It's a beautiful evening on the Missouri River. Rainbows are rising to tricos, caddis and hoppers. The water is warm enough to wet wade and every riffle and pocket is packed with anglers casting toward swirling dimples, hoping to land one of the legendary fish.

Locals and tourists alike try their luck, but more often than not the discerning rainbows and browns turn their noses at the imitations and slurp the naturals. An occasional unsuspecting whitefish may try to score an easy meal; if he does he finds himself in a net with a pair of forceps tugging at a hook from his fat lips. Seconds later he's back where he started - in the river.

Around the state - on the Madison, the Yellowstone, the Big Hole, almost every river and stream - it's a similar scene. Anglers vying for a perfect hole. Trout snubbing their offerings. It's a battle of the wits, which the fish usually win. Unfortunately for anglers, the fish don't win every battle they face. Habitat loss, invasive species, drought and fish diseases take their toll. One fish disease, known as whirling disease, is a particular threat to Montana's wild trout.

This is not breaking news. Whirling disease has been in the United States since at least the 1950s. And it was more than ten years ago when the Madison River made a splash in national headlines, when it was announced that 90 percent of the river's rainbow trout fishery had been decimated and the media predicted the demise of the Montana fishing tradition. Since that time, whirling disease has fallen off the radar and anglers are out in record numbers. It's as if whirling disease never happened or had been miraculously cured. But that's not necessarily the case.

The worst fears about whirling disease did not come to pass, but whirling disease continues to spread. "If anything, it's just as severe a problem as it was in the 1990s," said Dave Kumlien, executive director of the Whirling Disease Foundation, a non-profit organization he co-founded in 1995 in Bozeman to primarily help develop a research plan and promote solid scientific research on whirling. A former fly-shop owner and guide, Kumlien wanted to give something back to the suffering resource that had helped him earn a living. "It's expanded. Whirling disease is in most western Montana watersheds."

But it's easy to forget about the disease because it's mostly invisible to the public, Kumlien added. "There's not a lot of big, dead bodies washing up on shore."

Indeed, the immediate victims of whirling disease are usually the juvenile 1- and 2-inch fish, the ones nobody really sees. The fingerling are highly susceptible to the parasite responsible for whirling disease - Myxobolus cerebralis, a Eurasian native.

M. cerebralis requires two hosts to complete its life cycle. The parasite's spores lie in a stream's sediment, where they are ingested by a tiny worm called Tubifex tubifex. In turn, T. tubifex releases a form of the spore - called TAMs - that are infective to fish. Once the parasite penetrates the fish's skin, it travels to the fish's head and cartilage, where it can cause deformities and death in severe infections. And unlike adult trout, juveniles' skeletal structures are mostly cartilage, which is where TAMs can thrive, thus making the young trout more susceptible to the disease. Adult trout, however, are not immune to the disease. And once the infected fish dies, mature spores are released during decomposition and the entire cycle starts over again.

The whirling disease parasite has been found in 24 states, from New York to California. In some Montana streams severity of the disease was so high that the scale used to measure it had to be redefined. It's clear whirling disease has established its presence in the United States, and unfortunately, as Kumlien put it, "There is no silver bullet for getting rid of the parasite."

There is good news, however. While the parasite may be prolific and indestructible, researchers are discovering new tools to manage the disease and slow down the spread of infection. They are also taking lessons learned to rethink future studies.

"The infection rates may be getting higher, but not everywhere where it's high is it bad for the fish," said Richard Vincent, whirling disease coordinator for Montana Fish, Wildlife and Parks. "The infection rate can be high, but the impact isn't high."

In the Madison River, for instance, some fish may be showing resistance to the disease. According to Vincent, whirling disease coordinator for Montana Fish, Wildlife and Parks, young rainbows are surviving despite high levels of infective TAMs.

"Historically, we saw in the mid '90s a lack of young fish in the population," Vincent said. "Things started to change in the late '90s. [1998] and '99 were higher water years; we had above average spring runoff. It diluted the infective TAMs, and infection rates dropped off significantly."

