On One Hand

November 15, 2009

Cancer is contagious — wha!?

Filed under: science — ononehand @ 3:14 am

The first time any of us witnessed a cancer patient – a bald-headed peer in our elementary school, an aunt or grandmother with an appointment for a mastectomy, or a grandpa who got a dark, funny-looking mole removed – a reassuring adult explained that cancer isn’t contagious like other sicknesses. Cancer is our body’s own growth turned against us, sparked by a precise series of genetic mutations that debilitate a cell’s self-regulation but fail to kill it. It escapes immune detection because it is our body’s own cell; a cancer cell transfered to a new body would be recognized as foreign and pulverized by vigilant white blood cells.

Ther’s also the problem of getting an intact cancer cell into another person’s healthy tissue in the first place. Viruses spread because they are are extremely small packages of genetic material that can float through the air or wait on dry surfaces, hardy because they were never really “living” from the start. Bacteria are contagious because they can grow outside the body, can wait on skin, thrive in saliva or feces, and can often survive drying and re hydration by turning themselves into hardy spores. But a cancer cell is, first and foremost, a dependent tissue cell from a multicellular organism, which needs to be inside an organism to survive. It would have to be carefully detached alive, kept moist and immediately implanted directly into the deep tissue of another organism to recover and begin growing. When cancer spreads through a single person’s body in a malignancy, it does so by traveling through her or his own nourishing blood or lymphatic channels, never emerging outsisde the body.

There are a few kinds of cancers caused by a virus, including cervical cancers, genital cancers (usually caused by the Human papillomavirus) and Kaposi’s Sarcoma, found in people with advanced HIV infections and the elderly. Clusters of cancer cases have sprung up in rare anomalies where a virus is thought to have been the cause. Many common viruses lead to increased cancer risks – for example mononucleosis, caused by the Epstein-Barr virus which virtually everyone gets, is linked to an increased risk of developing leukemia. But in all these cases it is the virus that is contagious, not the cancer itself.

But there are two kinds of cancer that are actually contagious in and of themselves, spreading as living cancer cells from one organism to another. One is responsible for landing the Tasmanian devil on the endangered species list and threatens to wipe out that species.

Tasmanian facial tumor disease, image from the Public Library of Science

Tasmanian devils have the unfortunate habit of biting each other on the face while feeding and mating, often drawing blood. That allows living cancer cells of devil facial tumor disease to be implanted directly into the facial tissue of another Tasmanian devil. The tumors are fast-growing and kill the animal by overwhelming its face and preventing it from eating, leading to starvation. The disease has already affected between 20 and 50 percent of the Tasmanian devil population and has mutated into several different strains. But upon a genetic analysis of one of those tumor cells, you would find neither a fungus nor bacteria, but rapidly-growing Tasmanian devil cells from another individual.

The other kind of contagious cancer is the Canine transmissible venereal tumor, affecting dogs, foxes and coyotes. It spreads by sexual intercourse and affects the genitals and occasionally the face. It is estimated to have originated from a couple hundred to a couple thousand years ago, meaning that the original host’s own cells long outlived it; its cells continue to survive, as a pathogen, in other canines to this day.

Transmissible cancers break the rules of cancer by spreading from individual to individual, but are also unique among contagious diseases because they originate in an animal among its own cells. It would be as if a bump on your skin, part of your body, grew into a disease that started planting itself in other people and killing them. Anyone who got infected with your cancer would contain all of your your mutated DNA in their tumors – they’d essentially be dying of you, as a parasitic disease.


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