CANCER IN WILD ANIMALS

Introduction to Cancer in Wild Animals

Cancer seems to affect all animals, from anteaters to zebras.  Much less is known about the cancers that affect wild animals for the obvous reasons (they move around and may not be easily observed for long periods of time).  The cancers that have been studied are very interesting and may prove useful in the study of human cancer.  As an example, Tasmanian devils have a type of cancer that can be spread from animal to animal by biting!

Tasmanian Devil Facial Tumor Disease

In 2008, the International Union for Conservation of Nature officially declared the Tasmanian devil an endangered species

The animals were driven to extinction on the Australian mainland thousands of years ago, after humans introduced dingoes to the continent. The remainder of the wild population has since inhabited the Australian island-state of Tasmania.  In the mid 1990's the population reached an estimated 150,000 devils.⁽¹⁾ Today, however, the animals are plagued by an infectious cancer known as Tasmanian devil facial tumor disease (DFTD).  Since the emergence of the disease in 1996, the population has declined by more than 60%.⁽²⁾ As a result, what was once the largest surviving population of marsupial carnivores is now threatened with extinction.

This type of cancer is very unusual.  The great majority of cancer cases in humans and animals arise from a series of mutations in a single precursor cell and its daughter cells.  The process occurs over a period of years and does not involve contact with any other individuals.  DFTD develops differently.  It's transmitted from animal to animal and the cancer cells themselves are the infectious agent.

Researchers describe this phenomenon as allograft transmission .⁽³⁾   An allograft is the term for the transfer of cells/tissue from one individual to another.  An example in humans is organ transplantation.  The movement of cancer cells between animals has been confirmed by cellular and molecular studies.  A normal devil cell contains 14 chromosomes.⁽³⁾   DFTD tumor cells contain several very distinctive genetic changes and have only 13 chromosomes.  Importantly, the tumors from every animal tested appear identical.⁽³⁾  Researchers in Tasmania also found a devil with an unusual chromosomal abnormality in its non-tumorous tissue that did not appear in its tumor cells.⁽³⁾  These findings strongly suggest that the cancer did not arise from the animals' own cells. 

A cancer similar to DFTD occurs in dogs, and is known as Canine Transmissible Venereal Tumor (CTVT).  The immune system of dogs is capable of overcoming the disease, but devils do not seem to be able to do so.  Researchers have hypothesized that low genetic diversity among Tasmanian devils results in close kinship and reduces their immune responses.^{(4) (5) (6) (7)}   As a result, transplanted cancer cells are more likely to survive, grow, and spread.

Transmission can occur by biting, feeding on the same material, aggressive mating, and other social interactions.  DFTD tumors mostly form on the face and/or in the oral cavity.  The cancer can also metastasize to other areas of the body.  Nearly 100% of infected devils die within 6 months of the onset of clinical signs.⁽³⁾ Death results from an inability to feed, secondary infection, or symptoms associated with metastases. 

If researchers are able to develop a vaccine for DFTD, it could halt disease spread significantly.  Efforts are also being made to capture and relocate healthy animals to repopulate disease-free areas.  The Tasmanian government is working with conservation specialists to reduce the impact of the disease.

Shark Cartilage and Cancer: Overview

Shark cartilage has been used by many patients to complement traditional cancer treatments.   The effectiveness of the treatments is unclear.  Shark cartilage has not been approved by the FDA for the treatment of any disease, but it is available in the form of dietary supplements .⁽¹⁾ The use of shark cartilage in cancer treatment is based on the mistaken belief that sharks don't get cancer.  An extension of this belief is that shark body parts (i.e. cartilage) can prevent or treat cancer in humans.  However, benign and cancerous tumors can in fact develop in sharks; George Washington University Medical Center researchers reported over thirty tumors found in elasmobranchs, a group of animals that includes sharks, rays, and skates.⁽²⁾ The belief that sharks do not get cancer is not based on the known facts.

Another question that has been posed is whether or not sharks have reduced amounts of cancer.  This is a difficult question to answer because data concerning cancer rates in wild animals (including sharks) are limited, and the information that is available isn't conclusive.  There are some data which suggest that sharks may have a reduced occurrence of cancer.⁽³⁾  Some people interpret this to mean that sharks have some form of resistance to the development of cancer.  If sharks do have lower cancer rates, the reasons are not understood.

Why Cartilage?

The choice of cartilage, instead of some other body part is not random.  The first reason is the availability of the product.  The entire endoskeleton of sharks is composed of cartilage, amounting to about six percent of their body weight.⁽⁴⁾The amount of cartilage that can be obtained from a large shark is therefore quite large.  The second reason is based on some proven biology.  Cartilage, in both sharks and humans, lacks blood vessels.  It has been suggested that sharks resist cancer because of something in their cartilage that inhibits blood vessel growth (angiogenesis).  Angiogenesis inhibitors have been found in the cartilage of other animals.⁽⁵⁾

Growing tumors require angiogenesis to bring them oxygen, sugar and other nutrients.  Without a blood supply, tumors cannot grow beyond a very small size.  Blood vessels also serve as a "highway" for cancer cells to move to distant parts of the body (metastasize).  Angiogenesis inhibitors from other sources are already used to treat cancer patients.  If they exist, it is possible that the angiogenesis inhibitors in shark cartilage could be used create a drug to combat cancer in humans.  

Shark Cartilage and Cancer Cells in the Laboratory

Human studies with shark cartilage have been disappointing. Animal studies have provided some evidence to support its anti-angiogenic activity.  In one study, 0.6 grams cartilage/kilogram of body weight was administered to rats for 4.5 days.  Angiogenesis was then induced in the abdomen of the animals with chemicals.  The animals continued to receive the cartilage treatment for two weeks.  The results showed a notable decrease in blood vessel formation in the rats that had been fed shark cartilage compared with those that were not.⁽⁶⁾In another study, cancer cells were implanted into the corneas of rabbits.  A pellet of highly purified shark cartilage was placed next to the cancer cells.  After twenty days the tumors near to the cartilage showed reduced blood vessel development with zones of complete inhibition around the pellets themselves.⁽⁷⁾

Shark Cartilage and Human Cancer

Because of the results of shark cartilage in animals, several studies were performed in the late 1990's to examine the effectiveness of shark cartilage in treating cancer patients.  The results were disappointing.  In one study, twenty prostate or breast cancer patients received 1 gram/kg/day of an oral shark cartilage powder for twenty weeks.  Both survival times and quality of life were unchanged compared with a placebo control group.⁽⁸⁾   In another study, shark collagen was orally administered to sixty patients with different advanced cancers for six weeks.  The study noted no complete or partial responses.  The outcomes were similar to patients who had received only supportive care.⁽⁹⁾   Clinical trials with a shark cartilage extract, Neovastat® (AE-941), in patients with renal or lung cancer showed only limited or no benefit to patients.^(10) (11)