Comprehensive Study Guide on Transmissible Venereal Tumor (TVT) in Canines

Introduction and Comprehensive Nomenclature of Transmissible Venereal Tumor

Transmissible Venereal Tumor (TVT) is defined as a contagious, neoplastic disease that is transmitted through the transplantation of viable tumor cells, typically occurring during coitus. This condition is unique due to its infectious nature among canine populations. It is known by a variety of synonyms across veterinary literature, including Canine condyloma, Contagious lymphoma, Contagious venereal tumor, Infectious sarcoma, Sticker's sarcoma, Transmissible venereal sarcoma, and Venereal granuloma. Within clinical settings, it is most frequently referred to by the acronym TVT.

Epidemiology, Breed Predisposition, and Demographics

The prevalence of Transmissible Venereal Tumor varies according to several demographic factors. There is a documented higher prevalence of the disease in mixed-breed dogs. In terms of age, the disease primarily affects young, sexually active animals, with an average age of presentation between $4$ and $5$ years old. While TVT affects both sexes, it is observed with higher frequency in females compared to males. The epidemiology of the disease is also influenced by seasonality and the presence of stray animal populations, which serve as reservoirs for the tumor cells.

Etiology, Pathophysiology, and Mechanisms of Transmission

The etiology of TVT involves the direct transmission of tumor cells during coitus. The primary site of infection usually involves the external genital organs, with the potential for the disease to spread to the internal genitalia. The transmission process is facilitated if there are pre-existing abrasions or breaks in the integrity of the mucosal surfaces. Beyond sexual contact, the social behavior of canines can result in extragenital TVT. This occurs when tumor cells are transplanted into the skin, eyes, or the oral and nasal cavities through sniffing or licking. While the tumor is locally aggressive, metastasis is considered rare. In terms of contagion, the disease is strictly contagious among members of the family Canidae; it has not been identified as a zoonotic disease and poses no risk to human health.

Patient Presentation and Clinical Signs

Patients presenting with TVT often have a history of persistent or intermittent bloody genital discharge, which is frequently foul-smelling. Owners may report constant licking of the genital area, obvious genital malformation, or localized swelling. As the disease progresses, visible masses can be seen protruding from the vulva in females or the penis in males. These clinical signs typically persist for a duration of $40$ to $100$ days, though in some instances, the duration may be longer.

Physical examination findings typically reveal nodular lesions located on the external genitalia. These may appear as solitary or multiple masses. These lesions often progress to become cauliflower-like, papillary, multilobulated, or pedunculated masses. The typical appearance of a TVT mass is gray or pinkish-gray. The texture is characterized as firm but friable, meaning the masses bleed easily upon manipulation or trauma.

Extragenital Disease Forms and Associated Disorders

TVT can manifest in genital and extragenital forms. The oronasal presentation is characterized by epistaxis (nosebleeds) and frequent sneezing accompanied by epistaxis. Other signs include halithosis (bad breath), ptyalism (excessive salivation), dyspnea (difficulty breathing), and the eventual loss of teeth. Physical examination of the oral cavity may reveal ulcerative lesions on the gingiva and the palate. The cutaneous presentation involves nodular masses that are often ulcerated, frequently accompanied by the enlargement of regional lymph nodes.

Several associated disorders can arise due to the presence of the tumor. In females, TVT masses located at the vestibulovaginal junction can lead to urinary tract infections (UTIs). In males, the tumor can cause phimosis, a condition where the animal is unable to extrude the penis from the prepuce. Because the tumors are friable, they are easily traumatized and ulcerated, which commonly leads to secondary bacterial infections.

Diagnostic Procedures and Differential Diagnosis

The diagnosis of Transmissible Venereal Tumor is initially suspected based on the patient's history, the anatomical location of the lesions, and the physical appearance of the mass. To confirm the diagnosis, an initial laboratory database is required, including a cytologic examination of impression smears. For a more definitive confirmation, a polymerase chain reaction (PCR) assay or the histologic classification of a biopsy specimen is utilized.

It is critical to distinguish TVT from other round cell tumors, as they can appear similar. Furthermore, the clinical signs of TVT—such as genital discharge or swelling—can be confused with other conditions including prostatitis, estrus, urethritis, and cystitis. Therefore, confirmatory testing is essential to rule out these differential diagnoses.

Treatment Modalities and Management Strategies

Several treatment options exist for TVT, with varying degrees of efficacy. Surgical excision may be effective for small, localized masses; however, the excision of large tumors is frequently followed by tumor regrowth. Pharmacological management is highly effective, specifically the use of intratumoral or systemic chemotherapy. Weekly injections of vincristine are administered for a period of $6$ weeks or until complete tumor regression is achieved. Interleukin-2 may also be used in conjunction with these treatments. For cases of TV-vincristine-resistant TVT, electrochemotherapy with bleomycin has been utilized.

Orthovoltage radiotherapy is considered highly effective, boasting a $100\%$ cure rate with a single radiation dose of $10\,Gy$. During the treatment period, breeding must be strictly avoided to prevent contagion. Furthermore, owners should be aware that excitement leading to an erection and preputial extrusion of a penis affected by a TVT mass may result in paraphimosis, where the penis cannot be retracted.

Complications, Prognosis, and Prevention

Therapeutic efforts, particularly chemotherapy, carry risks of complications. Extravasation injuries, which occur if vincristine is injected perivascularly rather than intravenously, can range from simple irritation to severe tissue necrosis and sloughing. When vincristine is used in combined chemotherapy protocols, the frequency and severity of side effects typically increase compared to using vincristine as a monotherapy. Despite these risks, the prognosis for TVT is considered excellent when the recommended therapies are followed.

Prevention and control focus on reducing exposure. Pet dogs should be kept away from contact with stray dogs, especially in endemic areas. Client education emphasizes that pet dogs should be confined and spayed or neutered. These measures significantly reduce the risk of disease transmission by limiting the behaviors that lead to the transplantation of tumor cells.