The idea of “catching cancer” is pretty horrifying, right? Cancer is devastating as it is, but the idea of the disease being contagious is hard to imagine. There are certain viruses, such as Human Papillomavirus (HPV) and Kaposi’s sarcoma-associated herpesvirus, that can cause cancer in humans but to date, there has been no evidence that human tumor cells themselves are contagions. Traditionally, cancer is thought to be the result of host cells mutating to grow continuously into tumors that become malignant. However, a recent Cell paper reported that a leukemia-like cancer that has been observed in marine bivalves since the 1970s is transmitted among individuals by the tumor cells themselves.
Although this cancer in marine bivalves has been observed for decades, the origin and method of spread among populations remained unknown. The investigators of this current study previously identified a DNA segment that is highly expressed in tumor cells of clams. Taking advantage of this identified sequence, the Columbia University-based team performed genetic analyses to reveal that tumor cells isolated from diseased individuals did not match other cells from the same host, indicating that tumor cells are not related to the other cells of their host. On the contrary, the investigators astoundingly found that all of the clam leukemia cells are nearly identical to each other. The investigators looked at samples collected from all over the eastern seaboard, and their results remained consistent. That means that this cancer derived from one individual and spreads by clonal transmissible cells that devastate clam populations along the east coast. This “patient zero” remains unknown.
Clams are not the only organism threatened by contagious forms of cancer. Tasmanian facial tumor disease has already knocked out the vast majority of Tasmanian devil populations. Tasmanian devils are actually quite different from how you may remember tornado-spinning Taz from the Looney Tunes. Now only found on the Australian island of Tasmania, Tasmanian devils are carnivorous marsupials known for being smelly, sharp, and vicious. First observed in the late 1990s, Tasmanian facial tumor disease results, as the name implies, in facial tumors that grow to become fatal to those infected. The vicious quality of this animal is what makes the most plausible mean of transmission of cancerous cells between individuals biting. Models predict that this disease could lead to the extinction of the Tasmanian devil in the next few decades, and researchers are eager to develop diagnostic tools and containment plans to preserve the few remaining individuals of this species rapidly shrinking in size.
 Michael J. Metzger, Carol Reinisch, James Sherry, Stephen P. Goff. Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clams. Cell, 2015; 161 (2): 255
 McCallum, H. 2008. Tasmanian devil facial tumour disease: lessons for conservation biology. Trends in Ecology and Evolution 23:631–637.