Tumors and lesions pock the flesh of schools of otherwise healthy fish skirting through the murky waters of the Elizabeth River outside Norfolk.

Under ordinary circumstances, during a process called phagocytosis, the millions of macrophages circulating in the immune systems of these fish would have engulfed and neutralized the bacteria causing these infections.

But for reasons that both interest and concern toxicologist Steven Holladay, the fishes' immune systems are impaired, leaving them susceptible to opportunistic infections caused by routine bacterial agents.

"The immune system appears to be among the organ systems most sensitive to a variety of environmental contaminants," says Holladay, a immuno-toxicologist at work in the Virginia-Maryland Regional College of Veterinary Medicine.

He speculates that the immune suppression plaguing the fish in the Elizabeth River may result from wastes released into the water by nearby industrial operations. A likely culprit: polycyclic aromatic hydrocarbons, ubiquitous, highly toxic compounds associated with petroleum and asphalt products, cigarette smoke, and internal combustion engines.

The work he is doing in the college's Aquatic Medicine Laboratory with fish exposed to the contaminants that are known to exist in high levels in the Elizabeth River is important on two accounts. First, the fish may emerge as a useful non-mammalian laboratory animal for assessing immuno-toxicity.

More important, the fish may be the perfect sentinel for determining how environmental contaminants are affecting animal and human populations.

After all, he reasons, many chemicals in use today eventually end up in aquatic ecosystems.

Advances in chemical engineering are responsible for the increased agricultural productivity and industrial processes that support the nation's high standard of living. But these advances, he says, may have hidden costs. "We produce a tremendous amount of toxic chemicals that we have no idea how to detoxify or effectively contain," he says.

For example, dioxin, a by-product of some chemical manufacturing processes, was defined in 1985 by the American Academy of Clinical Toxicologists as the most toxic synthetic chemical known to man. Dioxin is so potent that EPA regulations limit its presence in industrial wastes to 13 parts per quintillion. A good way to visualize 13 ppq would be to compare one leaf on one tree to every leaf on every tree in North America, he says.

Holladay says, those limits likely should be more stringent. With toxins that potent, "you don't have to put much out there to pollute the planet," he says. For example, measureable dioxin levels can be detected in Beluga whales swimming in the Arctic Ocean, far from the developed world.

Chlorinated hydrocarbons such as poly-chlorinated biphenyls — commonly used as insulators in electric capacitors and transformers and in many other industrial applications — are another class of toxicants that bio-accumulate in living organisms.

"The enzyme systems that exist in nature to break down molecules can handle naturally occurring molecules," says Holladay. However, "to varying degrees, these enzymes are unable to handle the man-made ones."

The extent to which these compounds that are tucked away in distant reaches of the environment and in living tissues affect human and animal health now, and in the future, is an area that requires significant research, he says.

While considerable information exists regarding the carcinogenic effects of some toxic chemicals, little is known about their immunosuppressive effects on humans. By characterizing the diminished immune function in fish exposed to environmental contaminants, Holladay hopes to develop immune function assays to warn of environmental contamination in other species.