Whirling disease infection rates are measured on a scale of 0 to 5, with 5 being the highest rate of infection. Rates of 2.8 and higher usually show severe consequences; rates under that are usually benign. In the Madison, the rate dropped into the low 2s during the high water years, Vincent said, which explains why there were good years for young-of-the-year survival. But, predictably, once the water level returned to average in 2005, the infection rate rose to the mid 4s. But something mysterious happened.

"There was good recruitment," Vincent said, meaning the fingerling that would usually be highly susceptible to the disease survived. Was this a sign of whirling disease resistance?

To find out, biologists took some of the Madison rainbows' eggs to a lab near Pony. They hatched the eggs, raised the fish to 60 days of age and then artificially exposed them to high levels of whirling disease. The fishes' infection rate was in the 1s. In a similar study conducted in 1998, the exposed fish had infection rates in the 4s, Vincent said.

"We don't know why that happened," Vincent said. "We have a lot more young fish 6 to 10 inches in length than we did during the peak of whirling disease. We've never witnessed anything like it in any other place in the U.S. We're looking into why."

Another mystery unfolding on the Madison is why some of these same trout that seem to be exhibiting resistance to whirling disease aren't surviving the winters between their second and third years of life.

"We don't know if it has anything to do with whirling disease," Vincent said.

Elsewhere, research is having positive results in managing the disease.

There is continuing research with a strain of trout called the Hofer rainbow trout. This fish, which has been cultivated for more than 100 years in a German hatchery, exhibits high levels of resistance to whirling disease. One theory is that the Hofer strain, because it was developed in an area where whirling disease has existed for some time, has developed a natural resistance to it, similar to the reason brown trout are not as susceptible to the parasite.

In Colorado, fisheries biologists are crossing wild trout with the Hofer strain and are testing their descendants for survival and angler performance. Pilot projects are under way to introduce these crosses into the North Gunnison River.

"In Montana, we're not interested in the Hofer," Kumlien said. "We manage for wild trout and everyone is extremely sensitive to the wild trout population."

In order to help manage wild trout fisheries in the presence of the whirling disease parasite, the Whirling Disease Initiative was established by an Act of Congress in 1997. The Initiative, administered by the Montana Water Center and housed on the Montana State University campus in Bozeman, funds and conducts whirling disease research.

According to , several grants have just been awarded for new studies in the coming year. Since its inception, the Initiative has funded more than 130 projects and through those studies has been able to answer some of the major biological questions about whirling disease, Stromberg said. The fact that there is no effective vaccine or medical treatments for the disease is one of those answers. Also, studies have shown that the optimum temperature for TAM production is between 10 and 15 degrees Celsius, which suggests that temperature manipulation might be helpful in preventing the spread of the parasite.

"Now we're starting to answer big management questions," she added.

For the 2006-2007 funding cycle, the Initiative selected three new projects, and three continuing projects. Dr. Billie Kerans of MSU and her team will study the specific links between spatial and temporal patterns and the spread of whirling disease. The result will be a Montana-based statewide whirling disease risk assessment. A second study team comprised of researchers from the New Mexico, Arizona, Utah and Colorado Cooperative Fish and Wildlife Research Units, will study the prevalence of the disease in the southwest and how landscape patterns and environmental factors impact the transmission of the parasite. Several rare trout species will be involved in the study including the Gila trout, the Apache trout and the Rio Grande cutthroat. The third study, which will be conducted by researchers at the University of California-Davis, will take a look at the different strains of T. tubifex worms and the effects of substrate. It's thought that not all sub-species of T. tubifex worms produce equal numbers of the parasite. The study will try to determine why.

In her office at the Initiative, Stromberg scrolls through a photo archive of infected fish. Some have lumps on their heads, some have crooked tails, some are just an odd color. She's new to the Initiative, but tackles her job with youthful enthusiasm and a master's degree from the University of Idaho in fisheries.

"It's not pretty," she says while watching her computer screen. "People forgot whirling disease is a problem. It's time to get them interested in it again."