Can Your Dog Get Cancer Just by Sniffing?

Source: Benutzer Hundehalter/Wikimedia Commons CC BY-SA 3.0

Some common behaviors can get your dog into trouble. In this case, the problem is cancer, which can be passed from dog to dog simply by saying "Hello" in a canine manner.

Transmissible cancers are passed between individuals by the actual transfer of living cancer cells. Such diseases are very rare in nature, and only three transmissible cancers are known. One is a form that is found in marine mollusks, such as some species of clams and mussels. Another is found in the Tasmanian devil. Unfortunately, the third form is found in domestic dogs, and it is called Canine Transmissible Venereal Tumor (CTVT).

There is a clear behavioral component to CTVT since it is usually spread by the transfer of cells between dogs during mating. As its name implies, it can cause tumors on the external genitalia of both male and female dogs. However, this disease can also affect other areas of the body, commonly the nose and mouth, and in these cases, the route of transmission of the disease seems to be simply sniffing or licking—such as in the common greeting behaviors between dogs. Furthermore, some new data seems to suggest that male dogs are much more susceptible to these cancers of the nose and mouth.

An unusual disease

In many respects, CTVT's history is quite unique. The hypothesis that researchers have developed is that around 11,000 years ago, in southern Asia, a dog developed this form of cancer, perhaps starting with one single mutated cell which then spread through the dog's body. Instead of these cancerous cells disappearing when the dog died, they were passed on to other dogs through sexual activity or simply through sniffing. From there, the disease spread worldwide, concentrating in those countries and regions where dogs are mostly free-roaming. Unfortunately, with the importation of dogs from those regions, the disease is now beginning to show up in more Western nations where dogs are mostly pets.

Searching for cancer that enters through the nose

A team of researchers led by Andrea Strakova from the University of Cambridge's Department of Veterinary Medicine decided to look at the appearance of CTVT exclusively in the mouth and nose of dogs (since these are most likely to be the cases that are transmitted by simply sniffing). This investigation really involved two separate studies. In the first one, the cancer data was collected between 2009 and 2020 by more than 100 participating veterinarians, often at mass spay/neuter clinics, scattered across 59 countries. This resulted in the main databank of 1,916 dogs with verified CTVT cancer. Of these, close to 2 percent had the CTVT confined to the mouth and nose. But at this point, the researchers encountered a surprise. Of all of those dogs with CTVT in the mouth or nose, incredibly, 84 percent of them were male.

A sex-related revelation

In a press release, Strakova said, "We found that a very significant proportion of the nose and mouth tumors of canine transmissible cancer were in male dogs." This is a bit of an understatement since only 16 percent of the amassed CTVT cases involved females. She then added in explanation that "We think this is because male dogs may have a preference for sniffing or licking the female genitalia, compared to vice versa."

This is a sensible conclusion since many studies have shown that males seem to be much more attentive to scent stimuli than females. Presumably, the males are always cruising around, looking for a female to mate with, and the way that males check on whether a female is receptive is by sniffing the pheromones in her genital region. This would make males more likely to inhale one of the cancerous CTVT cells and thus develop the disease. However, the size of this male-female difference was so large that the researchers felt that it was necessary to run a second study to verify the results.

To do this, they conducted a literature review by searching for published reports of nasal or oral CTVT in scientific journals, using the PubMed and Google Scholar databases. They managed to find 65 cases that fit their criteria. The results confirmed the findings of their main study since the published reports indicated that again the majority of the cases (82 percent) were male.

That means that in the first study, the odds of a male dog getting CTVT were five times higher than that of a female, and in the second study using already published literature, the male susceptibility was a bit over four times higher than that observed in females. Although male dogs form the overwhelming majority of CTVT sufferers, females are not immune. The authors write: "It follows that, presumably, females also engage in transmission behavior, including sniffing and licking genitalia of males or other females, but perhaps less frequently than males."

Don't panic

When viewing results like these, some dog owners are apt to overreact and immediately try to stop their dogs when they attempt to engage in normal canine greeting behavior when encountering other dogs. After all, such greetings often include mutual sniffing of each other's genital regions. However, in the Western world, CTVT is still relatively uncommon, and the good news is that, if correctly diagnosed, treatment is very effective. Chemotherapy with Vincristine usually results in the majority of infected dogs recovering.

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