Question

Although cancer is not a contagious disease in humans or other vertebrates, there have been rare cases in which cancers have spread from one organism to another. Describe three cases of these contagious cancers and what conditions might have led to their appearance. For an introduction to this topic, see http:// www.cancer.org/cancer/cancerbasics/is-cancer-contagious.

Short answer

Question: Describe three cases of contagious cancers in organisms and analyze the conditions that might have led to their appearance. Answer: Three cases of contagious cancers in organisms are: 1. Tasmanian Devil Facial Tumor Disease (DFTD) occurs in Tasmanian devils and spreads through biting during aggressive behaviors or social interactions. The small population size and low genetic diversity among Tasmanian devils might have led to a weakened immune system that cannot recognize and attack the cancer cells, enabling the disease to spread. 2. Canine Transmissible Venereal Tumor (CTVT) affects dogs and is transmitted through close physical contact, mainly during mating or sharing food. CTVT likely originated from a single dog thousands of years ago, and its ability to evade the host's immune system is possibly due to the deletion or alteration of specific genes that would otherwise trigger an immune response. 3. Bivalve Transmissible Neoplasia is observed in mussels, clams, and other bivalves. It spreads through environmental water filtering, where cancer cells from affected organisms enter the water and are ingested by healthy ones. The bivalves' method of ingesting surrounding water and their less complex immune systems might have facilitated the spread of cancer cells between individuals.

Step-by-step solution

Understand contagious cancers

Contagious cancers are rare cases where the disease can spread from one organism to another, usually through direct physical contact. Typically, cancer is not contagious in humans or other vertebrates. It is essential to differentiate contagious cancers from viral cancers, where a virus plays a role in causing cancer in the infected individual.

Research examples of contagious cancers

Visit the provided link (http://www.cancer.org/cancer/cancerbasics/is-cancer-contagious) and gather additional information on contagious cancers. Search for other reliable sources to identify three cases of contagious cancers in different organisms.

Describe the three cases of contagious cancers

1. Tasmanian Devil Facial Tumor Disease (DFTD): This contagious cancer affects Tasmanian devils and is transmitted through biting during aggressive behaviors or social interactions. DFTD causes large facial tumors that make it difficult for the affected devils to eat and leads to their death. 2. Canine Transmissible Venereal Tumor (CTVT): This contagious cancer affects dogs and is transmitted through close physical contact, mainly during mating or sharing food. CTVT causes tumors on the dog's genital areas, which might spontaneously regress or lead to paralysis and death. 3. Bivalve Transmissible Neoplasia: This contagious cancer has been observed in mussels, clams, and other bivalves. It is transmitted through environmental water filtering, where cancer cells from affected organisms enter the water and are ingested by healthy ones. It results in the replacement of healthy tissue with cancerous tissue, eventually leading to death.

Analyze the conditions that may have led to the appearance of these contagious cancers

1. Tasmanian Devil Facial Tumor Disease: The small population size and low genetic diversity among Tasmanian devils might have led to a weakened immune system that cannot recognize and attack the cancer cells, enabling the disease to spread. 2. Canine Transmissible Venereal Tumor: CTVT likely originated from a single dog thousands of years ago and has since been transmitted to other dogs globally. The ability of CTVT cells to evade the host's immune system is possibly due to the deletion or alteration of specific genes that would otherwise trigger an immune response. 3. Bivalve Transmissible Neoplasia: The bivalves' method of ingesting surrounding water to filter food particles might have facilitated the spread of cancer cells between individuals. Additionally, these organisms' immune systems may not be as complex as those of vertebrates, making it harder for them to recognize and attack the cancer cells.

Key concepts

Tasmanian Devil Facial Tumor Disease

Tasmanian Devil Facial Tumor Disease (DFTD) presents an intriguing yet tragic instance of cancer transmission in the animal kingdom. This disease specifically afflicts the Tasmanian devil, an iconic marsupial native to Australia. It's transferred between individuals through biting, which is a common behavior during social interactions and mating contests.

DFTD manifests as large, disfiguring facial tumors, which seriously compromise the devil's ability to feed and eventually leads to death. A notable factor contributing to the spread of DFTD is the low genetic variability within the Tasmanian devil population, rendering their immune system less effective at recognizing and fending off the cancer cells.

The Unique Genetics of DFTD

Since the affected cells can evade detection by the devil's immune system, they spread like a contagion. Moreover, the disease's impact is also heightened by the Tasmanian devils' naturally aggressive behavior, which leads to frequent bites and, subsequently, transmission of DFTD.

Canine Transmissible Venereal Tumor

Another illustration of contagious cancers is the Canine Transmissible Venereal Tumor (CTVT), which affects domestic dogs. Unlike DFTD, CTVT is typically transmitted through sexual contact, but can also spread via licking, biting, or sniffing tumor-affected areas.

CTVT is notable for tumor growths primarily in the genital region, yet it can sometimes affect other areas like the mouth or skin. Interestingly, these tumors can at times regress spontaneously, indicating that a dog's immune system can occasionally overcome the cancerous cells.

Survival and Evolution of CTVT

CTVT's resilience is attributed to its ability to suppress and evade the host's immune response, which likely stems from alterations or losses of key immunogenic genes. It's theorized that CTVT has ancient origins, possibly dating back to a single canine ancestor, allowing it to adapt and persist across dog populations worldwide.

Bivalve Transmissible Neoplasia

Transmissible cancers are not restricted to terrestrial animals. Bivalve Transmissible Neoplasia (BTN) makes this evident as it affects marine bivalves like mussels and clams. BTN spreads in a distinctive manner compared to DFTD and CTVT; the cancer cells are released into the water and subsequently ingested by other bivalves during their filter feeding process.

Once ingested, these cancerous cells integrate and overtake healthy tissues, thereby compromising the health of the host. BTN's transmission is facilitated by the bivalves' style of feeding and possibly by their more rudimentary immune systems, which are less capable of identifying and destroying the foreign cancer cells.

Bivalves' Feeding and BTN Spread

The very means bivalves use to nourish themselves—filtering water—unwittingly exposes them to circulating cancer cells, and their lack of a sophisticated immune system akin to vertebrates makes them vulnerable hosts for these transmissible cancers.

Cancer Transmission in Animals

The concept of cancer transmission in animals is distinct from the notion of contagious diseases commonly understood by the public. It is an exceptional phenomenon wherein cancer cells from one individual are capable of being transmitted to another and establishing growth within the new host.

In most species, including humans, cancers don't spread from one individual to another; the immune system typically recognizes and attacks foreign cells. However, contagious cancers like DFTD, CTVT, and BTN bypass these defenses due to a combination of unique evolutionary adaptations, genetic mutations, and sometimes species-specific behaviors or physiological characteristics that enable the spread of these malignancies.

Implications for Conservation and Veterinary Medicine

The study of contagious cancers is vital for conservation efforts, particularly for impacted species like the Tasmanian devil, and provides insights that could influence the understanding and treatment of cancer in veterinary and possibly human medicine.