Since the early 1970s, biologists in the United States have been tracking the rise of a mysterious cancer plaguing sea lions. Now, a recent necropsy has revealed the disease may be affecting more species than we thought. 

In what appears to be the first such case on record, a carcinoma – a cancer that originates in the skin and the lining of internal organs – has been confirmed in a Steller sea lion (Eumetopias jubatus). The 15-year-old male was euthanised back in January after stranding on a Washington beach, and a post-mortem revealed tumours in the animal's penal shaft.

While the cancer was not the cause behind the stranding, the find is still troubling local scientists.

"This animal was geriatric," officials said in a summary of the necropsy results. "He had acute bronchopneumonia, severe fibrosis of lung and liver and also age-related changes in the brain. He was dying of old age. While [cancer] is an incidental finding in this case, it does have important ramifications for the overall population."

Nearly a quarter of the closely related California sea lions (Zalophus californianus) that wash up along the Pacific coast are affected by the disease, so information gained from this necropsy could be helpful in mapping its spread.  

We've already begun to crack the case on its origins. Recent research conducted in California points to a viral culprit: otarine herpevirus-1. (Despite some comments online, that means Fukushima radiation is not a likely contributing factor. More on that here). 

We don't know exactly how the virus triggers cancer, but tumour-stricken California sea lions are known herpesvirus carriers. The tumours originate in the cervix or penis – and while these genital growths aren't life-threatening, they can metastasise to the lymph nodes and spine. At that stage, the disease causes partial paralysis, which inevitably prevents sea lions from being able to swim and hunt. 

"There's just masses of yellow, cancerous tissue [in these animals]," Marine Mammal Center senior scientist Dr Frances Gulland told bioGraphic Magazine earlier this year. "When you see a sea lion coming in with cancer and they're in pain... it's not only distressing but it also compels you to understand why it's happening."

Herpesvirus can be transmitted sexually in sea lions, but it's also possible that mere close contact between sick and healthy animals allows it to "jump" between individuals. Figuring out why certain populations are more susceptible than others could be the first step in solving this puzzle.

Gulland's analysis of infected California sea lions revealed a troubling potential link: their blubber contained higher-than-normal levels of pollutants like DDT. In whales and other marine mammals, the pesticide is known to affect the immune system. It's possible that sea lions feeding in areas where water quality is compromised become infected more easily.

At this stage, however, it's too too early to draw conclusions, and other factors – like genetic bottlenecking caused by inbreeding – could be at play.

There's no doubt that curbing the spread of the virus is going to be a tricky task, but each case scientists are able to examine is an opportunity to learn more about the workings of the disease.


Top header image: Mick Thompson/Flickr