The Tweet: “Sharks are actually immune to all known diseases.”

Sharks are not immune to ‘all known diseases’. Like all other fish, sharks are susceptible to parasites and other infections. More commonly this myth refers to cancer and this is perhaps more interesting.

There are a number of possible reasons for the perpetuation of the myth that sharks are immune to cancer. Firstly, documented cases of sharks with cancer are lower than we would expect. This is entirely based on anecdotal evidence from caught sharks and doesn’t mean they have any immunity to disease. The idea that sharks don’t get cancer originates from the book Sharks don’t get cancer by I. William Lane, published in 1992. Misleadingly, the author doesn’t argue that sharks are immune to cancer, but does refer to the observation that they are caught with cancer in lower than expected frequencies.

But more worryingly, researchers from the Johns Hopkins University, Baltimore, warn of the effects of shark cartilage ‘cures’ on perpetuating this myth. Peddlers of cartilage extracts to be used as cures for human cancer often cite the observed lower frequency of cancerous sharks and the antiangiogenic properties of cartilage as evidence for its therapeutic benefits. But tumours rarely grow in cartilage of any species because tumours need blood vessels to continue to grow and cartilage antiangiogenic factors prevent blood vessel formation. Gary Ostrander, the lead author on this paper, warns of the effect of pseudoscientific cures on overfishing of shark populations: “Until this century, it was difficult to imagine that anthropogenic activities would endanger the existence of an entire class of animals in the open sea. A combination of efficient fishing technologies, susceptibility of the public to erroneous arguments, and the power of television to rapidly shape opinion has now contributed to depletions of shark populations.”

Even if shark cartilage is less susceptible to cancer there is no evidence to say that a cartilage-based cure will offer any therapeutic benefits: “The fact that people think shark cartilage consumption can cure cancer illustrates the serious potential impacts of pseudoscience,” says Ostrander in his paper. “Although components of shark cartilage may work as a cancer retardant, crude extracts are ineffective.”

If there was a lower instance of cancer in sharks it could be explained by a phenomenon called Peto’s Paradox, named after the cancer epidemiologist Sir Richard Peto. Peto’s paradox observes that large animals would be expected to have higher instances of cancer than smaller animals because they live for longer and have more cells, and therefore are more likely over their lifetime to develop tumours by random chance. However, lifetime risk of cancer in mammals varies little with body size. A paper published in July 2014 suggests larger bodied mammals may have evolved mechanisms to limit the activity of tumourigenic endogenous retroviruses, offering one explanation of Peto’s Paradox. Whether this phenomenon is also present in sharks has not been observed.

“We don’t know that they don’t get cancer,” says Ostrander. “Any suggestion that they get it at a lower rate than humans or other fish, is premature—because there haven’t been any carefully conducted systematic studies. I have not seen anything in the scientific literature that gives any confidence, with certainty, that sharks get cancer at a lower rate than fish or other species.”