Transmissible Venereal Tumour (TVT) in Dogs
Transmissible venereal tumour (TVT) in dogs also known as infectious sarcoma, venereal granuloma, transmissible lymphosarcoma or stickler tumour.
Transmissible venereal tumour (TVT) is a benign reticuloendothelial tumour of the dogs that mainly affects the external genitalia and occasionally the internal genitalia. Since, it is usually transmitted during coitus, it mainly occurs in young sexually mature animals. The transmissible venereal tumour cells contain an abnormal number of chromosomes ranging from 57- 64 in contrast to the normal 78 of the species.
Etiology
TVT arises from allogenic cellular transplants and the abnormal cells of the neoplasm are the vectors of transmission.
Exfoliation and transplantation of neoplastic cells due to physical contact during mating or licking of affected area is responsible for spread onto genital, oral or nasal mucosa.
The implantation of the tumour is facilitated by the presence of any mucosal lesion or by the loss of mucosal integrity.
Growth of tumour occurs 15 to 60 days after implantation. Metastasis may occur in less than 5-17% of cases.
Gross and Histopathology
Small pink to red, 1 mm to 3 mm diameter nodules can be observed 2 or 3 weeks after transplantation.
Initial lesions are superficial dermoepidermal or pedunculated. Multiple nodules then fuse together forming larger, red, hemorrhagic, cauliflower-like, friable masses of 5 to 7 cm in diameter which then progress deeper into the mucosa as multilobular subcutaneous lesions with diameters that can exceed 10-15 cm.
Tumours bleed easily and while becoming larger, normally ulcerate and become contaminated.
Exfoliative cytology shows discrete cells that are round to oval, with moderately abundant pale blue cytoplasm, an eccentrically located nucleus, with occasional binucleation and mitotic figures.
Single or multiple nucleoli are often observed. The most characteristic feature of TVT cells is the presence of numerous discrete clear cytoplasmic vacuoles.
Clinical Signs
- Clinical signs vary according to localization of the tumours.
- Discharge may be confused with urethritis or cystitis.
- In bitches the tumours are localized in the vestibule and/or caudal vagina, protrude from the vulva and frequently cause a deformation of the perineal region. Rarely, the tumour masses interfere with micturition.
- A considerable hemorrhagic vulvar discharge may occur and can cause anaemia if it persists.
- The discharge can attract males and the condition of the bitch can be mistaken for estrus by the owners. Infrequently, TVTs can localize in the uterus.
In cases with extra genital localization of the TVT, clinical diagnosis is usually more difficult because TVTs cause a variety of signs depending on the anatomical localization of the tumour, eg., sneezing, epistaxis, epiphora, halitosis and tooth loss, halitosis and tooth loss, exophthalmos, skin bumps, facial or oral deformation along with regional lymph node.
Diagnosis
Diagnosis is based on:
- Physical examination.
- Cytological findings of TVT in exfoliated cells obtained by swabs, fine needle aspirations or imprints of the tumours.
Treatment
Several treatments including surgery, radiotherapy, immunotherapy, biotherapy and chemotherapy have been applied for TVT.
Surgery has been used extensively for the treatment of small, localized TVTs, although the recurrence rate can be as high as 50-68% in cases of large invasive tumours. Contamination of the surgical site with TVT cells is also a source of recurrence.
Transmissible venereal tumours are radiosensitive and orthovoltage as well as cobalt have been used for this purpose.
Biotherapy studies have also been reported. The intra tumoural application of CalmetteGuĂ©rin’s bacillus (BCG) has been used for three weeks with sporadic success.
Recurrences have been described after immunotherapy using Staphylococcus protein A, BCG or a vaccine made from tumoural cells. Biotherapy has unfortunately also resulted in a high rate of recurrence.
Chemotherapy has been shown to be the most effective and practical therapy, with vincristine sulfate being the most frequently used drug.
Vincristine sulfate is administered weekly at a dose of 0.5 to 0.7 mg/m2 of the body surface area or 0.025 mg/kg IV. Complete remission takes 2-8 injections and occurs in more than 90% of the treated cases. Vincristine can cause myelosuppression and gastrointestinal effects resulting in luekopenia and vomiting. A complete WBC count is recommended prior to each administration. When the WBC count is below 4,000 m3, further administration should be delayed 3 to 4 days and its dose reduced to 25% of the initial dose. Local tissue lesions can be caused by extravasations of the drug during IV.
Cyclophosphamide @ 5 mg/kg PO for 10 days as a single drug therapy or in combination with prednisolone @ 3 mg/kg, for 5 days; vinblastine @ 0.1 mg/kg IV for 4-6 weeks; methotrexate @ 0.1 mg/kg PO every other day can be given alone or in combination for treatment of TVT. There is no advantage of combination therapy over vincristine administration alone.
In cases that fail to resolve with chemotherapy, electro-cauterization, cryo-cauterization or radiotherapy has found to yield good results.
