A technique used to amplify minute amounts of DNA can detect the parasite that causes whirling disease in trout earlier and more reliably than other available methods, a Colorado State University study has found.
The finding marks an advancement that could help devastated trout populations in major rivers throughout Colorado and in 20 other states.
Colorado State microbiology Professor Bob Ellis applied the polymerase chain reaction test, known as PCR, to DNA isolated from the Myxobolus cerebralis parasite that causes whirling disease. The test, often used in research projects to quickly replicate strands of DNA, flags the presence of the parasite in tissue samples taken from fish prior to being released into Colorado waters.
Studies led by Ellis and funded by the Colorado Division of Wildlife confirm the test can detect the parasitic infection in fish three weeks after exposure. Microscopic tests currently in use typically can’t detect the disease in fish until at least four months, and with high accuracy 10 months, after exposure.
An equally important finding is the test also picks up the parasite in tubifex worms, which live on muddy river bottoms and serve as intermediary hosts to the parasite. With existing tests, the parasite cannot be detected accurately in tubifex worms.
In one study, Ellis applied the DNA test to 29 fingerling trout collected three weeks after being placed in waters testing positive for whirling disease. Samples from the tails and heads of the fish were processed separately and DNA from the whirling disease parasite was isolated and replicated using PCR. Of the 58 samples, three taken from tails tested positive for whirling disease.
"From a sensitivity standpoint, this method is much more effective as a diagnostic tool for whirling disease," Ellis said. "We could take a cross-section of fingerlings from the hatchery and determine if they are infected before they are even released into Colorado rivers. That can’t be done with current methods." By detecting the parasite much earlier and keeping infected trout out of Colorado waters, other species susceptible to infection–such as cutthroat trout–have a better chance for survival, Ellis said. The diagnostic tool also gives fishery biologists more clues that could lead to ways to interrupt the parasite’s complex life cycle.
That cycle begins when an infected trout or salmon dies, triggering a release of spores into the water. Some of these spores fall to the bottom of the river, where they are ingested by tubifex worms. Inside the worms, the spores are converted into a three-pronged form that resembles a pitchfork. They float in the water, attach to a passing trout and inject it with spores.
Once inside, the parasite deforms cartilage, causing the fish to swim erratically. As a result, the fish can no longer find food and becomes a target for predators. An early sign of infection is a black tail, the site where the parasite typically attaches. Because the parasite specifically targets cartilage, young trout with undeveloped skeletons are more susceptible to the disease.
All of the major rivers in Colorado are infected with whirling disease, including the Colorado, Poudre and South Platte rivers. Likely spread initially by infected hatchery trout released into the water, the disease also has appeared in major rivers and streams in 20 other states, including Montana and New York.
Ellis points out that findings from the PCR test, created by scientists at University of California-Davis, are promising but more work is needed before it can be solely used to detect the whirling disease parasite. An area of emphasis is improving the test’s sensitivity on tubifex worms. In the test’s present form, only a few worms can be processed in a sample because the concentration of other worm DNA produces an unreliable reading.
"The results so far have been very promising," Ellis said. "In the future we hope to refine the techniques we are currently using to derive the most ideal way of detecting and preventing the spread of the disease."