CTVT – A contagious cancer that spreads from dog to dog

Our boy has had his chemo and seems fully recovered: swelling is gone, bleeding and reverse-sneezing have stopped, appetite is back to normal, all signs of lethargy and malaise are gone. Hopefully he will remain tumour-free.

The "lie" (which was not a lie at all) that you were told actually meant that you can't get cancer from being around someone with cancer. That's still true.

The rare human cancers that are transmissible like cervical cancer get covered in sex-ed, and interventions like condoms are effective.

The reason most cancers aren't transferrable, is the same reason that organ transplants are so hard. Most individuals in most species have unique antigen profiles. The immune system attacks any foreign bodies without matching antigens. An internally generated tumor works because it's cloned from an individual's own cells.

But a foreign tumor, is very unlikely to have a matching antigen profile. Therefore the immune system will attack and destroy with extreme prejudice. However a highly inbred population may lack the sufficient genetic diversity to have individually unique antigen profiles. In the extreme case if we were all clones we could pass tumors back and forth with ease.

The short answer is that it would be virtually impossible to see any widespread cases of clonally transmissible cancers in non-immunocompromised humans.

> Many carcinogens mutate DNA in distinctive ways: Sunlight, for example, creates a very different pattern of mutations than cigarette smoke. Murchison’s team found that CTVT contains a lot of those sunlight signatures, especially at lower latitudes, where the sun is stronger. For example, CTVT in Mauritius has more sunlight-induced mutations than the same tumor in Russia.

Epigenetics can cause different parts of DNA to be more or less exposed in different parts of the body. There are also indirect effects from chemicals- some carcinogens can cause changes in proteins, or be bound to proteins, which can cause them to be around certain regions of DNA more.

Mostly though you see that kind of thing more in viral/bacterial cancers- a sizeable percentage of all tumors are caused this way.

> “CTVT is like its own organism,” Murchison says. “It isn’t really a dog. Is it a cancer, still?”

> Its future is uncertain, though. While failing to acquire beneficial mutations, CTVT is also failing to weed out harmful ones. That’s understandable, because much of its DNA comprises genes for building a dog

> To clarify, several cancers can be caused by infectious entities; cervical cancer, for example, is the work of HPV, a sexually transmitted virus.

Oddly, the main panic we see around HPV is anti-vaxxers going after the Gardasil vaccine.

Per Wiki:

> There are no reported human cases of clonally transmissible cancers that can independently survive and be transferred in the manner of CTVT and DFTD. However, instances of one human passing cancer cells to another through physical contact have been reported. These cases typically involve organ or hematopoietic stem cell transplants or cancers transferred from a pregnant mother to her fetus.[66] The difference of these incidences from a true clonally transmissible cancer, however, is that the cases of human transfer were one-off events — the actual tumor did not have properties allowing it to be continually transferred throughout the population, and therefore the tumor clone went extinct with the death of the host individuals.