Clinical Exposures: Canine transmissible venereal tumor: The cytologic clues

A 3-year-old castrated male Labrador retriever with a history of blood dripping from its penis was referred to the Foster Hospital for Small Animals at the Cummings School of Veterinary Medicine at Tufts University. The dog had been adopted from a breed rescue group about six weeks before presentation. Before being acquired by the rescue group, the dog had lived in Texas and Tennessee. Bloody penile discharge had been noted since the adoption, although the new owners reported that it had initially seemed to resolve without any intervention.

DIAGNOSIS

On physical examination, a large mass was noted on the shaft of the penis (Figure 1). The remainder of the physical examination findings were unremarkable.

Figure 1

Cytologic examination of impression smears made from tissue excised from the mass revealed a markedly cellular sample with a predominant population of homogeneous, discrete round cells with mild anisocytosis and anisokaryosis (Figure 2). These cells had a moderate amount of pale-blue cytoplasm, often containing a few small punctate vacuoles. The nuclei were round with coarse chromatin and a single prominent nucleolus. Moderate mitotic activity was observed (0 to 2/50X field) (Figure 3). Occasional neutrophils, lymphocytes, plasma cells, and squamous cells were noted in the background, as were variable numbers of erythrocytes. The cytologic diagnosis was transmissible venereal tumor.

Figure 2

TREATMENT

The dog was treated weekly with intravenous vincristine. A dramatic reduction in tumor size occurred after the first dose, with complete tumor remission after seven doses. The dog was tumor-free at its last recheck one month after the last vincristine injection.

Figure 3

DISCUSSION

Canine transmissible venereal tumor, also called Sticker's sarcoma, is a naturally occurring, horizontally transmitted round cell tumor found in domestic dogs and potentially other canids such as gray wolves and coyotes.1,2 Although there have been reports of this tumor in most parts of the world, a high incidence seems to exist in temperate climates. The tumor is seen most commonly in young, sexually active, intact dogs allowed to roam freely (including stray dogs).1-3 These tumors are usually spread during coitus or other social behaviors such as sniffing and licking.1,2 Thus, the typical locations of these tumors are the external genitalia and the nasal and oral cavities.1-3 Other less common locations include the anal mucosa and the skin and subcutaneous areas.3,4 A recent report of a multicentric, extragenital, cutaneous canine transmissible venereal tumor in a sexually immature 11-month-old virgin female mixed-breed dog suggests that transmissible venereal tumor cells can be inoculated into puppy skin lesions by the mother during social interactions such as grooming and other mothering behavior.4 Transmissible venereal tumor metastasis has been seen involving the lymph nodes, skin, eyes, liver, musculature, abdominal viscera, lungs, and brain.3-5

Etiology

The origin of this tumor has been extensively studied. Although a viral cause has been postulated but not verified, recent research confirms that the tumor is clonal in origin, and the development of this tumor requires transmitting the neoplastic cells (not cell-free filtrates or killed cells) from one dog to another.1,2 In addition, unique chromosomal gains and losses in tumors from distinct geographic regions suggest local lineages.1,2 Lastly, a diagnostic long interspersed nuclear element (LINE-1) inserted near the c-myc gene occurs in all tumors, even samples collected two decades apart.1,2 LINEs are long DNA sequences that represent reverse-transcribed RNA molecules (also called retrotransposons). These can be used to generate genetic fingerprints. The LINE-1 insertion appears to be a specific marker of canine transmissible venereal tumor resulting from either an insertion during the somatic evolution of the tumor or its presence in the germ line of the original host.2

The histogenesis of canine transmissible venereal tumor cells is inconclusive. Although the tumor is often regarded to be of histiocytic origin, more recent studies suggest an immature leukocyte (likely myeloid) origin.6

Cytologic characteristics

As with other round cell tumors, canine transmissible venereal tumor cells tend to readily exfoliate, and fine-needle aspiration usually yields high numbers of individualized round cells.5,7 These cells have abundant, lightly basophilic to sometimes deeply basophilic cytoplasm, often with small punctate vacuoles.5,7 The nuclei are round with coarse to clumped chromatin and one or two prominent nucleoli,5,7 and mitotic activity is often high.5,7

