Among fish found in Arizona, rainbow, cutthroat and brook trout are the most susceptible species. Brown trout and grayling are considered less susceptible, and the disease has not been documented in Arizona's two native trout, Apache and Gila, although biologists suspect they may be susceptible. Other species of game fish, such as bass or crappie, do not contract whirling disease. Those that survive until the cartilage hardens to bone can live a normal life span, but are marred by skeletal deformities. Worms that become infected release a fragile state of the parasite that must infect a trout within a few days or perish.
Infected trout carry spores that are very persistent, but the trout can survive for years.
The spores are released when an infected fish dies and decomposes. The whirling disease parasite can survive passing through the digestive tract of a predator and there is a remote possibility that the disease also can be transferred from place to place on muddy boots or equipment.