Do dogs get sexually transmitted cancers?

The gorgeous Rodney

Article by Colette Kase, originally published in Dogs Today March 2020
Uploaded by Editor, Beverley Cuddy

The first time I encountered sexually transmitted cancer in dogs was shortly after I moved from the UK to Belize, a small Caribbean nation in Central America. I started working on some humane education projects and struck up a friendship with the owner of a lovely little fuzzy dog named Melly, whom she had rescued from the streets.

Melly, typical of abandoned and neglected dogs, was in quite a mess and that was no surprise to me, but what I learned about the type of cancer she had – and had been cured of – both fascinated and disturbed me. I had no idea there was any such thing as a ‘contagious’ cancer. Transmissible cancers are, it seems, very rare among cancers. They are only known to occur in dogs, Tasmanian devils and a type of clam.

The longer I lived in Belize and volunteered with various animal welfare groups, the more often I encountered canine transmissible venereal tumours (CTVT). Belize is one of the countries with the highest incidence of this disease. The more often I saw dogs, both male and female, with CTVT, the more I understood why the first vet I ever spoke to about this disease in Belize had told me that it ‘turns dogs’ genitals inside out’. There’s no way to avoid the fact that CTVT is horrible.


CTVT is most commonly passed between dogs during mating, which means that the cancerous growth often presents itself on the genitals. As the tumours grow, they become easy to see and very pronounced externally. The tumours often bleed, but many of the dogs are otherwise relatively healthy. There are theories that the potential anaemia that one would expect from the constant bleeding is counteracted by the behaviour of the tumour itself. After all, it must keep its host dog alive for its own survival.

The good news about CTVT is that it responds very well to chemotherapy with a relatively inexpensive drug called vincristine and, in most cases, goes into complete remission following treatment. Of course, the only dogs that are lucky enough to receive treatment are those that someone will take responsibility for and spend the time and money necessary to see it through. While welfare organisations do their best, many dogs are either in too advanced a stage or there simply are no funds, and so euthanasia is the result.

I moved to Mexico not long ago and had forgotten about CTVT. Then I adopted Rodney. Rodney came from the streets of Mexico City (about 1,000 miles from my home), which is by no means tropical and lies at an altitude of over 7,000 feet. There is a commonly held belief that CTVT is a disease that only exists in the tropics. I now know it is not.

The gorgeous Rodney

When I picked up Rodney at the airport, I noticed some discharge from his penis and assumed it was simply a urinary tract infection caused by the stress of the flight. He had been health-checked in Mexico City by a vet and came with a clean bill of health. The following morning, I took him to my local vet. As she pulled back the sheath of his penis to examine him, we both looked at each other as we recognised what we both knew to be a CTVT tumour.

Those at the rescue were very concerned and responsible in their response. They immediately offered to take Rodney back or, if I wanted to keep him, to help to pay for his chemo and to return my adoption donation. I know from experience how much small rescues struggle financially and I told them I would be keeping Rodney and paying for his treatment myself. It ended up being not so simple.


My partner and I were living in a rented flat with a 17-year-old dog with a very small garden. We were in the process of building a house (read: palace for our dogs), but until then, it was virtually impossible to separate the two dogs. You see, even though CTVT is primarily passed through sex and my elderly dog, Midas, was spayed, it can be passed in other ways. That’s because transmission isn’t really about the sex itself, but about the infectious cells entering the dog’s body through a damaged mucous membrane, which may happen during the act of mating. It’s believed that the prolonged tie that occurs after dogs have sex assists this process, but tumours can appear on any part of a dog’s body. Other than genitals, the nose, face and mouth are most common, probably caused by sniffing and licking.

We could not risk this with our elderly dog and so poor Rodney, after having been in rescue for months, then flown to a new place, was now going to spend the next four to eight weeks boarding at the veterinary clinic. We visited him twice daily to walk him, so we were able to at least get to know him and start training and socialising him.

For most dogs, three weeks of treatment is sufficient, but Rodney’s case was proving resistant. We really hoped he wouldn’t need more intensive chemotherapy, as he was coping so well with the once-a-week oral vincristine. Because the tumour was still reducing in size in response to the drug, the vet was confident that we should carry on. Another four weeks and he was cleared as cancer free and able to come home. We were thrilled and he’s turned into a healthy, happy and wonderful companion. We’re hoping he’ll eventually be a therapy dog.

Rodney’s case renewed my fascination with CTVT and so I started digging. One of my big worries was that other people, like myself, may be adopting infected dogs without realising it. If my own dog was cleared as healthy by a veterinary surgeon, I figured that this could be happening to new adopters in the UK. It turns out that it has, indeed, happened.


