Contagious Cancers: A Rare Phenomenon with Implications for Human Oncology

by Jeya Chelliah B.Vsc Ph.D.

Cancer, a word that evokes fear and concern, is typically associated with individual affliction. However, nature has its own peculiarities. In certain species, cancer is not just an individual’s battle but can spread contagiously from one individual to another. This phenomenon, although rare, offers a unique lens to understand cancer evolution, immune responses, and potential therapeutic strategies.

Along the east coast of North America, soft-shell clams are facing a lethal threat: a leukemia-like contagious cancer. This cancer, which seems to have originated from a single clam roughly 40 years ago, is now transmitted between these marine animals. The mystery deepens when considering how this cancer spreads among clams separated by hundreds of miles. Clams, being filter feeders, might ingest neoplastic cells released by afflicted peers. Additionally, human activities, such as relocating clams, could inadvertently aid in the disease’s spread.

The Salish Sea beaches have become a focal point for researchers, where the local Coast Salish people have traditionally consumed these clams. Here, scientists are observing an alarming rise in this leukemia-like cancer, termed disseminated neoplasia. Intriguingly, these cancer cells demonstrate a remarkable resilience in the marine environment.

But why can these cancers spread between clams? The answer might lie in the immune system. In vertebrates, the major histocompatibility (MHC) system plays a pivotal role in recognizing and rejecting foreign cells. Mollusks, including clams, lack this MHC system, potentially explaining the unchecked spread of tumor cells among them.

Contagious cancers aren’t exclusive to clams. The Tasmanian devil, a unique marsupial, is grappling with the devil facial tumor disease. These tumors, believed to originate from neural crest cells, manifest as head and neck cancers. They are aggressive, often spreading to lymph nodes and occasionally metastasizing to the lungs. The transmission mode? Bites during territorial disputes or courtship, transferring malignant cells directly into another devil’s wounds.

Dogs, too, aren’t spared. Canine transmissible venereal tumor, or Sticker’s sarcoma, is another example. While it shares similarities with the devil facial tumor disease in terms of transmission, its progression is distinct. Initially aggressive, it later regresses and is typically non-fatal. This behavior might be attributed to the modulation of MHC antigens, which initially cloak the tumor from the host’s immune system. However, once the MHC class 2 antigens are re-expressed, the canine immune system springs into action, rejecting the tumor.

For humans, these contagious cancers offer invaluable insights. While human cancers aren’t contagious due to our robust MHC system, understanding these rare cancers can illuminate the intricacies of cancer development, metastasis, and potential treatments. The contrasting outcomes observed in animal contagious cancers, from rapid fatality to complete regression mediated by immune responses, underscore the potential of immunotherapy in clinical oncology.

In conclusion, the narrative of contagious cancers in animals is both intriguing and enlightening. From the soft-shell clams of North America to the Tasmanian devils down under, these cancers provide a unique perspective on the interplay between cancer and the immune system. As researchers delve deeper, the hope is that these findings will not only benefit the affected species but also pave the way for advancements in human cancer treatments.

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