Figure 4A,B,C

Several of these features can be used to distinguish canine transmissible venereal tumors from other round cell tumors such as mast cell tumors, histiocytomas, plasmacytomas, lymphoma, and some melanomas. Canine transmissible venereal tumor cells typically have a lower nuclear to cytoplasmic ratio than lymphoma cells (Figure 4A). The characteristic sharply defined cytoplasmic vacuoles can be helpful in distinguishing this tumor from lymphoma (Figure 4A), plasmacytoma (Figure 4B), or histiocytoma (Figure 4C). Cells in a plasmacytoma or histiocytoma are also less likely to have the prominent nucleoli seen in canine transmissible venereal tumor cells (Figures 4B & 4C). Generally, cells from mast cell tumors and melanomas contain distinct cytoplasmic granules (Figures 5A & 5B). An exception may be observed with poorly granulated mast cells, which are round cells with a moderate amount of pale cytoplasm and central to slightly eccentric nuclei resulting in a typical fried egg appearance (Figure 6). Although these cells may lack the dense granulation of typical mast cells, close inspection will usually reveal some fine metachromatic granules, an important distinguishing characteristic.

Figure 5A&B

Cytologic examination can also help you determine the tumor's status. Canine transmissible venereal tumors are highly antigenic, and a dog's immunologic response plays an important role in inhibiting the growth and spread of the neoplasm.3 Similar to observations in histiocytomas, increased lymphocytes and plasma cells may be observed cytologically on impression smears and aspirates in regressing tumors, compared with progressing tumors.3

Figure 6

Therapy and prognosis

In animals that have an appropriate antitumor immunologic response, spontaneous tumor regression may occur after the tumor reaches a certain size. However, in animals that are unable to mount an appropriate immunologic response, the tumor may continue to grow and metastasize.3 Therapy is generally recommended when a canine transmissible venereal tumor is definitively diagnosed.

Several therapeutic modalities have been used with canine transmissible venereal tumors, including surgery, radiation therapy, biologic response modifiers (experimentally), and chemotherapy.3 The goal of biologic response modifiers, or biotherapy, is to alter the tumor-host relationship by acting on the host immune system or by directly affecting tumor cells or their microenvironment. Examples include tumor antigen vaccines, nonspecific immunomodulators, or growth factors.3 Surgery may be effective for small and localized cutaneous nodules, although adequate excision is usually not possible when external genitalia are involved, and the recurrence rate in these instances varies from 20% to 60%.3 Radiation therapy has also been effective.3

Chemotherapy is the most effective way to treat canine transmissible venereal tumor.3 Various chemotherapeutic agents such as vincristine, cyclophosphamide, doxorubicin, and methotrexate have been used, and vincristine is considered one of the most effective agents.3 It typically takes four to six once-weekly treatments (possibly more) to induce complete remission, and a complete cure can be expected in more than 90% of patients treated with this protocol.3

CONCLUSION

This article highlights the distinguishing cytologic characteristics of canine transmissible venereal tumors. History, signalment, and tumor location can also be important factors in diagnosing canine transmissible venereal tumors. Your index of suspicion should be high in intact, sexually active dogs that are allowed to roam, especially those living in or that have visited temperate climates. Clinical signs, signalment, and cytologic features are often sufficient for making this diagnosis. Biopsy and histologic examination are not necessary except in those cases with an atypical presentation.

This case report was provided by Rob Simoni, DVM, and Joyce S. Knoll, VMD, PhD, DACVP, Department of Biomedical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

REFERENCES

1. VonHoldt BM, Ostrander, EA. The singular history of a canine transmissible tumor. Cell 2006;126(3):445-447.

2. Murgia C, Pritchard JK, Kim SY, et al. Clonal origin and evolution of a transmissible cancer. Cell 2006;126(3);477-487.

3. MacEwen G. Transmissible venereal tumor. In: Small animal clinical oncology. 3rd ed. Philadelphia, Pa: WB Saunders Co, 2001;651-655.

4. Marcos R, Santos M, Marrinhas C, et al. Cutaneous transmissible venereal tumor without genital involvement in a prepubertal female dog. Vet Clin Pathol 2006;35(1):106-109.

5. Henson KL. Reproductive system. In: Atlas of canine and feline cytology. Philadelphia, Pa: WB Saunders Co, 2001;296-297.

6. Goldschmidt MH, Hendrick, MJ. Tumors of the skin and soft tissues. In: Tumors in domestic animals. 4th ed. Ames: Iowa State Press, 2002;115-117.

7. Zinkl JG. Cytology of the male reproductive tract. In: Diagnostic cytology and hematology of the dog and cat. St. Louis, Mo: Mosby Inc, 1999;236-237.