Dr Andrea Strakova is a postdoctoral researcher at the University of Cambridge’s Department of Veterinary Medicine, working on CTVT together with her colleagues from the Transmissible Cancer Group. I told her about Rodney and asked her if cases of CTVT were being seen in the UK.

She replied, “We have certainly come across a number of cases of CTVT in the UK – the majority of these dogs being adopted from Romania, and like you said, these dogs were cleared by vets as healthy before import.” As CTVT is not common in the UK, it’s very unlikely that vets will be looking for the disease in otherwise healthy patients. It’s certainly not something that appears to be covered widely in veterinary schools.

I was lucky enough to find out about the important research being done by Andrea Strakova through my contacts with Clara Lee Arnold, an extraordinary animal welfare advocate and the director of Hopkins Humane Society, a small animal welfare organisation that offers neutering clinics in Belize. She had liaised with the Cambridge researchers when they visited Belize to learn more about the disease and collect local samples.

The more I was discovering about CTVT, the more interested I was becoming. Not only are studies of CTVT giving us information about the disease, but it’s informing us about the history of dogs and the people who they travelled with thousands of years ago to make their way around the world.

Transmissible cancers are, it seems, very rare among cancers. They are only known to occur in dogs, Tasmanian devils and a type of clam

Some of this research is coming up with curious results. For example, it has led to the discovery that breeds like the Xoloitzcuintli and the Catahoula Leopard Dog, both thought to be direct descendants of indigenous dogs of the Americas, may not come from that ancient lineage at all. The DNA history of dogs held within the genomes of CTVT cells shows that the indigenous dogs of the Americas all died out, for the most part, and there are only tiny traces of their genes remaining in modern dogs.

All of this and so much more is being learned from studying CTVT because the genomes of this disease carry the mitochondrial DNA of some of the dogs that it has infected since it originated in one single dog between 4,000-8,500 years ago. This cancer carries the story of the relationship of humans and dogs in its very cells and the work of those studying the historical DNA of dogs carried in CTVT cells, such as Dr Strakova and Professor Elizabeth Murchison, who is leading the Transmissible Cancer Group, are continuing to unravel this compelling tale. Possibly, the very success of this disease over time is because of its ability to pick up the DNA of some of its hosts along the way.


It all started with that one dog, somewhere in Asia. The mitochondrial DNA from that dog, carried by CTVT, enabled researchers to follow how this dog’s DNA moved around the world, carried by infected dogs, either by crossing land themselves or by being brought by humans over land and by sea. The Cambridge team has identified five different dogs who donated their DNA to CTVT and this has enabled them to trace its journey.

There are no records of CTVT naturally appearing in wild canids. Dr Strakova said, “This could be due to the lack of opportunity for physical transmission of CTVT cells or it is possible that the immune systems of other wild canids differ significantly from the dog so CTVT cells are destroyed before they develop into a tumour. But the fact is that we do not know.”

Very few people in the UK have heard of CTVT, but with it now appearing in dogs imported from abroad, it is important to be aware that this is a common disease throughout the world, which often doesn’t present with obvious symptoms until it is quite advanced.

This doesn’t mean people need to panic about an epidemic of CTVT among dogs in Britain. Quite clearly, that is unlikely due to good, modern, dog management and control. But those rescuing dogs from countries where CTVT is endemic should be aware of the symptoms, so that they can start lifesaving treatment early. But, even more than that, this emphasises the need of those who care about animal health and welfare worldwide to prioritise the importance

“I had her checked out by a vet physio and injury was ruled out. My vet is an eye specialist, so she had the full test for PRA, which was positive.”

So, whether you always make sure not to step on the cracks in the pavement, or your dog does a little tippy tappy dance every time he’s fed, it seems superstitious behaviour is just another thing that we have in common with our beloved companion animals.

Remember that these behaviours may be based on fear or anxiety or, as in Bella’s case, may be the symptom of something more sinister. So, if you have any concerns about sudden changes in your dog’s behaviour, make sure to talk to your vet and, if necessary, get a referral to a qualified behaviourist.


  • CTVT is endemic in about 90 countries and it is most prevalent in countries that have large populations of free-roaming dogs and little effective management in terms of dog control laws and sterilisation programmes.
  • CTVT is older and more widespread than any other type of cancer.
  • CTVT existed in the UK and the first ever records of its existence come from London in 1810. It seemed to decline in the UK in the late 19th to 20th centuries, as a result of animal control laws.


Article by Colette Kase, originally published in Dogs Today March 2020
Uploaded by Editor, Beverley Cuddy


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