Story at-a-glance:

While cancer is generally rare in wild animals, 18% to 23% of California sea lions have cancer — one of the highest cancer prevalence in mammals.
The sea lions are primarily suffering from urogenital carcinoma (UGC), which has previously been associated with organochlorines such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethanes (DDTs).
Chronic dumping of persistent organic pollutants took place off the California coast prior to their ban.
In the sea lions, the risk of cancer was found to be 43.57 times higher in those infected with the otarine herpesvirus-1 (OtHV1), along with 1.48 times higher for every unit increase in contaminant concentrations in their blubber.
A synergism between the pollutants and later infection with OtHV1 may be driving up cancer rates in the animals, with serious implications for humans.

Animals often act as sentinels for what’s to come — the canaries in the coal mine for a number of potential environmental threats. The dubious sentiment tragically now applies to sea lions in California, which are being struck with cancer at an alarming rate.

About 250,000 California sea lions live along the west coast of North America. These long-lived marine mammals have a lifespan of 20 to 30 years, which gives time for environmental contaminants to accumulate in their bodies.

While cancer is generally rare in wild animals, 25% of California sea lions have cancer — one of the highest cancer prevalence in mammals, according to a team of researchers with the Marine Mammal Center in Sausalito, California. The study’s lead author, veterinarian Dr. Cara Field, called the finding “extremely alarming” and “unprecedented in wildlife.”

Environmental contaminants to blame

When examined post-mortem, it was found that 18% – 23% of the sea lions were primarily suffering from urogenital carcinoma (UGC), which has previously been associated with organochlorines such as PCBs and DDTs, and infection with OtHV-1. A virus, Otarine herpesvirus-1

After conducting post-mortem exams on 394 animals collected over a 20-year period, the risk of cancer was found to be 43.57 times higher in sea lions infected with OtHV-1, along with 1.48 times higher for every unit increase in contaminant concentrations in their blubber. Sadly, the sea lions live in an area that’s heavily polluted. Writing in Frontiers in Marine Science, the researchers explained:

“The central California coast and its food web are exposed to high levels of potentially carcinogenic persistent organochlorines following dumping of industrial waste DDTs (dichlorodiphenyltrichloroethanes) in the 1960s and urbanization and industrialization of the coast increasing run-off of newer chemical contaminants.”

Previous studies have tied pollutants with cancer in marine mammals, such as the beluga, while California sea lions with cancer are known to have higher levels of PCBs and DDTs in their blubber than sea lions without cancer. Higher levels of PCBs were linked to eight times the cancer risk in sea lions while DDTs raised the risk sixfold.

Synergism between virus and pollutants causing cancer

The featured study suggests exposure to OtHV1 in addition to early exposure to pollutants could be driving cancer rates up in sea lions, with potential implications for humans. People in the Huaihe River Basin in China, for example, have a high rate of esophageal carcinoma that is associated with human papillomavirus and exposure to the highly carcinogenic polycyclic aromatic hydrocarbon 3-methylcholanthrene.

Sea lions are exposed to PCBs and DDTs in utero, as the chemicals travel across the placenta, as well as via milk early in life. It’s likely, the researchers suggested, that a synergism between the pollutants and later infection with OtHV1, which is sexually transmitted and localized to the reproductive tract where the cancer is occurring, may be driving up cancer rates in the animals:

“As carcinogenesis is a multistep process requiring multiple mutagenic events and may necessitate replication and fixing of the DNA damage caused by promoters, this study supports the hypothesis that carcinoma in California sea lions is a multifactorial disease …

“ … Thus, as there is often a long latency period between the initial DNA damage and the onset of neoplasia, this study suggests early exposure to persistent organic pollutants, followed by infection with OtHV1, are important in the pathogenesis of UGC in wild sea lions.”