The adaptive immune system should prevent cancer cells passing from one individual to another, in much the same way that it protects against pathogens. However, in rare cases cancer cells do not die within a single individual, but successfully pass between individuals, escaping the adaptive immune response and becoming a contagious cancer. There are two naturally occurring contagious cancers, Devil Facial Tumour Disease (DFTD), found in Tasmanian devils, and Canine Transmissible Venereal Tumour (CTVT), found in dogs.3
The canine transmissible venereal tumor (CTVT), also referred to as Sticker’s tumor, venereal granuloma, transmissible venereal sarcoma, infective venereal tumor, and transplantable lymphosarcoma, is a contagious cancer that is naturally transmitted between dogs during coitus and results in the appearance of tumors most often associated with the external genitalia of male and female dogs. Transmission can also occur through dogs’ normal social behavior when they lick, sniff, scratch, or bite at their own genitalia (autoimplantation) or if they lick the genitalia of an affected dog. CTVT has been reported in dogs worldwide. It is a common tumor in tropical and subtropical regions where there is a high population of stray, malnourished dogs.1
These tumor cells are not the patient’s own cells transformed into cancer cells. The TVT is a tumor that grafts itself from one dog’s body onto another dog’s body. Ulceration of the surface of the tumor with secondary inflammation is common. Transmissible venereal tumors also bleed easily and often have a malodorous discharge especially if they become secondarily infected. The bloody discharge can be confused with signs of estrus or cystitis in females or prostatitis in male dogs. Metastases occur in less than 5% of cases and most commonly affect regional lymph nodes. However, metastasis to other internal organs such as the skin and subcutaneous tissue, pharynx, tonsils, liver, spleen, kidneys, brain, eyes, and lymph nodes has been reported. Tumors of the third eyelid are rare, seen in older dogs, and are typically malignant.1,2
Vincristine sulfate, as a single agent given IV once weekly is the most common treatment protocol for TVT. Four to 8 doses are usually necessary to effect a cure. Medical treatment options for TVT include other chemotherapy drugs such as monotherapy with doxorubicin or vinblastine or combination chemotherapy protocols.
- A case of ocular canine transmissible venereal tumor
- Immunology of naturally transmissible tumours
- A tale of two tumours: Comparison of the immune escape strategies of contagious cancers