Canine Transmissible Venereal Tumor (TVT) – NAVLE Study Guide

Overview and Clinical Importance

Canine Transmissible Venereal Tumor (TVT), also known as Sticker's sarcoma, transmissible venereal sarcoma, or infectious sarcoma, is a unique, naturally occurring transmissible neoplasm affecting canids worldwide. TVT is one of only three known transmissible cancers in mammals (the others being Tasmanian devil facial tumor disease and contagious reticulum cell sarcoma in Syrian hamsters).

TVT represents an extraordinary example of a clonally transmissible cancer where the tumor cell itself is the infectious agent. Genetic analyses reveal that all TVT tumors worldwide are derived from a single ancestral cancer that originated approximately 6,000-11,000 years ago in an ancient dog or wolf. This makes TVT the oldest known continuously surviving cancer lineage in nature.

TVT is highly prevalent in tropical and subtropical regions where free-roaming dog populations exist, and represents a high-yield NAVLE topic due to its unique transmission mechanism, characteristic cytologic appearance, and excellent treatment response to vincristine chemotherapy.

Etiology and Pathogenesis

Transmission Mechanism

TVT is transmitted through direct physical transfer of viable tumor cells between dogs. The primary transmission route is during coitus, but transmission can also occur through:

• Licking of tumor-affected areas
• Sniffing or nuzzling genital regions
• Biting or scratching tumor masses
• Dam-to-pup transmission during grooming or parturition

Successful transmission requires abrasions or microtrauma to the mucosal surface, which facilitates implantation of exfoliated tumor cells. A minimum quorum of neoplastic cells is required for successful transplantation.

Genetic and Chromosomal Features

TVT cells exhibit unique cytogenetic characteristics that differentiate them from host cells:

• Abnormal chromosome number: 57-64 chromosomes (normal canine karyotype: 78 chromosomes)
• XO sex chromosome complement regardless of host sex
• LINE-1 insertion: Specific long interspersed nuclear element (LINE) inserted upstream of the c-myc oncogene
• Clonal origin: All TVT tumors worldwide share genetic identity from a single ancestral tumor

Epidemiology

Geographic Distribution

TVT has a worldwide distribution but is most prevalent in:

• Tropical and subtropical regions: Central and South America, Southeast Asia, Middle East, parts of Africa, Caribbean
• Areas with large stray dog populations: TVT is enzootic where breeding is uncontrolled
• Southern United States, Southern Europe, Ireland, Japan, India: Sporadic cases occur in temperate climates

Risk Factors and Signalment

Clinical Presentation

Gross Appearance

TVT masses are characteristically described as cauliflower-like, pedunculated, nodular, papillary, or multilobulated. Key gross features include:

• Color: Pink to red, often hemorrhagic
• Texture: Soft, friable, easily bleeds on contact
• Surface: Often ulcerated with superficial bacterial infection
• Size: Ranges from small nodules (5 mm) to large masses (greater than 10 cm)
• Exfoliation: Tumor cells exfoliate easily, facilitating cytologic diagnosis

Anatomical Locations

Diagnosis

Cytology: The First-Choice Diagnostic Tool

Cytology is the gold standard for TVT diagnosis due to its simplicity, minimal invasiveness, and excellent diagnostic accuracy. TVT cells have pathognomonic features that allow confident diagnosis in most cases.

Characteristic Cytologic Features of TVT

Differential Diagnosis: Round Cell Tumors

TVT must be differentiated from other round cell tumors on cytology:

Advanced Diagnostics

Histopathology

Histopathology may be performed when cytology is inconclusive, particularly for extragenital TVT. Features include:

• Sheets of round to polyhedral cells with indistinct cell borders
• Finely vacuolated cytoplasm
• Round nuclei with stippled chromatin and prominent nucleoli
• Delicate fibrovascular stroma

Immunohistochemistry

TVT has an immunohistochemical profile consistent with histiocytic origin:

Molecular Diagnostics: LINE-1/c-myc PCR

The LINE-1/c-myc PCR assay detects the specific long interspersed nuclear element (LINE) insertion upstream of the c-myc oncogene. This test provides 100% sensitivity and specificity for TVT diagnosis and is particularly valuable for: (1) atypical cytologic presentations, (2) extragenital TVT, (3) differentiating TVT from histiocytoma, and (4) determining when to discontinue chemotherapy.

Tumor Behavior and Staging

Natural Tumor Progression

TVT undergoes a predictable tumor cycle:

• Progressive Phase (P phase): Initial rapid growth period lasting 4-6 months
• Stable Phase: Tumor growth plateaus
• Regression Phase (R phase): Spontaneous regression may occur (immune-mediated), though not all tumors regress

Metastasis

TVT generally exhibits benign biological behavior with a low metastatic rate (less than 5-15%). Metastasis is more likely in:

• Immunocompromised dogs
• Puppies/neonates
• Dogs with large tumor burden
• Plasmacytoid cytomorphologic subtype (most aggressive form)

Common metastatic sites:

• Regional lymph nodes (most common)
• Skin and subcutis
• Eye and orbit
• Brain (rare - poor prognosis)
• Liver, spleen, kidney (rare)
• Lung (rare)

Treatment

Vincristine Chemotherapy: Treatment of Choice

Vincristine sulfate is the gold standard treatment for TVT, with excellent response rates (greater than 90% complete remission).

Vincristine Side Effects

• Myelosuppression: Monitor CBC; neutropenia nadir typically 5-7 days post-treatment
• GI toxicity: Vomiting, diarrhea, anorexia (usually mild)
• Peripheral neuropathy: Rare in dogs (more common in humans)
• Perivascular extravasation: SEVERE tissue sloughing/necrosis - must use proper IV technique

Alternative and Rescue Treatments

Prognosis

The prognosis for TVT is EXCELLENT with appropriate treatment. Key prognostic factors include:

Prevention and Control

• Spay/neuter programs: Reduces roaming and mating behavior
• Pre-breeding examination: Inspect animals before breeding to exclude active infection
• Stray dog population control: Reduces TVT prevalence in endemic areas
• Isolation during treatment: Prevent contact between affected and unaffected dogs until treatment complete
• Zoonotic risk: NONE - TVT does not affect humans