Californian sea lions are dying of cancer at a high rate

Medical exam of California sea lion at the Marine Mammal Center in Sausalito, California, credit: Reuters/Nathan Frandino
Medical exam of California sea lion at the Marine Mammal Center in Sausalito, California, credit: Reuters/Nathan Frandino

A very ill female adult sea lion was recently found in San Luis Obispo County in California. She was thin and underweight, moving her head back and forth, and did not flee as people approached, said Cara Field, medical director at the Marine Mammal Center in Sausalito.

Once the sea lion was sedated, she could be examined by veterinarians. They discovered she had urogenital cancer, an untreatable diagnosis, and was euthanized. She had a very, very bad kidney, Field said.

Almost 25% of adult California sea lions end up with this cancer, according to a study published in December in Frontiers in Marine Science. “This is extraordinary and really quite awful,” Field said. “This is an unprecedented rate of cancer in wildlife.”

Herpes virus combined with chemicals
The study found two leading causes of the high cancer rates. One is the chemical pesticides found in the sea lions’ blubber tissue, such as dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyl (PCB).

And the other is the presence of a cancer-causing herpes virus known as Otarine herpes virus (OtHV-1). “Very few animals that had cancer did not have the virus,” said Padraig Duignan, director of pathology at the center and a co-author of the study.

The virus combined with the amount of chemicals in their body increased the probability of cancer by 30% to 80%, Duignan added.

Duignan pointed to the mammals’ breeding grounds near the Channel Islands in southern California, where toxic chemicals were dumped for years. A Los Angeles Times investigation found “thousands of barrels of acid sludge” laced with DDT dumped near Santa Catalina Island in the years after World War Two.

DDT was banned in the United States in the 1970s, and PCB was forbidden in 1979 after being linked to cancer and other health problems. But the chemicals made their way through the food chain and eventually into the blubber of sea lions, which is now killing them.

“The strong association with the herpes virus and pollutants in the environment is a huge warning to us to pay attention to what’s happening in the oceans,” Field said.

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