Long-term Observation of Cancer in a Shark

Sharks don’t get cancer. That’s the lore we’ve all heard. Some take this claim even farther. Eat a shark and you won’t get cancer, or the cancer you have will be cured. Many sharks are killed for the supposed curative powers of their cartilage, among other health claims. Sharks do, of course, get cancer, and numerous examples of shark cancer have been documented. Why isn’t cancer seen more frequently in sharks? Likely for the same reason we can’t tell if a person walking down the street has cancer – it is a largely internal disease, often up to the point of death. Sick individuals are easily predated by other animals, and since dead sharks sink to the ocean floor, rather than float or beach as do marine mammals, shark cancer deaths are rarely detected.

Despite there being substantial evidence in the scientific literature of sharks with cancer, I am always interested in new documentation of these cases. Brunnschweiler et al, have now done something unique – they have followed cancer progression in a free-living wild shark. They document a cancerous lesion on the jaw of a bull shark (Carcharhinus leucas), resident in a marine reserve, over seven years. The shark was photographed in 2010 with a fracture to the lower jaw, likely resulting from a fishing hook found embedded in the tissue. This same animal was observed 67 times in the reserve through 2016. With time, the embedded hook was lost, and a growing mass became evident at the injury site. Deformation of the jaw worsened over time, and in 2014 a second fishhook in the same area suggested additional damage to the tissue from repeated fishing attempts.

The authors propose that ongoing tissue damage to the area resulted in inflammation and cellular overgrowth that became cancerous. This disease course can be seen in humans, for example in the way chronic hepatitis can lead to liver cancer or inflammatory bowel disease results in an increased risk of colon cancer. The affected bull shark, as of the publication date, was still alive, and monitoring of the tumor progression is ongoing.