Vaginal Neoplasia (Leiomyoma, Transmissible Venereal Tumor - TVT)

Vaginal neoplasia encompasses a diverse group of benign and malignant tumors arising from the vaginal and vulvar tissues in female dogs and, less commonly, cats. The most frequently encountered benign tumor is the leiomyoma, a smooth muscle neoplasm originating from the tunica muscularis of the vaginal wall. Malignant tumors include leiomyosarcoma, fibrosarcoma, and squamous cell carcinoma, but the most clinically significant and unique neoplasm is the transmissible venereal tumor (TVT), a contagious round cell tumor transmitted by direct contact during coitus. TVT is an allogeneic transplant of tumor cells, typically affecting the external genitalia of both sexes. Vaginal neoplasms can cause significant morbidity through mechanical obstruction, hemorrhage, secondary infection, and, in malignant cases, metastasis. Accurate diagnosis and staging are essential for appropriate therapeutic intervention and prognostic counseling.

The etiology of vaginal neoplasia varies by tumor type. Leiomyomas are hormonally responsive tumors, with growth influenced by estrogen and progesterone. They often arise during diestrus or pregnancy and may regress after ovariohysterectomy, indicating a strong hormonal dependence. The exact molecular mechanisms involve the expression of steroid hormone receptors (estrogen receptor alpha, progesterone receptor) on smooth muscle cells, promoting proliferation. TVT is caused by the direct transmission of viable neoplastic cells, which are histiocytic in origin, through coitus, licking, sniffing, or biting of affected genitalia. The tumor cells harbor a unique genomic integration of a transposable element (LINE-1) near the c-myc gene, leading to uncontrolled proliferation. Other risk factors include chronic inflammation, viral infections (e.g., papillomavirus in some squamous cell carcinomas), and genetic predispositions. In cats, viral-induced fibrosarcomas and lymphosarcomas may involve the vagina, but primary vaginal tumors are rare.

Vaginal neoplasia is uncommon in dogs, accounting for approximately 2-3% of all canine tumors. Leiomyomas are the most common benign vaginal tumor, typically occurring in middle-aged to older intact female dogs (mean age 8-10 years). Breeds such as Boxers, German Shepherds, and Golden Retrievers may be overrepresented. TVT is endemic in tropical and subtropical regions, including parts of the Caribbean, South America, Africa, and Asia, but can occur sporadically worldwide. It affects sexually active, free-roaming dogs of both sexes, with a higher incidence in young dogs (2-5 years). No breed predisposition is recognized. Feline vaginal tumors are extremely rare, with lymphosarcoma and fibrosarcoma being the most frequently reported. Spayed females are less likely to develop hormonally dependent tumors, but TVT can occur in any dog regardless of reproductive status.

Leiomyomas arise from the smooth muscle of the vaginal wall and are typically well-circumscribed, slow-growing, and non-metastatic. They are hormonally sensitive, with growth stimulated by estrogen and progesterone. During proestrus and estrus, elevated estrogen levels promote tumor cell proliferation; during diestrus, progesterone may also stimulate growth. The tumors can become large, causing mechanical obstruction of the vaginal canal, leading to dysuria, tenesmus, and dystocia. Malignant transformation to leiomyosarcoma is rare but possible. TVT is a transmissible round cell tumor that is histologically similar to histiocytic tumors. The tumor cells are transmitted as allografts, evading the host immune system through downregulation of MHC class I and II molecules. They proliferate locally at the site of implantation, typically on the vaginal mucosa, and can metastasize to regional lymph nodes, skin, and internal organs in immunocompromised animals. The tumor is characterized by rapid growth, ulceration, and secondary bacterial infection, leading to a purulent, hemorrhagic vaginal discharge.

Intrinsic factors include age (middle-aged to older dogs for leiomyomas; young dogs for TVT), intact female status (for hormonally dependent tumors), and genetic susceptibility (breed predispositions). Extrinsic factors include exposure to intact males for TVT transmission, overcrowded kennels or free-roaming lifestyles, poor hygiene, and immunosuppression (e.g., concurrent diseases, chemotherapy). For leiomyomas, exogenous steroid administration (e.g., progestins for estrus suppression) may promote tumor growth. Nulliparity has been suggested as a risk factor for some reproductive tumors, but evidence is limited. Chronic vaginitis or trauma may predispose to squamous cell carcinoma, though this is rare.

