A Transmissible Venereal Tumor (TVT), also known as a Sticker Tumor or Canine Transmissible Venereal Tumor (CTVT), is a unique type of cancer found primarily in dogs. Unlike most cancers that arise from a mutation within an individual’s own cells, TVT is an infectious disease. The tumor is spread between animals, establishing itself primarily on the external genitalia of both male and female canines.
The Unique Biology of Transmissible Venereal Tumors
The biology of TVT defies the typical understanding of cancer development. It is caused by the physical transfer of living cancer cells from one animal to another, a process known as an allograft. The transferred cells are genetically distinct from the host, acting almost like an infectious parasite. TVT represents a single, ancient cell lineage that has been continuously propagated for thousands of years, surviving countless host animals.
This cell line is estimated to have originated in a single dog between 6,000 and 11,000 years ago, making it the oldest known somatic cell lineage. Genetic analysis shows that all TVT tumors worldwide share a common genetic blueprint. The cancer cells possess an abnormal number of chromosomes, typically 57 to 59, significantly fewer than the 78 chromosomes found in a normal dog. This chromosomal difference confirms the tumor is a clone of the original ancient cell, and the tumor cells themselves are the infective agent.
Transmission and Affected Populations
The primary mechanism for TVT transmission is physical contact with an existing tumor mass. The disease is transmitted most commonly during coitus between dogs, where friction and abrasions allow the implantation of living cancer cells into the mucous membranes of the external genitalia.
Transmission is not strictly venereal, however, and can occur through common social behaviors. Dogs can spread the tumor by licking, sniffing, scratching, or biting affected areas. This non-coital transfer can lead to tumors appearing in extragenital locations like the nasal cavity, oral cavity, or eyes. TVT is found globally, but it is most prevalent in tropical and subtropical urban environments, such as Central and South America, Africa, and Asia. The highest rates of infection are seen in sexually active, free-roaming, and stray dog populations. Although dogs are the main host, other canids like foxes and coyotes can occasionally be affected.
Recognizing the Clinical Signs
The clinical presentation of TVT is typically a noticeable mass on the external genitalia. In males, tumors usually appear on the penis or prepuce, while in females, they are found on the vulva or within the vagina. These masses often have a cauliflower-like or nodular appearance.
The tumors are friable, meaning they are easily broken and tend to bleed readily upon contact. A common sign is a bloody or serosanguineous discharge from the preputial or vulvar area. Depending on the location and size, the tumor can cause difficulty with urination or defecation. Tumors in less common locations, such as the nasal cavity, can cause chronic nasal discharge, sneezing, or nosebleeds. Diagnosis is reliably confirmed by cytology, which involves examining cells collected from a fine-needle aspirate or an impression smear of the tumor.
Treatment and Prognosis
The standard and most successful treatment for TVT is chemotherapy, using vincristine sulfate as the agent of choice. Vincristine is an antimitotic agent that works by disrupting the cancer cells’ ability to divide, arresting their growth. The medication is typically administered intravenously once a week for several weeks, continuing until the tumor has completely regressed and stabilized.
TVT is recognized as one of the most treatable canine cancers, with a highly positive outlook. The prognosis for complete remission with vincristine chemotherapy is excellent, reaching a cure rate of 90 to 95%. Surgery to remove the mass is sometimes attempted, but recurrence is common if chemotherapy is not also used, due to the tumor’s friable nature. Radiation therapy or alternative chemotherapy drugs like doxorubicin are reserved for rare instances where the tumor is large or resistant to vincristine.