Cancer is a horrible disease, but thankfully the majority are not contagious meaning other people cannot catch the cancer from a person. However, did you know that there is a cancer in dogs that not only is contagious, but sexually transmitted? Today we are going to discuss canine transmissible venereal tumours, and how vets manage this unusual cancer.
Transmission and Prevalence
Canine transmissible venereal tumours (TVT) is a malignant cancer (a harmful cancer that spreads) found only in dogs. The cancer is most known for being spread via sexual reproduction – when dogs mate, cancer cells from the tumour (often located on the genitals) from the affected dog are rubbed off and infect the non-affected dog. However, the cells can also be spread via other close-contact between dogs, including licking, scratching, sniffing and biting. It can also be spread from mother to pups.
It can be found worldwide, but is more prevalent in Central and South America, Asia, and Africa. It is less common in Northern Europe, the USA and Australia, possibly due to widespread neutering of dogs and fewer strays. Sporadic cases are often seen in these countries from dogs imported from abroad. The disease is considered absent from the UK, aside from these isolated imported cases.
Tumour Behaviour
The main symptom of TVT is the formation of nodular (lumps), pedunculated (on a stalk) or multi-lobed (multiple sections) growths. The tumours are often delicate and easily bleed. The majority are found on the external genitals of dogs (due to the main transmission method) but can also be located within the vagina or nasal cavity if they colonise these regions. The tumours can often appear small and benign initially, but can grow large over four to six months, particularly in young, elderly or immunosuppressed dogs. They rarely spread (around 5-15%) to other parts of the body; when they do, it usually is to the nearby lymph nodes, the kidneys, spleen, eye, brain and skin.
Clinical Signs
The main clinical signs are associated with the tumour location. Tumours on the genitals can lead to pain, bleeding (often reported as spotting in female dogs with subtle or internal tumours), excessive licking of the area, and discharge from the urinary tract. Male dogs often have their glans permanently extruded from the prepuce. Skin infections are common. Dogs with genital tumours can be at a greater risk of urinary tract infections. Rarely, large tumours may block the urethra, leading to bladder or kidney damage. Nasal tumours can result in sneezing, watery eyes, nosebleeds, eyeball bulging, bad breath, dental disease and facial deformities.
More general symptoms reported include general weakness, anorexia, constipation and weight loss. It has been reported that symptoms in male dogs are either less severe, or less recognised by owners – many may be considered asymptomatic.
There are rarely changes in blood parameters, with a mild increased in white blood cells and anaemia the most reported.
Diagnosis and Differential Diagnoses
When the tumours are large and located on the genitals, a vet may be able to make a presumptive diagnosis based purely on appearance. However, definitive diagnosis usually requires either cytology (aspiration of cells via a needle or pressing the tumour onto a slide) or histopathology (removal of tissue), both of which are looked at under a microscope for specific features of TVT cells. TVT cells can appear similar to other round cell tumours, like lymphosarcomas. Advanced genetic testing can also confirm a TVT – fun fact, TVTs are a cell line that seem to have originated in a single dog as much as 11,000 years ago!
Once a TVT is diagnosed, vets may wish to perform further tests to determine if it has spread, such as ultrasonography of the abdomen, and x-ray or CT of the chest. Blood and urine tests may identify any associated diseases, like a UTI.
Other diseases that may appear similar to TVTs include cancers like lymphosarcomas, mast cell tumours or nasal tumours. Other differentials for smaller lesions include dermatitis, other skin cancers, wounds, parasitic infections or bacterial skin infections.
Treatment
TVT can be treated in three main ways, sometimes in combination; surgery, chemotherapy, or radiotherapy.
Surgical therapy
This involves the removal of some or all of the tumour under anaesthetic. Depending on the size of the tumours, these surgeries can be challenging, and may involve partial or complete removal of parts of the genitals.
Chemotherapy
Chemotherapeutic drugs are often effective at shrinking or even completely eliminating the disease. The most common drug is vincristine, given intravenously once a week until the tumour has been eliminated. Resolution of the cancer can take up to 6 weeks of treatment. Other drugs used include doxorubricin and L-asparaginase for those unresponsive to vincristine. These drugs are powerful, and can have side effects such as anorexia, weight loss or white blood cell reduction. However, the very aggressive approach used sometimes in humans is almost never performed in dogs, and if significant side effects occur, the protocol will be altered in order to maintain quality of life.
Radiation therapy
This high-tech approach involves blasting the tumour with radiation to kill the cancer cells. This can be used alone but is often used after chemotherapy for tumours that are unresponsive. This can be done in multiple smaller session, or one session with a larger dose of radiation.
Outcomes
In some cases, TVT may spontaneously resolve, likely due to the dog’s immune system destroying the cancer. However, this tends to occur early in the disease, and is unlikely to happen in dogs that have had TVT for a year or more. Some vets may also recommend ‘benign neglect’ (doing nothin to treat the TVT) if there are no symptoms and the dog is well. This risks the tumour causing local or systemic disease, but in some cases a dog can have a normal quality of life. Care must be taken to ensure the dog does not infect other dogs. The same outcome can apply to dogs that have treatment without a complete resolution.
If caught early and given appropriate treatment, the prognosis for TVT is generally good, with many dogs having a complete resolution. However, those that have spread to other parts of the body, especially the brain or eyes, have a worse prognosis. Dogs can become reinfected after resolution.
Prevention
The risk of a dog catching TVT in the UK is very low. Nevertheless, you should prevent unknown dogs from interacting with your dog if possible, particularly any sexual interaction. Those who breed dogs should ensure that all dogs are free of disease before breeding them, especially those from abroad. Neutering your dog will reduce (but not eliminate) the risk of sexual interaction with other dogs.
The majority of recent cases of TVT in the UK have been in imported dogs. We would encourage anyone importing a dog from abroad to ensure they have had a complete health check before travel, including an examination of the genitals. Once in the UK, they should be isolated from other dogs until a subsequent health check can be made. Any non-neutered dog should be neutered. In general, we would discourage adopting a dog from abroad to prevent entry of dogs with diseases like TVT and others.
Further Reading
Canine Transmissible Venereal Tumour – MSD Vet Manual
Canine TVT–Clinical Findings, Diagnosis and Treatment – VIN
TVT In Dogs – Pet Cure Oncology
Transmission of canine transmissible venereal tumour between two dogs in the UK – JSAP
Canine transmissible venereal tumour (CTVT) – University of Cambridge
Transmissible Dog Cancer Genome Reveals the Origin and History of an Ancient Cell Lineage
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