"Also, the majority of those mutations, including almost all of the recent ones, are neutral. They don’t seem to benefit the tumor in any way. Again, that’s very different from most cancers, which are constantly adapting to outfox their hosts’ immune systems, or to grow a little faster, or to spread to new areas. Not so with CTVT. “We think that the cancer is not adapting anymore,” says Adrian Baez-Ortega, who was one of the main researchers on the study. “Maybe it doesn’t really need to.”
Perhaps, early on in CTVT’s history, it found such thorough ways of escaping the immune system and jumping into new hosts that no further tweaks were necessary. Or perhaps it no longer imposes enough harm on its hosts to warrant a counterattack. It’s rarely fatal, after all, and if dogs are otherwise healthy, it doesn’t grow very fast. “The tumor and the dogs aren’t competing against each other anymore,” Baez-Ortega says. “They’re coexisting. The cells behave like cancer cells, but the ecology of the tumor is that of a parasite.” And a fairly benign one at that."