Clinical signs vary depending on tumor size, location, and malignancy. Small leiomyomas may be asymptomatic and discovered incidentally during routine examination. Larger tumors can cause vaginal discharge (serous, mucoid, or hemorrhagic), vulvar swelling, excessive licking of the perineum, dysuria, stranguria, tenesmus, and constipation. In breeding animals, dystocia may occur. TVT typically presents as a friable, cauliflower-like, pedunculated or nodular mass on the vaginal mucosa, often with ulceration and bleeding. A sanguineous or serosanguineous vaginal discharge is common. The tumor may be visible protruding from the vulva. In males, TVT affects the penis and prepuce. Metastatic TVT can cause systemic signs such as lethargy, weight loss, and lymphadenopathy. Malignant tumors may invade locally, causing pain and obstruction.

Differential diagnoses for vaginal masses include: (1) Vaginal polyps (fibroepithelial polyps) – benign, often pedunculated, non-neoplastic growths; (2) Vaginal prolapse or hyperplasia – associated with estrus, reducible, and hormonally driven; (3) Vaginal hematoma – history of trauma, acute onset; (4) Vaginal abscess – painful, fluctuant, with systemic signs; (5) Granulomatous vaginitis – chronic inflammation, often due to foreign body or bacterial infection; (6) Ectopic ureter – congenital, causing urinary incontinence, not a mass; (7) Squamous cell carcinoma – malignant, invasive, often in older animals; (8) Fibrosarcoma – malignant, firm, locally invasive; (9) Lymphosarcoma – systemic involvement, cytology/histopathology; (10) Mast cell tumor – rare in vagina, cytology with metachromatic granules. Definitive diagnosis requires cytology, histopathology, and imaging.

The diagnostic approach begins with a thorough history and physical examination, including a vaginal examination (digital palpation, speculum examination, vaginoscopy). If a mass is identified, the following steps are recommended: (1) Vaginal cytology – impression smears or fine-needle aspiration to identify cell types (e.g., round cells for TVT, spindle cells for leiomyoma). (2) Biopsy – incisional or excisional biopsy for histopathology to confirm tumor type and grade. (3) Imaging – abdominal ultrasonography to assess for metastasis (especially for TVT) and to evaluate the reproductive tract; thoracic radiographs for malignant tumors. (4) Staging – for TVT, assess regional lymph nodes (palpation, cytology) and consider PCR for the LINE-1 insertion to confirm diagnosis. (5) Hormonal assays – serum progesterone and estrogen levels may be useful in hormonally dependent tumors. (6) Complete blood count and biochemistry to assess overall health and rule out systemic disease.

Hematology and biochemistry are often unremarkable unless secondary infection or metastasis is present. In TVT, mild anemia may occur due to chronic blood loss. Leukocytosis may be seen with secondary bacterial infection. Serum progesterone and estrogen levels may be elevated in intact females with hormonally dependent tumors, but are not diagnostic. Vaginal cytology from TVT typically shows large, round cells with abundant cytoplasm, distinct cell borders, and multiple nucleoli, often with a background of neutrophils and red blood cells. Leiomyoma cytology may show spindle-shaped cells with elongated nuclei. Histopathology is definitive: leiomyomas are composed of interlacing bundles of smooth muscle cells with minimal atypia and low mitotic index; TVT shows sheets of round cells with a high mitotic index and characteristic cytoplasmic vacuoles. Immunohistochemistry can be used to differentiate tumors (e.g., vimentin positive, cytokeratin negative for TVT; smooth muscle actin positive for leiomyoma).

Abdominal ultrasonography is useful to evaluate the vaginal mass, assess the uterus and ovaries, and detect metastasis (e.g., enlarged lymph nodes, liver/spleen lesions). Leiomyomas appear as well-defined, hypoechoic masses within the vaginal wall. TVT may appear as a hyperechoic or mixed echogenic mass. Thoracic radiographs are indicated for malignant tumors to rule out pulmonary metastasis. CT and MRI provide detailed anatomical information and are valuable for surgical planning, especially for large or invasive tumors. Vaginoscopy allows direct visualization of the mass, assessment of its origin, and guided biopsy.

Cytology of TVT is highly characteristic: large, round to polyhedral cells with abundant pale blue cytoplasm, distinct cell borders, and round nuclei with prominent nucleoli. The cells often contain small cytoplasmic vacuoles. A background of neutrophils, lymphocytes, and red blood cells is common. Histopathology of TVT shows sheets of densely packed round cells with a high nuclear-to-cytoplasmic ratio, frequent mitotic figures, and areas of necrosis. The tumor cells are positive for vimentin and negative for cytokeratin, CD3, and CD79a. Leiomyoma histopathology reveals well-differentiated smooth muscle cells arranged in interlacing fascicles, with minimal nuclear atypia and low mitotic activity. Immunohistochemistry for smooth muscle actin and desmin is positive. Malignant leiomyosarcoma shows increased cellular atypia, high mitotic index, and invasion.

