Cancer in wild animals may be triggered or accelerated by environmental contaminants, some researchers argue
Crystal Gammon and Environmental Health News, Scientific American 27 Aug 09;
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.
The carcinomas in California sea lions are caused by a herpes virus. It’s unclear how PCBs may contribute to the cancer, but researchers speculate they may suppress their immune systems, allowing the herpes virus to replicate unchecked. Previous research showed that PCBs in fish destroyed the immune cells of another marine mammal – harbour seals – and contributed to a European die-off from a distemper-like virus.
Although the United States banned PCB production in 1979, PCBs are still found in electrical equipment, and they sometimes leak into the air or water.
PCB levels along the California coast will likely pose a threat to the sea lions for decades, wildlife experts say.
“Mothers dump their contaminant loads to their first born pups,” said Gina Ylitalo, a research chemist with the National Oceanic and Atmospheric Administration in Seattle, Washington, who led the study.
Up to 90 percent of the PCBs in a mother’s body can be transferred to her first pup, meaning that PCB loads decrease only slightly from generation to generation. High levels of PCBs are also passed to pups through milk.
While these cancers haven’t impacted the overall number sea lions – the population has grown steadily by about six percent each year – they suggest that people might also be exposed to dangerous levels of pollutants from consuming the same fish. In California, state officials warn anglers against eating some fish caught in San Francisco Bay and in waters off the Los Angeles area because of the cancer risk posed by PCBs and DDT.
In Ohio’s Black River in the 1980s, brown bullhead catfish were nearly wiped out by liver cancers caused by contaminants from a coking facility. The population rebounded within four years of the facility closing in 1983.
Belugas in the St. Lawrence Estuary have drawn the most attention because of the estuary’s proximity to aluminum smelters. The smelters released 20 tons of polycyclic aromatic hydrocarbons into nearby waters every year. One of the substances, benzo(a)pyrene, which is classified as a probable human carcinogen, accounted for nearly a ton of the smelters’ yearly emissions.
The compounds accumulated in sediments and were absorbed by mussels and other invertebrates, which are the main food source of the one-ton whales. One study found that blue mussels transplanted into the estuary increased their benzo(a)pyrene levels 200-fold.
When Martineau and his group began analyzing beluga carcasses in the early 1980s, they noticed that many of the whales had intestinal tumors. Over the next 20 years, the group found cancer to be the major cause of death in adult belugas—a surprising finding given the rarity of the disease in wildlife.
In particular, small-intestinal tumors seemed to be especially prevalent and deadly for the animals: 27 percent had died of cancer, and 30 percent of the cancers were found in their small intestines. Colon cancer is common in humans and other animals, but small-intestinal cancers are relatively rare.
The 27 percent rate of cancer deaths for the estuary’s belugas is similar to the 23 percent rate for humans in the Western world, Martineau noted. McAloose called that similarity “very interesting … Similar diseases caused by similar circumstances often have similar outcomes.”
In 2004, two years after the beluga study was published, the aluminum smelters near the St. Lawrence estuary closed.
But, five years later, the belugas that first caught Martineau’s attention have not recovered. And he is not surprised.
“Cancer is the consequence of a lifetime of accumulating mutations,” said Martineau, who added that the deadly disease “is exactly what you would expect to find in animals that are eating from these sediments.”
The beluga population, he suspects, won’t begin to recover for at least half a lifetime – 35 years, in the case of these long-lived whales. Fewer than 1,000 belugas, which are listed as a threatened species in Canada, remain in the estuary.
Researchers like Martineau and McAloose continue to stress the importance of studying wildlife diseases driven by pollution. Developmental disorders and reproductive problems in animals may also be linked to industrial pollutants and other contaminants.
“Cancer may just be the easiest endpoint to get our hands on,” McAloose said. “We need to continue try to see connections between pollutants and disease, but currently there just aren’t a lot of people looking.”
This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.
