Marin research sheds light on sea lion cancer mystery
Many of the sick animals seen by Marine Mammal Center
Researchers say they have solved a three-decade mystery as to why so many California sea lions brought to the Marine Mammal Center in the Marin Headlands are dying of an aggressive cancer.
The findings recently published in the scientific journal Animals concluded that the sexually transmitted herpes virus was causing the high rates of cancer among female sea lions. About one of four dead sea lions examined by the Marine Mammal Center was found to have died of the cancer known as sea lion urogenital carcinoma, which aggressively spreads through the affected sea lion.
Dr. Alissa Deming, director of clinical medicine at the Laguna Beach-based Pacific Marine Mammal Center and the study’s lead author, said the findings could help researchers better understand how viruses trigger cancer development in other species including humans.
“Now that we know that this is a virally-induced cancer, we can use them as a model for studying that in all species,” Deming said on Tuesday.
Many cancers are triggered by viral infections in combination with other factors, Deming said. The human papillomavirus, known as HPV, is known to cause cervical cancer in women, especially in combination with other factors such as a history of smoking.
Researchers with the two mammal centers found in a study published earlier this year that contaminants such as the pesticides DDT and PCB were contributing factors in sea lions developing this cancer, especially in southern California. Now researchers plan to study how the virus and these other chemicals work to trigger cancers in some sea lions but not others.
Researchers at the Marine Mammal Center first diagnosed the cancer in 1979, but did not know the underlying causes
and why it so acutely affected California sea lions. Between 18% and 23% of all sea lions admitted to the center’s hospital at the Marin Headlands between 2002 and 2017 died of the cancer, according to the study.
Study coauthor Dr. Pádraig Duignan, director of pathology at the Marine Mammal Center, said the “cancer begins in the sea lion’s genital tract and aggressively spreads throughout the sea lion’s body, resulting in death, often from kidney failure.”
Often the center’s hospital only receives stranded sea lions that are already at the end stages of the cancer.
At that point, these sea lions are like “swimming tumors,” Deming said.
“It’s quite incredible to see how much it’s taken over these animals’ bodies,” Deming said. “Their lungs just have no normal lung tissue left and their livers are just riddled with tumors.”
What’s also uncommon is the high rates of the cancer among younger sea lions of about 5 to 7 years old when they begin breeding. For other animals, cancers typically develop when the organism ages and its immune system becomes compromised.
“It’s kind of an abnormal thing and has pretty serious population-level impact if you’re taking out a younger, just-starting-to-reproduce population,” Deming said.
The researchers tested
for the herpes virus known as OtHV1 in tissues from 95 sea lions that had developed the cancer and 163 sea lions without the cancer. All of the cancer-afflicted animals had the virus, while only 36% of the healthy animals did.
Additionally, the cancerous sea lions had “exceptionally high” levels of the virus that were highly active in genital tumors, according to the study. In contrast, healthy sea lions showed no detectable levels of viral gene expression, which suggested the virus was dormant, according to the study.
Although the cancer was first recognized in the late 1970s, it wasn’t until the 1990s when Marine Mammal Center researchers such as Frances Gulland began collecting more data on these samples. The combination
of nearly three decades of sea lion tissue samples along with the advancement of DNA sequencing technology, especially in this past decade, allowed for the discovery of the link between herpes and the sea lion deaths, Deming said.
While the findings likely won’t allow veterinarians at the marine mammal centers to cure infected sea lions, they could hold the secret on how to block the aggressive spread of the cancer in other animals, Deming said.
“It really gives us an opportunity to learn from a bad situation that we really can’t do anything about to help future animals and potentially even come up with some treatments of animals in managed care like aquariums that develop this disease,” she said.