An introduced parasite in a wilderness

By Jeff Mitton
Trappers Lake, in the Flat Tops Wilderness, is known not only for its stunning beauty but also for its self-sustaining population of Colorado River cutthroat trout.

However, the cutthroats are being threatened by two introduced species. Brook trout, because they spawn in the fall, have a natural competitive advantage over cutthroat, which spawn in spring. Whirling disease causes developmental abnormalities with symptoms varying from disfiguring to lethal.

The introduction of whirling disease from Europe in the 1950s is shrouded in mystery, but one plausible hypothesis is that it was embedded in a shipment of ground fish to be used as fish food in a hatchery in Pennsylvania. Whirling disease was discovered by Colorado Division of Wildlife biologist Barry Nehring, who noted in 1991 to 1993 that nearly all of the young rainbow trout were dying in the Colorado River on the western edge of Rocky Mountain Park. Whirling disease moves easily from rainbow to cutthroat trout, so native cutthroat were also infected.

Whirling disease is caused by a parasite with a complex life cycle. The parasite requires two hosts, tubifex worms and trout, and two forms of spores to complete its life cycle. Tubifex worms inhabit the soft sediments of rivers, creeks and lakes. They grow to be a half-inch long and eat decaying matter. Cutthroat, rainbow and brook trout serve as hosts in Colorado.

Tubifex worms become infected with spores when they consume infected fish. Worms defecate the second type of spore into the water. This spore has a sinister appearance, with four spikes emanating from its center, like the caltrop used in medieval warfare. Caltrops were thrown on the ground by retreating troops to cripple pursuing horses. When a caltrop landed on the ground, three spikes would dig into the ground and act as legs, holding the fourth spike upright, ready to pierce a hoof.

The spiked spore attaches to a larval fish with three spikes, and the fourth spike pierces the skin. It injects cells that reproduce and burrow through the fish, seeking spinal or skull cartilage. In cartilage, they develop into the other spore form and wait as the cartilage turns to bone. When the fish dies, tubifex worms consume fragments, ingesting spores and completing the life cycle.

Spores in the cartilage of a growing and developing fish cause a variety of symptoms. Spores can create pressure on the spine and brain stem, causing the fish to swim in circles. Pressure can also be exerted on nerves that go to pigment cells, producing a black and shriveled tail. A heavy parasite load can deform the skull, shortening the snout and reducing the depth of the head, or produce a distinct kink in the spine. Individually or in combination, these symptoms can interfere with feeding and escape from predators, causing heavy mortality of fish fry.

Heavy infestations of whirling disease can diminish recruitment of young fish into a population and cause local extinctions of trout. Rainbow trout have been reduced in the upper Colorado River and in the Black Canyon of the Gunnison. Yellowstone cutthroat disappeared from Pelican Creek in Yellowstone National Park, and substantial declines of cutthroats have been noted in other creeks flowing into Yellowstone Lake.

Whirling disease was found in Trappers Lake in 2003 and was probably introduced in 1999. The White River, which flows from Trappers Lake, was infected, but a waterfall prevents fish from swimming up into the lake. Perhaps an eagle caught an infected cutthroat in the river and dropped fragments into the lake. Or perhaps fishermen inadvertently introduced spores carried on their waders or gear.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Jan. 22, 2010