A specific cancer of epithelial origin was first diagnosed in California sea lions at The Marine Mammal Center in 1979. Approximately 17% of adult stranded sea lions that die are diagnosed with cancer.
The Center collects samples from each of these patients to measure environmental carcinogen exposure. It has also discovered that these sea lions are infected with a herpesvirus similar to that which causes Kaposi's sarcoma in humans. Future research will determine the factors, such as pollutants or genetics, which allow the virus to invade sea lion tissues and cause cancer.
This is a very high prevalence for a population of wild animals. Cancer development is a multi-step process during which damage to the genetic material of cells (DNA) arises from the interaction between a number of factors. These may include environmental factors such as chemical contaminants, infection by tumor-promoting viruses and the animals' own genetic predisposition to develop the disease. People with this type of cancer generally have been smokers or exposed to toxins. The latter may apply to marine mammals. High levels of persistent organic pollutants such as DDT and PCBs have been found in the blubber of California sea lions.
The research the Center conducts on post mortem sea lions with cancer is invaluable as this kind of work is not possible with human subjects. The Center's findings show that sea lion pups are exposed to PCBs while in utero, a factor that has important health effects later in their lives as PCBs can alter organ development.
In 2010, The Marine Mammal Center along with an array from international scientific community formed the Sea Lion Cancer Consortium to highlight the scientific body of work currently underway in investigating urogenital cancer in sea lions.