Plague Of The Rainbow Trout

One of America's prime game fish, the rainbow trout, is facing an unprecedented threat: whirling disease, one of the worst plagues to hit any fish in North America. Dr. Karl Johnson, the codiscoverer of the deadly Ebola and hanta viruses and a former chief of the Special Pathogens Branch at the U.S. Centers for Disease Control in Atlanta, calls whirling disease "the AIDS of trout."

The sickness spreads easily and is especially prevalent in the Rocky Mountain states, which are celebrated for having the best trout fishing in the country. The disease has now been found in 21 states, with the worst outbreaks so far in Colorado and Montana.

Also susceptible are the steelhead rainbows, cutthroat trout, Chinook salmon, kokanee salmon and brook trout, all of which are salmonids native to North America. Whirling disease does not, however, affect all salmonid fishes: Coho salmon and lake trout, also native to North America, so far have been resistant. So too are brown trout, an import that originated in Eurasia, where the parasite that causes whirling disease is believed to have evolved. But these resistant species can serve as carriers.

Now the science adviser of the Whirling Disease Foundation in Bozeman, Mont., Johnson, 69, began studying the disease in 1995. The foundation has a $250,000 annual budget for research, funded by private donations. Johnson says that the disease is believed to have reached the U.S. in the 1950s, apparently in infected rainbow trout imported from Denmark. Those fish were descendants of trout sent from the U.S. to Europe roughly a hundred years ago.

Johnson's challenge is formidable. The whirling disease parasite, a microscopic metazoan named Myxobolus cerebralis, has such a bizarre life cycle that until 16 years ago, it was believed to be two species. The primary host is an inch-long aquatic worm, Tubifex tubifex, as many as 10,000 of which can thrive in a square yard of silty bottom. A soft-tissue stage of the parasite emerges from the worm and drifts downstream with what Johnson describes as "a shotgun shell for a head."

The head contains 64 parasites that inflame and destroy the cartilage of juvenile trout before it can turn to bone. The parasites burrow inside the fish through its mouth (which opens to take in oxygen from the water) or skin and travel along the nerve channels to get into the cartilage in the skeleton and head. They chew away at the cartilage and after 60 days make spores. The resulting skeletal deformities and heavy pressure on the fish's organs of equilibrium cause it to swim erratically, unable to feed or avoid predators.

When the trout dies, the heavier-than-water spores sink to the bottom. Even if the spores do not immediately enter a tubifex worm to start the life cycle anew, they can survive for 20 years in the mud. A wading angler who stirs up the bottom can send hundreds of thousands of spores downstream to infect fish. Some spores might also adhere to boots or gear, ready to be liberated in the next body of water in which the angler sets foot—even a continent away. That's cause for concern in Argentina, Chile, New Zealand and the Australian island of Tasmania, all of which got their rainbows from North America.

"We keep having to worry about the affected fish—seeing that hatcheries get cleaned up and that affected fish are not moved into waters that are not yet infected," Johnson says. "If the parasite gains entrance, the cycle is established, and it's going to be there forever unless we can find a way to break it."

What can be done? According to Johnson, a "good" species of tubifex worm—one that doesn't harbor the parasite—might out-compete the present "bad" worm for its niche and break the parasite's life cycle.

More promising is the fact that coho salmon, which are of the same genus as rainbow trout, are resistant to the parasite. "We think the resistance lies in a set of genes known as MHC that turns out to be conserved across fish and higher vertebrates over many millions of years," Johnson says. "These genes dictate whether you or I can exchange organs. They have to do with cellular immunity against viruses, bacteria, parasites and fungi. We want to do studies that isolate and define the differences between susceptible fish and related resistant fish in terms of their critical genes. The technology is out there, but it will take five to 10 years to prove."