Human cancers aren’t contagious, but dogs and other animals aren’t so lucky

Whenever Máire Ní Leathlobhair visits a new city, she calls up vets in the area. She wants to know if they’ve seen any dogs harboring ugly genital tumors, contagious lumps that contain one of the more bizarre cancers in the world. Back in her lab, a map hangs on the wall, dotted with pins where her team has tracked down cases of the cancerous tumors, known as canine transmissible venereal tumors, or CTVT.

The cancer isn’t triggered by a virus, the way Human papillomavirus can prompt cervical cancer in people. Instead, CTVT spreads between dogs by the transfer of cancerous cells themselves. “It’s like if you were to get a graft of different tissue or an organ transplant. But in this case, it’s a tumor,” explains Ní Leathlobhair, a geneticist who is part of the Transmissible Cancer Group at the University of Cambridge.

Rather than rejecting the tumor as an invader, dogs’ immune systems will ignore the foreign cells. Sadly, they’re not the only animals whose immune systems offer aid to cancerous interlopers. Tasmanian devils also grow disfiguring tumors that cover their face, eventually killing the marsupials in a grotesque, painful death. And more than a dozen strains of cancerous cells drift in seawater, infecting several species of mussels and clams.

This is not how cancer usually works. Cancer stems from mutated cells continually making copies of themselves inside an individual. If you had cancer, those cells would contain your own DNA, and when you died, your cancer would (usually) perish with you. Under normal circumstances, you wouldn’t be able to transfer cancer to others, and unless researchers made a particular effort to preserve them, the cancerous cells would die too.

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