Colorado State University Study Confirms New Test More Effectively Detects Whirling Disease In Trout
Monday, July 7, 1997
FORT COLLINS--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."
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