Scientists Are Concerned About Cancer in Animals

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's Honolulu 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.

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.

"The more we contaminate the environment, the more we will see problems," Gulland says. "If you dump a pollutant, it doesn't just go away."

In the icy waters of Canada's St. Lawrence Estuary, where researchers have been studying the local population of Beluga whales for decades, the connection between contamination and cancer is stark. The Saguenay River, which flows into the St. Lawrence Estuary, is lined with the smoking stacks of aluminum smelters, heavy producers of polycyclic aromatic hydrocarbons (PAHs). PAHs are toxic compounds formed by the incomplete burning of anything carbon based, and are long-proven carcinogens in both man and beast. Not surprisingly, the humans working the smelters along the river have shown high rates of lung cancer, while those in the vicinity who drank from taps supplied with surface water developed stomach and intestinal cancer.

Out in the estuary, researchers note a similar trend among the dead Belugas found beached or drifting out to sea. Research published in a 2002 edition of Environmental Health Perspectives found that the second leading cause of death (in 18 percent of the Beluga carcasses) was cancer, largely of the gut. That fits with the animal's feeding patterns. Belugas run their wide mouths along the sandy estuary floor, eating invertebrates like the blue mussel. The concentration of the PAH benzo[a]pyrene was 200 times higher in blue mussels in the Sanguenay River portion of the Beluga habitat than in adjacent areas.

The Belugas of Quebec are considered an endangered species, brought down by environmental pollution. The devils of Tasmania are similarly at risk, but for much different reasons.

A contagious cancer and the Tasmanian devil's penchant for fighting may be the species' death knell. In the 1990s the devil population was estimated at 150,000; by 1996 the population had been halved. The killer: devil facial tumor disease (DFTD), which affects 65 percent of the remaining population and has an unflinching mortality rate of 100 percent.

Devils, the largest marsupial carnivore, are notoriously territorial and temperamental. During their riotous, squeal-infused battles, they bite and scratch each other's faces, which dislodge microscopic bits of cancerous tissue in infected animals. Those bits are thought to seed the skin of the uninfected devils through a rare process called allograft transmission. The result is a fast-metastasizing cancer that attacks the face and neck and quickly spreads into the lymph nodes and lungs.

"Animals with large tumors can be severely disfigured, with complete erosion of the jaw and eye sockets," says Hamish McCallum, head of the Griffith School of Environment in Australia and a lead researcher of DFTD. "The tumors are an angry red color."

Researchers attribute the cancer partly to a loss of genetic diversity, thanks to the animals limited, island habitat. In the "multi-hit theory" of carcinogenesis such a lack of diversity could be one of many reasons for the cancer's aberrant and uncontrollable cell reproduction, along with other factors like viruses and, yes, the presence of man-made contaminants. The only recourse is drastic mitigation: culling the infected, isolating the healthy and hoping for a vaccine.

The good news for animals suffering from pollution-induced cancer is that when contaminants are taken out of the environment, some species have shown marked drops in carcinogenesis. For example, catfish living in Ohio's Black River had cancer rates ranging from 22 percent to 39 percent in the early 1980s. The disease killed virtually all fish before they reached five years of age. After a steel plant's coking facility closed in 1983, PAH levels dropped significantly; cancer rates dropped 75 percent and the amount of fish living past five years has tripled.

Despite these spectacular and telling cases, animal cancer is still largely a mystery.

A key reason for the dearth of scrutiny is biologists' difficulty in performing animal autopsies before the jackals, crows and maggots destroy the carcasses. "When animals die they simply disappear," says William Karesh, director of WCS's Global Health Program. When specimens are found, cancer ranks much lower in mortality rates than the trauma, predation or infectious diseases so rife in the wild.

McAloose, like many wildlife pathologists, sees the study of the intersection between the environment and cancer in wild animals as a clear way to better understanding how cancer affects humans. "Now we need collaboration and cooperation across conservation organizations, public-health communities, as well as governments to make changes that have positive outcomes for animals and the planet," she says. "Because that will have positive impacts on the human population."

For the turtles of Hawaii, which like California's sea lions suffer from tumors linked to a herpes-associated virus, such collaboration may someday mean relief.

In Molokai, Thierry Work had little choice. "With such heavily tumored animals, surgery would be like butchering the turtles alive," Work says. Instead he euthanized the turtle, and like he has done with hundreds of turtles since, he conducted a necropsy, hopefully bringing scientists one step closer to figuring out the riddle of cancer—animal and human.