine-ly granular and contained evidence of 1-2, prominent, varia-bly sized nucleoli. The cytoplasm was moderately abundant, devoid of granulation, and contained variable numbers of frequent punctate vacuoles, typically in a perinuclear loca-tion. Occasional mitotic figures were evident and a variable degree of inflammation was noted. Histologically the tumors were composed of round cells arranged in packets or solid areas (Fig. 2). The cells exhibited round-to-oval nuclei, 2-fold anisokaryosis, 1 or 2 prominent nucleoli, moderate-to-abundant eosinophilic cytoplasm, and some cells contained cytoplasmic vacuoles. The mitotic rate was generally high (2-7 mitotic figures per high-power (400 X) field). The round cells were intermingled with a few small lymphocytes and occasional neutrophils and eosinophils.

TVT is a horizontally-transmissible round cell tumor of dogs that is endemic in warm temperate regions such as the Caribbean, Central and South America, parts of Africa, southern Europe, and Asia. A recent survey indicates the tumor is globally distributed and endemic in at least 90 countries. The tumor is sexually transmitted by allogeneic transfer of neoplastic cells from one dog to another during coitus. The neoplastic cells are genetically distinct from the canine host and analysis of the tumor genome suggests the cancer arose from a canid somatic cell several thousand years ago. TVT prevalence is high in free-roaming, sexually intact dogs in endemic regions. The tumor is rare or non-existent in countries with established animal control pro-grams. The external genitalia are the most common primary sites of tumour formation but lesions may also develop in other locations such as the nose, mouth, and face. A bloody or serosanguineous discharge often accompanies the tumors.

A presumptive diagnosis may be indicated by tumor location, geographic origin, or travel history, but a definitive diagnosis requires cytologic and/or histologic examination. The prognosis and response to treatment are generally very good. The 3 dogs of this report all experienced tumor regres-sion following vincristine therapy.