TVT, or transmissible venereal tumor, is a contagious cancer in dogs that spreads through direct physical contact, primarily during mating. Unlike nearly every other cancer in nature, the tumor cell itself is the infectious agent. When an affected dog comes into contact with a healthy one, living cancer cells transfer from one animal to the other, take root, and begin growing. It is one of only a handful of cancers known to science that can spread this way, and the cell line responsible has been continuously propagating for roughly 6,000 years.
How TVT Spreads Between Dogs
The most common route of transmission is sexual contact. During mating, tumor cells from an infected dog’s genitals physically implant onto the mucous membranes of the other dog. This makes intact (not spayed or neutered), free-roaming dogs the highest-risk group by a wide margin.
TVT can also spread through non-sexual contact. Dogs that lick, bite, or sniff tumor-affected areas on another dog can pick up cancer cells on their own mucous membranes. This is why tumors sometimes appear in the mouth, nose, or around the anus rather than the genitals. Owned, supervised dogs that don’t roam freely are more likely to develop these non-genital forms when they do get TVT, because their exposure tends to come from social sniffing and licking rather than mating.
TVT only spreads between canids. There is no documented evidence that it can infect humans, cats, or other species.
What TVT Looks Like
The tumors most often appear on the external genitalia. In male dogs, you may notice a fleshy, red, cauliflower-like mass on the penis or prepuce. In females, the tumor typically grows on the vaginal mucosa or vulva. The masses are fragile and bleed easily, so one of the first signs many owners notice is bloody discharge from the genital area. Dogs may also lick the area excessively.
When TVT develops in non-genital locations, the signs depend on where the tumor grows. Nasal tumors can cause sneezing, nasal discharge, or difficulty breathing through one nostril. Oral tumors may cause drooling, difficulty eating, or visible masses along the gums or lips. Skin tumors appear as raised, reddish nodules. In rare cases, TVT can metastasize to internal organs, though this is uncommon.
Diagnosis
Veterinarians can often suspect TVT based on the tumor’s appearance and location, especially in regions where the disease is common. To confirm, a vet typically takes a small sample of cells from the mass using a needle or by pressing a glass slide against the tumor’s surface. Under a microscope, TVT cells have a distinctive round shape that experienced vets and pathologists can identify relatively quickly. This makes diagnosis straightforward in most cases, without the need for invasive surgical biopsies.
Treatment and Success Rates
Chemotherapy is the standard treatment for TVT, and it works remarkably well. The drug used is a chemotherapy agent given intravenously once a week, typically for four to six sessions. A systematic review and meta-analysis found that this protocol produces a complete response rate of 93.1%, meaning the tumor disappears entirely. Extending treatment to eight weekly sessions pushes that rate to 98.9%.
Most dogs tolerate treatment well. The tumor visibly shrinks with each session, and vets continue weekly treatments until no visible mass remains. In one reported case series, a treated female dog went on to mate, carry a pregnancy, and deliver healthy puppies with no complications.
Surgery alone is less reliable. While tumors can be removed surgically, recurrence after surgery is a known problem. In clinical reports, dogs treated surgically have sometimes needed to return for additional treatment when the tumor grew back. For this reason, chemotherapy has become the preferred first-line approach. Radiation therapy and alternative chemotherapy drugs exist as second-line options for the rare cases that don’t respond to the standard protocol, though these are seldom needed given the high success rate.
Prognosis After Treatment
The outlook for dogs treated with chemotherapy is excellent. Once complete regression is achieved, recurrence is rare. Dogs that finish their full course of treatment typically remain cancer-free and return to normal health. TVT is considered one of the most treatable cancers in veterinary medicine, with a cure rate that far exceeds most other malignancies in dogs.
Where TVT Is Most Common
TVT exists worldwide, but its prevalence varies dramatically based on how well dog populations are managed. The disease is most common in tropical and subtropical regions where large numbers of free-roaming, intact dogs live and breed without oversight. In countries with strong stray-dog control programs and high spay/neuter rates, TVT has become rare or has disappeared entirely.
The United Kingdom offers a striking example. TVT was once a common canine disease there, but a series of dog management laws starting with the Dogs Act of 1871, which required dogs to be “under proper control” and allowed stray dogs to be detained, appears to have unintentionally eradicated the disease over the following decades. On Koh Phangan Island in Thailand, a sterilization campaign launched in 2001 nearly eliminated TVT from the local dog population.
Prevention
Spaying or neutering your dog is the single most effective way to reduce TVT risk. It eliminates the drive to roam and mate, which cuts off the primary transmission route. Keeping your dog supervised and preventing unsupervised contact with stray or unknown dogs adds another layer of protection. In areas where TVT is prevalent, avoiding dog parks or communal areas with large populations of unmanaged dogs is a practical precaution.
At a population level, the pattern is consistent across countries and regions: where more dogs are spayed or neutered and fewer dogs roam freely, TVT prevalence drops. Free-roaming intact dogs serve as the reservoir that keeps the disease circulating in a community.