Businesses that sell shark cartilage as a cancer cure or preventive have claimed for years that sharks never get cancer, but last week scientists from the Johns Hopkins and George Washington universities presented a detailed history of benign and malignant tumors found in sharks and related fishes.
At a news conference at the annual meeting of the American Association for Cancer Research, scientists noted that sharks can even get chondromas--cancers of the cartilage now being sold as a cancer cure. Their results come from a survey of data in the National Cancer Institute's Registry of Tumors in Lower Animals at George Washington University. Using very strict diagnostic criteria, scientists were able to find 40 cases of tumors in sharks and related fishes like skates and rays.
"People are out there slaughtering sharks and taking shark cartilage pills based on very faulty data and no preventive studies to show that it works," says Gary Ostrander, Hopkins professor of biology and comparative medicine. "That's not only giving desperate patients false hope based on misinterpreted data, it's also taking a top-level predator out of an ecosystem, which could cause major disruptions in the ecosystem."
Ostrander and lead author John Harshberger, a professor at George Washington University who directs the Registry of Tumors in Lower Animals, note that the new study can't rule out the possibility that scientists may one day find a useful cancer treatment in cartilage from sharks or other animals. They also acknowledge that folk and alternative medicine can sometimes offer legitimate insights to the biomedical sciences.
For now, though, no proof exists that shark cartilage can have positive effects on cancer, and the pills' cost to patients, potential for interference with proven cancer treatments, and the possibly devastating impacts on marine ecosystems have scientists like Ostrander and Harshberger deeply concerned.
"Cancer exists throughout the phylogenetic tree, science's system for classifying the many forms of life," Ostrander explains. "The idea that there's some animal out there that never gets cancer and never expresses it just really doesn't resonate well with people who work in the field."
Scientists have not yet conducted a comprehensive study of cancer rates among sharks, and currently available evidence does not allow them to assess the causes of the cancers they have found. The myth that sharks do not get cancer sprang up in part because of their isolation from humans, which reduces their exposure to carcinogenic pollutants and the likelihood that sharks with tumors will be caught, Ostrander explains.
Proponents of shark cartilage also selectively used scientific results to bolster their case, he notes. For example, a single effort to give sharks tumors in a laboratory failed. But Ostrander points out that several different efforts to give tumors in the lab to English sole also failed, even though that fish has high cancer rates in habitats such as Puget Sound.
Shark cartilage advocates also seized upon scientists' efforts to suppress angiogenesis, a tumor's ability to encourage growth of new blood vessels. When researchers realized that angiogenesis allows tumors to obtain much of the raw material they need to fuel their own abnormal growth, they began to look for factors in the body that could stop angiogenesis. One of the first places they looked was cartilage, a tissue with relatively few blood vessels.
Sharks are animals with a great deal of cartilage. All those factors left sharks vulnerable to myth and misperception, Ostrander explains, but none of them changes the fact that sharks can get cancer or suggests that they must have a key to beating cancer.
"Part of the reason your own blood vessels are laid out as they are is because your body contains factors that stop them from growing," he explains. "And chicken cartilage, human cartilage and all other kinds of tissue have anti-angiogenic factors in them. Yes, there may be some others in shark, but to suggest they will be a cure-all for cancer based on available data is bogus."