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CORVALLIS - An interim report on the cause of fish deformities in a portion of the Willamette River south of Portland concludes that parasites are the primary cause of the problem, rather than toxic pesticides, heavy metals, other chemicals or organic pollutants.

The study, compiled by a group of researchers at Oregon State University and just presented to the Oregon Watershed Enhancement Board, should be considered good news for communities in the region that have expressed concern that the fish deformities reflect risks to human health, scientists say.

"In a very thorough analysis of 15,000 fish we've now considered almost all of the possible causes for these skeletal deformities that are found at a high rate in this portion of the river," said Larry Curtis, professor and head of the OSU Department of Environmental and Molecular Toxicology. "We studied physical conditions and chemical pollutants, including heavy metals, organophosphates and dioxins - a very wide range of things.

"At this point it's clear that virtually all of these deformities are being caused by two parasites, and we've demonstrated in laboratory studies how the deformities occur," Curtis said. "The parasites, which for some reason are more prevalent in this part of the river, pose little threat to human health, and they are destroyed either by freezing or cooking of infected fish."

The research was done in the Newberg Pool, a deep, slow-moving stretch of the Willamette River between Newberg and Oregon City, southwest of Portland. In the 1990s, researchers observed that fish deformities in that section of the river were far higher than in some other areas, with more than 50 percent of some species showing lesions and deformities.

Concerns about the possible causes of these deformities prompted the three-year, $500,000 study that is nearing completion at OSU, which included an interdisciplinary team of fisheries experts, microbiologists, environmental chemists and toxicologists from the OSU Agricultural Experiment Station. Other co-authors included Michael Kent, director of the Center for Fish Disease Research at OSU, and Douglas Markle, a professor of fisheries and wildlife.

Two species of minnow, the chisel mouth chub and northern pike minnow, have most often suffered skeletal deformities, researchers have found. The study revealed that most of the deformities are caused by a fluke, Apophallus donicus, which at various life stages passes through snails, fish, and fish-eating birds or mammals, and in one stage is a very tiny worm that can infect fish. A lesser number of fish are affected by a myxozoan similar to Myxobolus cyprini.

In fish, the flukes can penetrate to the bone, form a shell or "cyst," and in the process cause spinal deformities and unusual bone growth. They can be lethal, especially in very young fish. The Myxozoan is a relative of the parasite that causes whirling disease in salmon, and is apparently native to the Pacific Northwest. Bone deformities that resemble those caused by this parasite have been identified in museum specimens from the 1850s.

"There's some evidence that the levels of these parasites increased in the 1950s, but it's not definitive," Markle said. "It's also not clear just what environmental conditions in the Newberg Pool favor higher levels of the parasites. But the deformities they can cause are now very clear. Some fish that are born in the middle of the spawning season when fluke levels are highest have almost a 100 percent incidence of infection and deformities."

According to Kent, the fluke parasite could, in theory, infect people as well if they ate raw fish. Pets such as dogs might also be vulnerable. But in humans the parasite would only cause gastrointestinal upset, he said, and in any case any problems can be prevented by either freezing or cooking of infected fish. Consumption of fresh raw fish from the Willamette River is not advisable, he said.

Exhaustive research on other possible causes of this problem showed no other concerns, Curtis said. In comparing water from the Newberg Pool to other sites, the study found no significant difference in levels of heavy metals, dioxins, or other "legacy" pollutants from the past such as DDT or PCBs, he said, all of which were present but at extremely low levels. The data has not yet been finalized on organophosphates.

"The Willamette is a big river and there's a lot of water moving through it," Curtis said. "There actually is a huge dilution factor that tends to give most lower parts of the river very similar levels of water quality, other than a superfund site in Portland harbor."

Research will continue in efforts to determine why the Newberg Pool has higher levels of these parasites and the deformities they cause, the scientists said.