The Wild’s Creeping Killer

In 1999, wildlife disease specialist Thierry Work looked over the bow of his small whaler as it cut through a lagoon on the south side of Molokai, an island in Hawaii. On an emergent rock he saw a listless sea turtle, waiting to die.

"This guy was so weak that he just let us pick him up," says Work, who runs the National Wildlife Health Center’sHonolulu field station. "He was so emaciated that his ventral was completely disked in. You could fill him up with water and use him as a bowl." Like more than quarter of the green turtles Work has plucked from the water or found stranded on Hawaii's beaches, this one was covered with tumors on its eyes and mouth, dying from a poorly understood form of cancer.

 

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Work's turtle is one of a wide array of species afflicted by a range of cancers, according to a paper published in the July edition of Nature Reviews Cancer. "Wildlife Cancer: a conservation perspective," summarizes mounting evidence of human's contribution to carcinogenesis in wild-animal populations across the globe, thanks to man-made toxins dumped into wildlife's natural habitats.

"I am concerned that we as humans continue to impact the environment quite significantly," says Denise McAloose, the report's lead author and chief pathologist for the Wildlife Conservations Society's (WCS) Global Health Program. "As the human population continues to grow and utilize resources and damage the environment, I do believe that we will continue to see the emergence of disease, including cancer in wildlife."

On San Francisco's touristy Pier 39, the incessant barking of male sea lions and their harems of smaller females fills the air. Periodically Frances Gulland, the director of veterinary science at the Marine Mammal Center in neighboring Sausalito, receives calls from the pier reporting a sea lion crippled by tumors. Despite huge swelling on and around their hind flippers and anus, Gulland says the animals "mask their pain" in a last ditch effort to elude opportunistic predators. "You see that they have been struggling and struggling."

According to Gulland, 17 percent of the sea lions brought to the center die of renal failure or paralysis, caused when tumors linked to Otarine herpesvirus-1 travel up the genital tact and push against the kidney and spine. According to the Nature Reviews Cancer article, sea lions that died of genital carcinoma had an 85 percent higher concentration of toxic polychlorinated biphenyls (PCBs) in their system than other sea lions. (PCBs are toxic compounds used in coolants and electrical transformers.) Gulland points out that blubber samples of sea lions who died of cancer also show high concentrations of the pesticide dichlorodiphenyltrichloroethane (DDT), in part because many are born near the Channel Islands where 1,700 tons of the toxin were dumped prior to its ban in 1972.