One of the most promising developments, they say, is the discovery of wild rainbow trout that are naturally resistant to whirling disease. Another is the mysterious rebound of rainbow trout in the Madison River, the first Montana river where whirling disease was discovered.
"There's hope," said Montana State University ecologist Billie Kerans. "There's some hope for the trout in Montana. Not all drainages have responded the same way to whirling disease."
Dick Vincent, one of her longtime collaborators and recently retired as whirling disease coordinator for the Montana Department of Fish, Wildlife and Parks, said, "The worst case scenario was drawn and didn't happen in Montana."
The biggest surprise is that the rainbow trout population has started to rebound in the Madison River, Vincent said. He first noticed around 1991 that young rainbow trout populations were showing large declines in the upper Madison River. By 1994, they had fallen by 90 percent. Now, despite a high rate of infection and significant inbreeding, the rainbow trout population in the upper Madison is 60 to 70 percent of what it was before it started to crash.
He doesn't know of any other stream in the Intermountain West that has recovered on its own, Vincent said.
"Clearly, we got lucky," he commented. "If it hadn't been for resistance, it wouldn't be looking like it did. Who would have guessed?"
Four years ago, Vincent announced the discovery of rainbow trout that were somewhat resistant to whirling disease.
"They are not absolutely resistant, but they are significantly resistant," Vincent said recently.
The fish live in the Willow Creek Reservoir, three miles east of Harrison in Madison County. He thinks the fish somehow made it into the Madison River, but no one intentionally planted them there, Vincent said. He added that some of the Willow Creek fish were stocked in Hebgen Lake and Ennis Lake, so it's possible that some of the fish floated over the spillways into the Madison. Ancestors of the resistant fish came from California via Wyoming.
Researchers don't have a solution to whirling disease yet, but they understand much about it, Vincent continued.
They have learned that whirling disease occurs when a microscopic parasite called Myxobolus cerebralis latches onto the skin of young fish, usually rainbow trout. The parasite works its way into the fish and injects a series of infective spores into the muscle tissue where it migrates to its food source, which is cartilage. The damaged cartilage causes severe deformities in the fish. When the fish's body tries to attack the parasite, it causes inflammation and infection within the fish. If most of the infection is on one side of the fish's body and causes nerve damage, the fish's brain can only communicate with one side of the body. Instead of swimming forward, the fish starts to whirl.
In the process of dying or being eaten, rainbow trout release Myxobolus spores into the water, Vincent continued. Tubifex tubifex worms eat the spores, which can survive in river bottom sediment or dry mud for up to 30 years. Within three to four months of settling in the worm's gut, the spore opens and a new infective version of the parasite emerges. It's called a Triactinomyxon or TAM. When the worms excrete TAMS into the water, the free-swimming TAMS start latching onto fish again. The cycle continues.
Young fish are especially susceptible to whirling disease because, at that point in their development, their skeleton is mostly made up of cartilage instead of bone, Vincent said. Whirling disease flourishes when water ranges between 40 and 68 F. When the temperature reaches about 73.4 F, rainbows actually expunge the spores from their bodies. High water temperatures, in fact, may be the reason that whirling disease hasn't been as much a problem in the lower Madison as the upper Madison, Vincent said. Young fish are especially susceptible to whirling disease when the water temperature averages in the 50s F, especially in their first 90 days of life.
Scientists once predicted that the rainbow trout population in the Missouri River would crash, but it didn't, Vincent said. Not sure why, he said it could be related to the size of the river and the large number of tributaries it has. Some juvenile trout may be safe because they swim in uninfected areas of the river.
For more information about whirling disease in Montana, read "Has whirling disease come full circle?" in the spring 2009 issue of MSU's magazine, "Mountains and Minds." The article is located on the Web at http://www.montana.edu/mountainsandminds/spring2009/whirling/
Evelyn Boswell, (406) 994-5135 or firstname.lastname@example.org