These spores are then eaten by common aquatic worms of the genus Tubifex . While in the gut of these worms, the spores change into a new form which is infectious to salmonids. This transformation takes about 3.5 months, when the parasite will become a threat to fish by being released by the worm, or if the fish ingests the worm. The parasite may infect the fish through the digestive system, or directly through the skin. Larger fish may carry the spores, but not show any visible symptoms of the disease.
Young salmonids are the most vulnerable.
After their cartilege hardens, the fish will show few ill effects of infection. Precisely why trout die from WD has not yet been determined, whether from direct causes, or indirect, such as increased predation due to the whirling behavior which makes the small fish vulnerable. Lake Trout and splake are virtually immune to the disease. Whirling Disease was first described in Europe in 1893, and it probably entered the United States in 1956 from processed fish shipments from Denmark. Pennsylvania discovered the disease in 1956, and California in 1965.