Contagious cancer cells are spreading between different animals and even different species in the sea, according to new research which raises the prospect of the disease becoming infectious in humans.
Previously it was thought that catching cancer from another animal was extremely rare, although last year cancer cells from a tapeworm infected an Aids patient with a severely compromised immune system. Sexually transmitted tumours are also known to affect dogs, and the Tasmanian devil population has been devastated by a contagious facial cancer spread by biting.
However the new study, published in the journal Nature, suggests that infectious cancer is common among three different kinds of shellfish. There is no suggestion that humans would be at risk as our immune system would attack any alien tissue that entered the body.
Researchers found mussels, cockles and clams, collected off the coasts of Canada and Spain, that had been infected with tumours which originated in another individual.
“Our results indicate that transmission of contagious cancer cells is a widespread phenomenon in the marine environment, with multiple independent lineages developing in multiple species,” the paper said. “Cases of transmissible cancer appear to outnumber spontaneous disease, at least in the species investigated so far.”
The researchers added that the cancers usually spread between animals of the same species, but they had found "one example of cross-species transmission". "These transmissible cancers constitute a distinct class of infectious agent and show the remarkable ability of tumours to acquire new phenotypes [genetic types] that promote their own survival and propagation," the paper said.
Normally tumours consist of the body’s own tissue, which makes them particularly dangerous because the immune system fails to react effectively. Another organism’s tumours should pose little threat because the immune system would attack in the usual way if it is functioning properly. Molluscs are believed to have only a primitive immune system that may leave them particularly prone to a cancerous infection.
One of the researchers, Professor Stephen Goff, of Columbia University Medical Centre, said their findings had prompted him to look at the marine world in a different way.
“It’s interesting to note that the ocean is a sea of various bacteria and now [cancer] cells that are capable of being pathogens,” he told The Independent. “I guess it’s a kind of change of thinking, that there are contagious cells floating around in the sea that can colonise a susceptible host.”
The scientists now plan to study the genetic processes that allow tumours from one creature to infect another, which might shed new light on how cancer spreads within people.
Professor Goff stressed their research provided no reason to stop swimming in the sea or eating shellfish. “It’s only a problem if you are a mollusc. There’s really no evidence that tumours of molluscs have spread outside of molluscs. They are not likely to cause a problem because we do have an immune system that works,” he said.
In an article in Nature commenting on the research, Dr Elizabeth Murchison, a reader in comparative oncology and genetics at Cambridge University, said finding that cancers “can invade new host species” was significant. “The potential for cancer cells to become free-living infectious agents raises questions about the implications for cancer transmission in humans,” she wrote.
13 ways to help prevent cancer
13 ways to help prevent cancer
Stopping smoking. This notoriously difficult habit to break sees tar build-up in the lungs and DNA alteration and causes 15,558 cancer deaths a year
Avoiding the sun, and the melanoma that comes with overexposure to harmful UV rays, could help conscientious shade-lovers dodge being one of the 7,220 people who die from it
A diet that is low in red meat can help to prevent bowel cancer, according to the research - with 30 grams a day recommended for men, and 25 a day recommended for women
Foods high in fibre, meanwhile, can further make for healthier bowels. Processed foods in developed countries appear to be causing higher rates of colon cancer than diets in continents such as Africa, which have high bean and pulse intakes
Two servings of fruit and three servings of vegetables a day were given as the magic number for good diet in the research. Overall, diet causes only slightly fewer cancer deaths than sun exposure in Australia, at 7,000 a year
Obesity and being overweight, linked to poor diet and lack of exercise, causes 3,917 deaths by cancer a year on its own
Dying of a cancer caused by infection also comes in highly, linked to 3,421 cancer deaths a year. Infections such as human papilloma virus - which can cause cervical cancer in women - and hepatitis - can be prevented by vaccinations and having regular check-ups
Cutting back on drinks could reduce the risk of cancers caused by alcohol - such as liver cancer, bowel cancer, breast cancer and mouth cancer - that are leading to 3,208 deaths a year
2014 Getty Images
Sitting around and not getting the heart pumping - less than one hour's exercise a day - is directly leading to about 1,800 people having lower immune functions and higher hormone levels, among other factors, that cause cancers
2011 Getty Images
Hormone replacement therapy, which is used to relieve symptoms of the menopause in women, caused 539 deaths from (mainly breast) cancer in Australia last year. It did, however, prevent 52 cases of colorectal cancers
2003 Getty Images
Insufficient breastfeeding, bizarrely, makes the top 10. Breastfeeding for 12 months could prevent 235 cancer cases a year, said the research
Oral contraceptives, like the Pill, caused about 105 breast cancers and 52 cervical cancers - but it also prevented about 1,440 ovarian and uterine (womb) cases of cancer last year
2006 Getty Images
Taking aspirin also prevented 232 cases in the Queensland research of colorectal and oesophagal cancers - but as it can also cause strokes, is not yet recommended as a formal treatment against the risk of cancer
A key question is whether the contagious cancer cells have been around for thousands of years or are a new phenomenon. And, if they are new, what caused them to develop?
“It is possible that, like the canine transmissible cancer, these cancers are ancient cell lineages that have co-evolved with their hosts through the millennia; or perhaps their emergence is a relatively recent occurrence, possibly stimulated by infectious agents, environmental changes, aquaculture or other anthropogenic [human] activities,” Dr Murchison added.
Professor Mel Greaves, director of the Centre of Evolution and Cancer at The Institute for Cancer Research in London, stressed results of the study were “no cause for concern” about humans catching cancer from the sea.
The disease has been known to spread in humans from a mother to a baby in the womb, between twins in the womb or after an organ transplant.
“In all three cases, transmission was possible because a blood route for cancer cells was available and the immune system was compromised. This risk is very, very small indeed," Professor Greaves said in an email. “Regarding these new results in shellfish, the public should not be at all alarmed as the processes involved are different from those in people.
"The biology is, however, very interesting with implications for the evolution of both cancer cell clones and immune recognition within and between species.”