Student guest post: Cancer isn’t contagious…or is it??

Student guest post by McKenzie Steger

Off the southeastern coast of Australia lies a small island that in the 1700 and 1800’s was inhabited by the very worst of Europe’s criminals and is now the only natural home in the world to a species named after the devil himself. Decades later beginning in 1996 Tasmanian devils were going about their nocturnal lifestyle in normal devilish fashion feasting on small mammals and birds, finding mates and reproducing, occasionally fighting with one another and so on. (1) Just as criminals divvied up their booty hundreds of years before, the devils were sharing something of their own—only something of much less value. It turns out they were transmitting to one another a rare and contagious form of cancer known as Devil Facial Tumor Disease or DFTD. Once infected, facial tumors developed and the devil faced 100% mortality most often due to inability to eat or airway obstruction. Over the last 17 years the result of this highly contagious and fatal cancer has been the elimination of over half of the devil population throughout Tasmania. (2)

Source: http://www.discoverworld.ru/park-tasmanijskogo-dyavola-17528/

DFTD is not alone when it comes to transmissible forms of cancer. For over six thousand years dogs, jackals, wolves, and coyotes across the globe have experienced their own “contagious” cancer in the form of canine transmissible venereal tumor—C TVT and also called Sticker’s sarcoma. (2) CTVT is generally considered the first known cell line to be malignant having been described in the mid 1800’s. These unique growths like DFTD can spread from one individual to the next, but in the case of CTVT this most commonly occurs during coitus, licking, and biting infected areas. CTVT lesions usually establish in the genitals or in close proximity as a result. CTVT is unique in that only an estimated 7% of cases metastasize unlike in DFTD cases where 65% of them result in metastasis. CTVT rarely results in severe clinical illness but instead nearly always regresses on its own. (3)

So what is it that makes DFTD and CTVT so “contagious”? Essentially it boils down to host immunity. In the case of DFTD, devils pass on tumor cells when they are in close physical contact with others during mating or fighting. The Tasmanian devil population simply lacks the genetic diversity to be able to immunologically recognize and ward off the tumor and thus, these highly virulent and metastatic cells set up camp in the new host tissue and invade in no time. Interestingly, studies have shown that the DFTD cells are unique, containing only 13 pairs of chromosomes instead of 14 like most cells. Technology has also shown the very same cell line that began the DFTD devastation—thought to be of Schwann cell origin—is the very same one being transmitted throughout devil populations today. (2)

In contrast, CTVT, a histiocytic tumor (4), affects mammals rather than marsupials which have much greater diversity within the population and a more advanced capability to detect foreign and potentially invasive cells. This is due to the MHC-1 molecules or multiple histocompatibility complexes that help the body’s immune system to recognize foreign substances. CTVT is so effective in transmission because it down regulates these MHC-1 molecules effectively “hiding” the invasive cells from the body’s immune system. At some point however, this mechanism is overcome and the CTVT is recognized and killed by the body in animals that are immunologically sound. (2)

What about transmissible cancer in humans? The good news is that no comparable strain of such a killer contagious cancer has been recognized in humans compared to what devils in the “land down under” are experiencing. The bad news is that there are technically forms of cancer affecting man that result from contagious agents. Estimations attribute 15% of tumors world-wide to contagious pathogens including mainly viruses but also bacteria and parasites as well. Most documentation of cancer transmission cases in humans are reported in individual case reports, however, highlighting the rarity and unlikelihood of this occurrence. (2) Nonetheless, it still occurs. Hepatitis B and C viruses, herpes viruses, human immunodeficiency virus (HIV), and papilloma viruses are just a few examples of viruses that can develop into cancer in patients or predispose them to tumor formation. Bacterial etiologies include members of the Chlamydia, Helicobacter, Borellia, and Campylobacter families. There are also a few select parasites classified as Group I and Group II carcinogens including members of the Schistosoma, Opisthorchis, and Clonorchis families. So really, “contagious cancer” in humans is due to contagious or infectious etiologies and not necessarily direct contact transmission. Although there are documented and potential exceptions including cancer spread through tissue grafts, organ transplants, papillomavirus transmission during sexual intercourse and other isolated events. (1)

At the end of the day, the presence, history, transmission, and pathogenesis of transmissible cancers in Tasmanian devils, dogs, and the few cases documented in humans provides insight regarding the immune mechanisms that do and those that do not allow cancer to develop. The key difference here is mammals verses marsupials and the reality that mammals have a more advanced immune system allowing them to better overcome cancer and other foreign invasions. A better understanding of both CTVT and DFTD has and will likely continue to allow researchers better insight into mechanisms of immune system invasion of various types of cancer. (1)

Sources:

(1)   http://www.dpiw.tas.gov.au/inter.nsf/WebPages/BHAN-5358KH

(2)   Welsh JS. Contagious Cancer. Oncologist. 2011 January; 16(1): 1–4. Published online 2011 January 6. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3228048/

(3)   Belov K. Contagious cancer: Lessons from the devil and the dog. BioEssays: Volume 34 (4), pages 285–292, April 2012. http://onlinelibrary.wiley.com/doi/10.1002/bies.201100161/full

Picture: http://www.discoverworld.ru/park-tasmanijskogo-dyavola-17528/