Cancer is one of the most dreaded diseases of the modern world. In 2012, an estimated 14.1 million new cancer cases were diagnosed worldwide, and 8.2 million people died from the disease in the same year. Worse still, 21.7 million new cancer cases are predicted to appear annually by the year 2030, accompanied by 13 million deaths due to this disease. As of yet, even though a few infectious agents, like the Human Papillomavirus (HPV), are held responsible for certain types of cancer, the human cancer cells themselves are not known to be contagious in nature, and ultimately die with the death of the human host. Imagine what would happen if the cancer cells would not only metastasize from organ to organ within a single host, but also travel from one person to another? It would wipe out entire human populations in a single strike. Presently, there is no substantial evidence to prove that cancer is or can be contagious, or that it spreads through any form of contact between individuals, including contact with body fluids during sex, kissing, and sharing of meals. Even when a cancer cell from a patient enters a healthy individual’s body, the immune system of the individual immediately recognizes the cell as ‘foreign’ and spontaneously acts to eliminate it. Hence, while we can rest assured of the fact that there is no imminent threat of ‘contagious cancers’ affecting the human population, scientific research does not completely eliminate the future medical possibility of such cases, due to several recent findings in animals pointing in that direction.
Case Studies in Humans
Some dispersed pieces of evidence exist suggesting a contagious nature of cancer, including rare cases in the human world and some quite noticeable ones in the animal world. A letter published in the New England Journal of Medicine in 1986 reported the case of a 19 year-old laboratory worker who developed a tumor nodule in her hand after accidentally pricking her left hand with a syringe of colon cancer cells while injecting them into laboratory mice. The patient had no clinical history to suggest immune deficiency, which could have helped explain this unexpected result. Another case study indicating a similar situation was reported by the New England Journal of Medicine in 1996. While operating on a 32 year-old man with a malignant fibrous histiocytoma, a 53 year-old doctor accidentally injured the palm of his left hand while placing a drain. The patient died due to postoperative complications and, five months later, the doctor developed a tumor-like lesion on his left palm. Histologic and immunohistologic analyses, as well as genetic studies of the tumor cells from both the patient and the doctor, were compared and found to be identical. The scientists concluded that somehow the doctor’s immune system evaded the invading cancer cells, allowing the tumor to grow. Another case of cancer transmission was reported in Japan, where a 28 year-old Japanese woman died from leukemia a few months after she gave birth to a baby girl. In what came as a shock to doctors and scientists, at the age of 11 months, the baby too developed leukemia. Studies revealed that the baby had been infected in utero, and that the cancer cells evaded the immune response in the baby by being 'immunologically invisible'. The study was published in the Proceedings of the National Academy of Sciences on October 12th, 2007.
Contagious Cancer in Animals
Though in the above three case studies the spread of cancer was limited to transmission of cancer between two individuals and no further spread was involved, cancers of a more vicious nature have been detected in a number of animal species. The earliest known example of such disease is the Canine Transmissible Venereal Tumor Disease, which is transmitted between dogs during sexual intercourse. An extensive review of research on this cancer can be found in the review paper, “The Cancer Which Survived,” published in the journal Current Opinion in Genetics and Development in February of 2015, and authored by Andrea Strakova and Elizabeth P. Murchison of the University of Cambridge. The cancer probably came to existence 11,000 years ago as a single cell in a dog, and spread through the years via copulation. Some evidence also points to the fact that this cancer, like some viral diseases, might have evolved to modify canine sexual behavior to promote the spread of the disease in the population. Another, more dramatic case of contagious cancer is found in the Tasmanian Devils, mammals endemic to Tasmania in Australia.
Currently, the entire species is facing extinction due to the rampant death of its members affected by this deadly cancer. The Devil Facial Tumor Disease (DFTD), as the cancer is named, was discovered in 1996. The disease is transmitted when the animals bite each other during feeding or mating, with malignant tumors developing in the face making it impossible for the animal to continue eating, killing it by starvation. When geneticist Elizabeth Murchison of Cambridge University studied the cancer cells in 2012, she found them to belong to a single line of cancer cells, derived from a single female host. However, in 2014, Murchison’s colleague, Ruth Pye, discovered something far worse. A tumor, physically representing DFTD but genetically different, was found to infect a number of Tasmanian devils as well. Now, though the presence of one transmissible tumor could be shrugged off as a fluke of nature, the existence of a second one points to a more serious nature of contagious cancers. One possible explanation could be that the Tasmanian devils, due to a huge population crash, have lost enough genetic diversity to check their immune cells from recognizing invading cancer cells as ‘foreign’, allowing the cancer to flourish. Another case of contagious cancer in the animal kingdom has been found in Soft-shell clam populations along the East Coast of North America, where outbreaks of leukemia among the clams have devastated some regional populations.
Cancer has probably existed on our planet since the time of the development of multicellular organisms on Earth, as per the research study published in Nature Communications in 2014 by Croatian researcher Domazet-Lošo and German researcher Alexander Klimovich. However, the life spans of the longest-living organisms is perhaps not long enough to allow the cancer cells to evolve sufficiently to become transmissible like an infectious agent. Still, the discovery of scattered cases of cancer transmissions between two human individuals, and the existence of cases of contagious cancer in certain animal species, cannot completely negate the future possibilities of a cases of contagious cancers in the human population. Scientists thus believe that sufficient research must be focused on this issue to establish proper warning systems that will protect humans from the grasps of these prospective death-dealing diseases.