Headquarters, Seattle Laboratory
Myxobolus cerebralis is a metazoan parasite that penetrates the head and spinal cartilage of fingerling trout where it multiplies very rapidly, putting pressure on the organ of equilibrium. This causes the fish to swim erratically (whirl), and have difficulty feeding and avoiding predators. In severe infections, the disease can cause high rates of mortality in young-of-the-year fish. Those that survive until the cartilage hardens to bone can live a normal life span, but are marred by skeletal deformities. Lightly infected fish can, however reproduce without passing on the parasite to their offspring.
Whirling disease has become a highly significant problem for fisheries managers in federal and state agencies throughout the nation. Thought to have been introduced from Europe, whirling disease was first detected in the U. S. in 1958 and has now been reported from fish in more than 20 states. The potential for whirling disease to cause losses in natural stocks of trout became a national concern in 1993-1994 when losses of up to 90% of the wild rainbow trout (Oncorhynchus mykiss) in several trout streams in Colorado and Montana were attributed to whirling disease. Subsequently, a large, ongoing survey revealed that naturally spawning rainbow and brown trout in many western rivers were infected with the parasite with some of the fish showing clinical disease. Of additional concern was the presence of whirling disease in native stocks of cutthroat (Oncorhynchus clarki) and bull trout (Salvelinus confluentus) which are of special status due to their declining numbers. Improved diagnostic methods now provide epizoo-tiological information about strains of the parasite or the alternate host.