Sidebar by Ed Quillen
Whirling Disease – October 1999 – Colorado Central Magazine
Whirling Disease gets its name from its effect on fingerling trout. If they’re infected, they tend to swim in circles, like puppies chasing their tails — except it isn’t cute when trout do it.
WD most often affects trout when they’re young, less than four inches long. It attacks their soft cartilage before it hardens into bone, causing skeletal deformities and nervous-system damage (thus the whirling), and sometimes death.
A parasitic amoeba (Myxobolus cerebralis) causes WD when it invades a trout. The amoeba spreads through tiny spores — 4 million can fit on the head of pin — that are quite hardy.
When an infected trout dies, the spores are spread in the water, where they sink and are ingested by WD’s alternate host, a common bottom-dwelling tubifex worm. Spores incubate inside the worm’s gut and multiply rapidly.
As spores are released from the worm, they can infect fish by attaching to their bodies, or when fish eat the worms. The spores can remain viable for at least 30 years on dry land and seem immune to sun and drought.
WD originated in Europe, and arrived in America about 40 years ago. European fish, like the German brown trout, carry its spores but seem to have evolved a resistance to infection. Native cut-throats also appear more resistant than imported trout like the rainbow, originally a California breed. –Ed Quillen