Defense Systems Under Fire - Strong evidence that certain toxic pollutants make wildlife more vulnerable to disease may also apply to people

National Wildlife, Oct-Nov, 2000 by Peter Jaret

THE MASSIVE DIE-OFF was one of the worst marine biologists had ever seen. Over the course of seven months in 1988, almost 20,000 harbor seals in the Baltic and North Seas perished mysteriously-half the population thought to live in northern Europe's waters. Researchers quickly identified a previously unknown virus, called morbillivirus, that had infected the doomed seals. For a time, it seemed, the case was closed.

Then, within a year, a new mystery emerged. Seals in Canada had also been infected with the virus, scientists discovered, but none of the animals had died. Searching for clues, researchers noticed that the dead seals in northern Europe tended to be from colonies in areas known to be heavily contaminated with toxic pollutants.

Many of the animals had unusually high levels of substances like polychlorinated biphenyls, or PCBs, in their fatty tissues-chemicals that are known to suppress the immune system in fish and laboratory animals such as mice. So scientists thought these substances may have made the animals more vulnerable to infection. Still, so far there was only circumstantial evidence.

To test whether contaminants actually were implicated in the seals' deaths, research scientist Peter Ross of the Institute of Ocean Sciences in British Columbia began a simple experiment. In the early 1990s, he collected seal pups from an uncontaminated area in Scotland. Then he fed

one group of the pups herring from the Baltic Sea and a second group herring from Atlantic waters known to have very low pollution levels. After two years, seals eating the Baltic fish showed signs of dramatically weakened immune systems-a strong indication that pollution may have played a role in the massive die-off.

Beyond cancer: Today immunotoxicologists have evidence that contaminants widely dispersed in the environment are weakening the immune defenses of a wide variety of wildlife species around the world- from birds, to marine mammals, even to rats and cockroaches in particularly contaminated sites. The new findings are part of a growing understanding that the threats from such pollution can go way beyond cancer, the old standard for toxicity. And many experts, including Environmental Protection Agency scientists, are studying how toxic substances may impair animals, possibly including humans, in ways that range from fertility problems to neurological defects. Among the most troubling effects are compromised immune systems.

"What frightens me is that this could be a health time bomb," says immunologist David Ferrick of the Laboratory for Marine Mammal Immunology at the University of California at Davis. "The chronic accumulation of these chemicals may be making these populations more and more vulnerable until a pathogen or a change in the environment comes along and creates another massive die-off. And once that happens, it may be too late to do much about it."

Die-offs have continued to occur around the world, some of them linked at least in part to immune-suppressing contaminants in the environment. In 1990, morbillivirus struck again, killing more than 1,000 striped dolphins in polluted areas off the coasts of Spain, France and Greece. In 1994, bottlenose dolphins in heavily contaminated areas off the Texas Gulf Coast died of the same virus in record numbers. "And no doubt there are die-offs that we don't observe, that occur far out at sea," says Ferrick.

The potential threat goes well beyond these dramatic deaths, "We're also concerned about the more subtle effects of these chemicals, which may be making animals chronically sick, making them less able to fight off infections or less likely to reproduce," says Judith Zelikoff, an immunotoxicologist at New York University School of Medicine and a leading expert on the effects of pollution on fish.

Weakening the body's defenses: Among the substances known to be immunotoxic are PCBs, certain pesticides, mercury and a range of dioxinlike substances. Many of these chemicals can linger in the environment for decades and can spread to seemingly pristine regions of the world with remarkable ease. Some have been found in wildlife in both the Arctic and Antarctica.

"Even at very low doses-a single exposure of no more than one millionth of a gram, for instance-some of these chemicals have been shown to disrupt immune function," says immunologist Steve Holladay of the College of Veterinary Medicine at Virginia Tech. "And we know that many of these chemicals have made their way into wildlife, sometimes at astonishingly high concentrations." Last year, for example, a team led by research scientist Ross found high levels of PCBs in 47 killer whales roaming the Pacific waters off the U.S. coast. Transient males averaged 251 parts per million-400 to 500 times more than levels in humans.

Killer whales, Ross thinks, are now the most contaminated mammals in the world. But plenty of other animals also carry heavy burdens of these chemicals in their tissues, including beluga whales in the St. Lawrence River, fish, herring gulls, cormorants, seals, dolphins and sea lions. Marine life in many coastal locations is at high risk because many of the contaminants wash into the sea from rivers and runoff.