Dogs can contract cancer by doing one of their favorite things: sniffing each other’s crotches.
According to research done at the University of Cambridge, Canine Transmissible Venereal Tumors (CTVTs) spread between dogs by skin contact, with the cancer cells sticking to the new dog and “transplanting” themselves between the animals.
CTVTs usually only affects dogs’ genitals, spreading via mating, but because dogs also touch each other’s nether regions with their mouths and noses, these areas can also be affected.
A study published by the Cambridge University researchers in the journal Veterinary Record has shown that while genital cases of CTVT are equally seen between male and female dogs, males are between four and five times more likely than female dogs to be infected with the oronasal version of the cancer.
Using a database of almost 2,000 cases of CTVT, researchers found 32 CTVT tumors affecting a dog’s nose or mouth, of which 27 cases—82 percent—were in males.
According to the researchers, this is because male dogs spend more time sniffing and licking female dogs’ genitalia than the females do to them in return.
“Male dogs recognize estrous females by sniffing the genitalia, and male dogs sniff vaginal secretion odor more frequently than female dogs,” said the authors in the paper. “Furthermore, female genital CTVT tumors tend to be more exposed and accessible for licking or sniffing than those of males, which are usually enclosed within the prepuce. Thus, the external location of female genital CTVT tumors, coupled with a likely male preference for licking or sniffing female genitalia, may contribute to increased risk of oronasal CTVT in males. It is not known whether CTVT itself is attractive to dogs; we can only speculate that its odor may mimic estrus bleeding, which may attract males.”
Dogs may also contract this disease when they’re smelling each other’s anal glands, the unique smell signature of which allows the dog’s powerful nose to determine if they’ve previously met, and the other dog’s temperament.
CTVTs stem from a single dog who lived around 11,000 years ago, and have since spread across the world. They’re most commonly found in areas with many free-roaming dogs and strays. Other transmissible cancers can be found in hamsters, Tasmanian devils, and some species of clams. Tasmanian devils can contract a disease called devil facial tumor disease, which is transmitted between individuals via biting.
A figure of 32 cases of oronasal CTVT of a total 2,000 cases implying that transmission by sniffing or licking, while more common in male dogs than females, is a rare transmission pathway overall.
“The rarity of oronasal CTVT in the population, despite the likelihood that opportunities for licking and sniffing transmission behavior arise frequently, suggests that transmission of CTVT by sniffing or licking is an unlikely outcome. It is possible that sniffing and licking of CTVT tumors do not usually dislodge cancer cells or that, if dislodged, these cells are unlikely to establish tumors in the recipient’s oral or nasal cavities. . Moreover, oronasal CTVT would appear to be an evolutionary dead-end for this contagious cancer lineage, offering limited opportunities for further transmission,” the report concluded.