Thirty years ago, a Canadian marine biologist noticed something mysterious was happening to beluga whales in the St. Lawrence Estuary. Decades of over-hunting had decimated the population, but several years after the government put a stop to the practice, the belugas still hadn’t recovered.
Two decades and hundreds of carcasses later, he had an answer.
“They were dying of cancer,” said Daniel Martineau, now a professor of pathology at the University of Montreal.
The white whales were victims of intestinal cancers caused by industrial pollutants released into the St. Lawrence River by nearby aluminum smelters.
Now research points to environmental pollutants as the cause of deadly cancers in several wildlife populations around the world. Normally rare in wildlife, cancers in California sea lions, North Sea flounder and Great Lakes catfish seem to have been triggered or accelerated by environmental contaminants.
Other animals, including Tasmanian Devils, sea turtles, woodchucks, eels and sperm whales, also have been stricken with cancers, although they appear to stem from natural causes, including viruses, spontaneous tumors, or genetic factors.
In some cases, the survival of a species and the stability and biodiversity of an ecosystem is jeopardized. The cancers also highlight the dangers that industrial activities pose – not just to animals, but to people in the same areas, exposed to the same compounds.
“We know that toxic compounds in the environment can cause cancer in humans, so it's not a far stretch to realize that pollutants can cause cancer in animals,” says Denise McAloose, a pathologist with the Wildlife Conservation Society in New York, who recently reviewed the topic in the journal Nature Reviews Cancer.
Animals have long been recognized as sentinels for human health hazards. Wildlife populations, such as the belugas, often interact with the same pollutants as people.
In the St. Lawrence region of Quebec, people who worked in smelters near the cancer-stricken belugas have reported many cases of lung and bladder cancers linked to coal tar exposure at the factories. Other residents have high rates of digestive tract and breast cancers.
Scientists say careful monitoring of wildlife populations can reveal cancer patterns that could send early warning signals to people. While human cancers arising from pollutants can take decades to appear, wildlife diseases often show up earlier.
Nevertheless, few resources have been dedicated to identifying wildlife cancers. Most cases go undetected.
Obstacles such as high altitudes or deep waters make monitoring and collecting sick animals difficult, and carcasses are often decomposed or destroyed by scavengers before researchers can collect them.
“Cancer, overall, is very infrequent in animals,” apparently less frequent than in humans, said Carol Meteyer, a wildlife pathologist with the National Wildlife Health Center in Madison, Wis.
Meteyer said the shorter lifespan of birds and small mammals means fewer tumors than in people, although there is little data estimating the prevalence. In the past 34 years, the center has examined over 100,000 wild animals. Only 22 had tumors, and cancer killed only a handful of them—a death rate about 5,000 times lower than that of human beings.
Even when sick animals are identified, it can be difficult to link their cancers to environmental causes. Tying tumors to specific pollutants is “very challenging,” Meteyer said, because of the small number of cases and the wide geographic range of many animals.
Many tumors are spontaneous, arising from a “wild cell type that takes off on its own,” she said. Most of the cancer cases she’s seen in her 17 years at the center involved spontaneous tumors.
“Only certain tumors can be indicators of environmental contamination and ecosystem health,” McAloose said.
Despite the obstacles, identifying animals at risk of cancer is essential for protecting these populations and their human counterparts, she said.
Some persistent organic pollutants are implicated in wildlife cancer clusters. These pollutants, including PCBs (polychlorinated biphenyls) and the pesticide DDT, build up in the environment and accumulate in the fatty tissues of wildlife.
Called POPs, these compounds contribute to cancers in a variety of ways. Often they interact directly with an animal’s DNA by disrupting its structure and leading to mistakes in replication. These mistakes accumulate over the animal’s lifetime, leading to tumors and, possibly, death.
In other cases, the chemicals attach to DNA and turn genes on or off. Pollutants can also contribute to cancers by distracting an animal’s immune system, allowing certain types of viruses to cause tumors.
Flounder from Germany’s contaminated Elbe estuary had higher rates of liver cancer than fish from unpolluted regions, according to a study published last year. Researchers found a link between higher levels of heavy metals and POPs and increased liver lesions in the flounder.
Also, sea lions along California’s Central Coast are dying from a cancer possibly associated with industrial pollutants.
A 2005 study found elevated levels of polychlorinated biphenyls, or PCBs, in the blubber of adult sea lions with reproductive tract cancers. Those with cancer had PCB levels 85 percent higher than those without cancer. One weakness of the study, however, is that sick or dead marine mammals often have higher contaminant concentrations in their bodies because they have less fat.