Story at a glance

  • Researchers analyzed 394 California sea lions that have died over the past 20 years and found that many suffered from an aggressive cancer.
  • After analyzing tissue samples from the animals, researchers say toxic manufacturing chemicals and a herpes virus are to blame.
  • According to the study, the prevalence of such cancer in California sea lions is one of the highest amongst mammals, with 18 to 23 percent of adult sea lions examined after death over the past 40 years having urogenital carcinoma.

Researchers say they’ve likely discovered why a large number of sea lions in California have been dying from an aggressive cancer for several decades. 

A study published December in the journal Frontiers in Marine Science has linked toxic manufacturing chemicals like DDT and PCBs, which were dumped in the California coast years ago, and a previously unknown herpes virus to the high prevalence of urogenital cancer among wild California sea lions. 


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Decades ago, scientists began detecting the cancer among sea lions as barrels of industrial trash, pesticides and oil refinery waste were dumped along the coast. According to the study, the prevalence of such cancer in California sea lions is one of the highest amongst mammals, with 18 to 23 percent of adult sea lions examined after death over the past 40 years having urogenital carcinoma. 

The cancer is typically found in the kidney, bladder and prostate among mammals. 

Over 20 years, the Marine Mammal Center in Sausalito, Calif., catalogued and analyzed tissue samples from 394 sea lions and were able to rule out inbreeding and other possible causes of the cancer. Researchers have now discovered the herpes virus was triggering the cancer, and sea lions with higher concentrations of DDT, PCBs and other toxic chemicals in their blubber were more likely to have cancer take over their bodies. 

“It is extraordinary, the level of pollutants in these animals in California. It is a big factor in why we’re seeing this level of cancer,” Pádraig Duignan, chief pathologist at the Marine Mammal Center and co-author of the study, told The Los Angeles Times

“With all the dumping since the Second World War, right up to the 1970s, that’s a lot of stuff out there,” Duignan said. 

Researchers noted that the U.S.’s largest DDT manufacturer dumped waste near the Channel Islands where a large number of California sea lions breed and feed their pups. 

DDT was used as an insecticide until it was banned in the U.S. due to its adverse effects on the environment and potential human health risks. The chemical was the subject of Rachel Carson’s landmark book "Silent Spring," which drew public attention to the damage of pesticides on endangered and threatened species.


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Published on Feb 02, 2021