The Contagious Nature of Cancer

 

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Picture Credit: Richard Harris | Soft-Shell Clams | 2015 | KERA News

 

If there is one word that strikes fear into peo­ple’s hearts and conveys a haunting image of sickness and death, it’s cancer. According to the Union for International Cancer Control (UICC) and the International Agency for Re­search on Cancer (IARC), this disease is a glob­al epidemic that kills 7.6 million people every year, 4 million of whom are below the age of 69. Even worse, experts predict that the death toll is projected to increase to 6 million lives per year by 2025. Scientists are continuing to pursue different fields of research that can shed new light on the nature of this deadly disease. Recently, experts have come across a horrifying discovery: cancer may be contagious.

With every new insight bringing us closer to putting an end to cancer, this finding proves to be a terrifying yet valuable piece of infor­mation. This discovery originated in the 1970s when scientists were puzzled by the outbreak of leukemia in soft-shell clams along the east coast of North America. They found that this type of cancer could be spread to healthy clams by injecting them with the blood of cancer-stricken clams. For decades, research­ers concluded that a virus was transmitting the cancer. It wasn’t until 2015 that a team of experts lead by Stephen Goff from Columbia University finally pinpointed the answer: the cancer itself was spreading to other clams. This meant that the clam leukemia originated from a single host and somehow gained the ability to survive and thrive in other hosts.

As the second leading cause of death in the U.S., a top cause worldwide, cancer was thought to have a single saving grace: its non-infectious nature. While a tumor may outwit all attempts to stop its growth in a patient, the cancer ul­timately dies with its host, unable to infect another victim. However, the idea of cancer being transferred to new hosts is nothing new. In 1964, researchers at the National Cancer In­stitute performed an experiment where they harvested cancer cells in hamsters and injected them into healthy hamsters to encourage the cancer’s evolution. After numerous cy­cles, the tumor developed into a “super tumor” that could spread from hamster to hamster, without a needle, through social contact.

Regarding human cases, there have been a handful of documented cases where doctors, surgeons, and laboratory work­ers accidentally pricked themselves with a sur­gical instrument infected with cancer cells and had tumors proliferate in the wounded area. In almost all these cases, the infected person had to undergo emergency surgery before the tumor grew out of control.

However, these examples were extremely rare, freak incidents caused by accidents and human tampering. Cancer isn’t known for spreading naturally. It may be triggered by a carcinogenic chemical, bacteria or a virus, but the actual cancer cells shouldn’t be able to move from host to host like a pathogen. Yet, with the discovery of the clam leukemia’s contagious nature, the number of known exceptions to this commonly-held belief has increased to three.

The other two exceptions belong to dogs and Tasmanian devils, an aggressive species of marsupial found in Australia. For dogs, the tumor cells are physically transmitted during sexual contact where the tearing of genital tissues provide a bridge for the cancer . This condition, called Canine Transmissible Venereal Tumor (CTVT), originated 11,000 years ago from a single dog and has been circulating ever since. With Tasmanian devils, a cancer known as Devil facial tumor (DFT) disease has been spreading as they fight and bite each other’s faces. Having emerged from a single source, this contagious cancer has been completely ravaging the Tas­manian devil population and has ultimately put them on the endangered-species list.

What makes the clam leukemia worrying is that, unlike dogs or Tasmanian devils, this type of cancer is not spread through physical con­tact. Instead, it’s speculated that the clams are drawing in floating cancer cells as they sieve food from the water. It may not be a quick or efficient way for cancer cells to transmit themselves to other hosts, but it’s bound to happen eventually. Goff and his team are already in search of other species that are affected by cancer spread in a similar man­ner. They have already found similar instances in other mollusks in European waters as well as a contagious cancer that affects cockles.

It is terrifying to imagine cancer evolving into a transmissible contagion, especially one that can get into our water supply and cause tumors through contaminated drinking water. However, scientists have relieved fears, stating that no case of cancer naturally transferring to humans has been observed and that transmissible cancers still remain very rare. In addition, natural immunity in humans prevents hu­man-to-human cancer transmissions.

However, what’s worrying is that this scientific revela­tion is just one addition to a growing trend of cases on contagious cancer. In 2013, a man from Medellin, Colombia was diagnosed with can­cer thanks to the spread of cancer cells from a cancer-ridden tapeworm inhabiting the man’s body. On November 2015, scientists study­ing DFT disease found a second type of con­tagious cancer in the Tasmanian devils, mark­ing the discovery of two transmissible cancers within just 30 years.

Whether or not cancer is truly contagious to humans, it’s important to keep track of the progress being made in this field of research. Any development may cause huge shock-waves in the scientific community and prepare us for a grim future ahead. Even if cancer can’t be spread from person to person, researching how tumors are spread in animals can provide more insights on its mechanism and prevention. Whichever direction this research takes, the scientific community should bring more focus on this issue and expand its efforts in finding answers. The idea of contagious cancer may be frightening, but more extensive study could ultimately yield new insights and perhaps even the eternally sought-after cure.

Originally published on March 2, 2016, in The Miscellany NewsResearch reveals implications of clam cancer

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