PKD has traditionally been perceived as a seasonal disease, outbreaks being largely limited to the warmer months of the year when the water temperature exceeds 12 ºC. As spores of T. bryosalmonae have been detected in infected waterways all year round, the temporal limits of disease manifestation have been attributed to temperature effects on parasite development in fish tissue. Although affected by water temperatures, the incubation period in the fish is approximately 7 weeks, with a self-limiting clinical course of usually 2-3 months. On infected farms, morbidity levels can approach 100% while mortality levels varying widely depending on the impact of secondary factors such as concurrent diseases, environmental conditions and management practices. In addition to affecting first season rainbow trout, older year classes which are naïve to T. bryosalmonae exposure can also become infected. Those fish which survive clinical disease prove resistant to subsequent challenge, signifying the development of protective acquired immunity.