Whirling Disease Results: No Resistance Found Among Rainbow Trout

Several years ago, scientists noted that some offspring of fish born after whirling disease was detected in the Colorado River had lower numbers of spores from the parasite that causes the disease.

Whirling disease attacks the cartilage of young fish, causing them to swirl in a circular pattern and fall victim to predators.

One hypothesis was that those fish may have developed some resistance to the disease, said study co-author Al Zale, assistant leader of the Montana Cooperative Fishery Research Unit at Montana State University in Bozeman.

But a closer look in the laboratory offered no such proof, Zale said.

Scientists saw no difference in the degree of infection among the three groups of fish tested: offspring of adults born before whirling disease was found in the Colorado River; fish whose parents were born after the disease was found; and a control group of domestic rainbows. Scientists analyzed such indicators as spore counts, microscopic disease lesions and swimming performance. Zale said he wasn't surprised by the results.

"We have never seen a fish rapidly develop resistance to a novel disease--ever," he said. By rapidly he means within a few generations.

"There may well be resistance eventually but we're not seeing evidence of it yet. It's something that needs to be looked at again," he added, probably in a few years.

Brown trout are resistant because they co-evolved with the disease parasite over many generations in Northern Europe, far longer than whirling disease has been present in North American rainbow and cutthroat fisheries, Zale said.

The study, which also involved MSU graduate student Eileen Ryce and R. Barry Nehring of the Colorado Division of Wildlife, was published in the Journal of Aquatic Animal Health (12:63-68, 2001) in March.

The study was funded by the U.S. Fish and Wildlife Service through the Whirling Disease Initiative of the National Partnership on Management of Wild and Native Coldwater Fisheries.