Cancer at Sea

California sea lions suffer an unusually high rate of an aggressive cancer that begins in the reproductive organs and can spread to lymph nodes and the spine, sometimes destroying vertebrae and causing paralysis. At the Marine Mammal Center in Sausalito, researchers found cancer in 18 percent of adult sea lions examined after death from the 1980s through the mid-1990s, rising to 26 percent between 1998 and 2012. The increase may reflect better detection as well as a real rise, but scientists say the disease is not declining. A suspected sexually transmitted virus appears to trigger the cancer, making sea lions a potential source of insight into how viral infections lead to cancer in humans and other animals.

The investigation unfolds amid broader pressures on sea lions. Unusually warm Pacific waters and a bloom of toxin-producing algae have disrupted the food web, while struggling mothers cannot find enough food to nourish their pups. Many pups have starved, and 80 to 90 percent reportedly died over a three-year period; the center’s admissions reached record levels. Scientists argue that environmental stress may worsen the effects of infection, much as stressors can influence disease in people. Although the sea lion population can currently absorb cancer losses, a decline caused by starvation and other threats could make the disease more consequential. The center’s rescue and necropsy work supplies researchers with animals and tissue samples to investigate how viral infection, environmental conditions, and cancer interact.