Plague Of The Rainbow Trout Whirling disease is threatening one of the most prized game fish in the U.S.

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
outcompete 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."

That's good news, but bad news, too. With the damage whirling
disease has already caused, 10 more years could be totally
devastating.

COLOR PHOTO: BETH MACCONNELL The parasite destroys the cartilage of the juvenile trout before it can turn to bone. [Less-than-3-inches-long baby trout on tape measure]