Treatment depends on tumor type, size, location, and presence of metastasis. For benign leiomyomas, surgical excision (vaginal mass resection) is curative. Ovariohysterectomy is recommended to remove hormonal stimulation and prevent recurrence. For TVT, the treatment of choice is chemotherapy with vincristine sulfate (0.025 mg/kg IV once weekly) until complete remission, typically 3-6 treatments. Vincristine is highly effective, with a remission rate of >90%. Alternative protocols include doxorubicin or a combination of vincristine and cyclophosphamide for resistant cases. Surgical debulking may be considered for large tumors but is not curative alone. Radiation therapy is also effective but less commonly available. For malignant tumors (e.g., leiomyosarcoma, squamous cell carcinoma), wide surgical excision is recommended, with adjunctive chemotherapy or radiation for incompletely excised or metastatic disease. Supportive care includes antibiotics for secondary infections, pain management, and nutritional support.

The prognosis for benign leiomyomas is excellent after surgical excision, with a low recurrence rate if ovariohysterectomy is performed. Fertility may be preserved if the tumor is removed without compromising the reproductive tract, but breeding is generally discouraged due to the risk of dystocia. TVT has an excellent prognosis with chemotherapy, with a cure rate of >90%. However, metastatic TVT carries a guarded prognosis, though chemotherapy is still effective in many cases. Malignant vaginal tumors have a guarded to poor prognosis, depending on the grade and stage. Early detection and complete surgical excision offer the best chance for long-term survival.

Post-treatment monitoring includes: (1) For TVT, weekly physical examinations and tumor measurements during chemotherapy; after remission, monthly rechecks for 3 months, then every 3 months for a year, and then annually. (2) For surgically treated tumors, recheck examinations at 2 weeks, 1 month, 3 months, and then every 6 months for 2 years. (3) Serial imaging (ultrasonography or radiography) to monitor for recurrence or metastasis. (4) For intact females, recommend ovariohysterectomy to prevent hormonal influence on residual tumor cells. (5) For breeding animals, delay breeding until complete recovery and consider artificial insemination to avoid trauma.

Pearls: (1) TVT is highly responsive to vincristine; always confirm diagnosis before starting chemotherapy. (2) Vaginal leiomyomas are hormonally sensitive; ovariohysterectomy is an essential part of treatment. (3) Vaginoscopy is invaluable for biopsy and surgical planning. (4) In endemic areas, TVT should be a differential for any genital mass in sexually active dogs. Pitfalls: (1) Misdiagnosing TVT as a bacterial vaginitis or a benign polyp, leading to inappropriate treatment. (2) Performing surgery alone for TVT, which often results in recurrence. (3) Failing to stage malignant tumors, leading to missed metastasis. (4) Using corticosteroids in TVT, which may cause immunosuppression and tumor progression. (5) Overlooking the possibility of a vaginal mass causing dystocia in a pregnant bitch.

For TVT: Vincristine sulfate (0.025 mg/kg IV once weekly) is the first-line protocol. Administer as a slow IV bolus, typically for 3-6 weeks until complete remission. If no response after 2-3 treatments, consider doxorubicin (30 mg/m² IV every 3 weeks) or a combination protocol (vincristine 0.025 mg/kg IV on day 1, cyclophosphamide 200 mg/m² IV on day 1, repeated every 3 weeks). For leiomyomas, no specific drug therapy is indicated; surgical excision and ovariohysterectomy are the treatments of choice. For malignant tumors, chemotherapy protocols may include doxorubicin or carboplatin (300 mg/m² IV every 3 weeks) as adjunctive therapy. Supportive care: antibiotics (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO q8-12h) for secondary infections, and analgesics (e.g., carprofen 2.2 mg/kg PO q12h) as needed.

Landmark studies have established vincristine as the standard of care for TVT, with reported remission rates of 90-100% (Amber et al., 1990; Das & Das, 2000). A study by Rogers et al. (1998) demonstrated that surgical excision alone for TVT has a high recurrence rate, emphasizing the need for chemotherapy. For vaginal leiomyomas, a retrospective study by Sontas et al. (2010) reported that ovariohysterectomy and mass excision resulted in excellent outcomes with no recurrence. The hormonal dependence of leiomyomas is supported by studies showing expression of estrogen and progesterone receptors (Klein et al., 2006). Consensus guidelines from the American College of Theriogenologists and the European Society for Small Animal Reproduction recommend chemotherapy for TVT and surgical management for benign tumors. Recent studies have explored immunotherapy for TVT, but chemotherapy remains the gold standard.