Researchers in California are struggling to explain why California sea lions are getting sick with cancer.


Fourteen years after veterinary experts first noticed sea lions becoming ill, scientists are studying 300 sea lions and examining three prime suspects: viruses, PCBs in the water and genetics. “Years of study have led researchers to think the answer lies not with any one culprit, but with several,” reports The New York Times.


“It’s such an aggressive cancer, and it’s so unusual to see such a high prevalence of cancer in a wild population,” said Dr. Frances Gulland, director of veterinary science at the Marine Mammal Center. In 1996, Gulland and colleagues at the University of California, Davis, found that 18 percent of deaths among stranded adult sea lions were related to cancerous tumors.


“That suggests that there’s some carcinogen in the ocean that could be affecting these animals,” Gulland said.


The first reports of sea lion cancer came 14 years ago, among rescued California sea lions. Today, the Marine Mammal Center sees 15 to 20 California sea lions with cancer each year.


“It’s pretty distressing to see,” Gulland said. During post-mortem examinations of the sick sea lions, doctors have found tumors in the animals’ genitals, lymph nodes, lower spine, kidneys, liver and lungs.


Few veterinarians monitor the incidence of cancer among wild animals. But about 18 percent of dead beluga whales stranded in Canada’s St. Lawrence River had intestinal tumors or other cancers that were linked to industrial pollutants. Among the California sea lion population, no diagnostic test for cancer exists.


Dr. Robert DeLong, a research biologist at the National Marine Mammal Laboratory in Seattle, has seen two to five sea lions a year with advanced tumors, out of some 100,000 animals in the Channel Islands, the birthplace for many California sea lions.


Environmental pollutants are prime suspects. From the late 1940s until the early 1970s, DDTs and PCBs were dumped in the Southern California Bight.


But, in 2000, researchers in Washington, D.C., found strain of a herpes virus in sea lions that was similar to lesions in AIDS patients. More recent studies show the virus, which lives in the reproductive tract, is two times as common in male sea lions as in females. Researchers now think the virus and chemical pollutants have been triggering tumors. Bad genes probably have something to do with it, too.


“We don’t have all the answers by any means,” said Dr. Linda Lowenstine, a veterinary pathologist at UC Davis who works with Gulland. But the plight of sea lions will affect humans, since they “eat a lot of the same things we do,” said Gulland, echoing a familiar refrain. “We really should start paying attention to what we’re putting into the oceans.” 


Also on MNN: Sea lions return to San Francisco pier after dog leaves 

Cancer strikes sea lions
Why are California sea lions getting sick with cancer? Researchers suspect viruses, PCBs in the water and genetics.