Evidence that infectious stages of Tetracapsula bryosalmonae for rainbow trout Oncorhynchus mykiss are present throughout the year

Proliferative kidney disease (PKD) is a hyperplastic condition of the lymph oid tissue of salmonids infected with the spores of Tetracapsula bryosalmon ae, a myxozoan parasite formerly designated PKX, which has recently been de scribed as a parasite of several species of bryozoans. The occurrence of PK D is generally associated with seasonal increase in water temperature, with research indicating that transmission of the disease does not occur below 12 to 13 degreesC. This suggested that the infectious stages are absent fro m about November to March/April. Here we document the transmission of PKD a t water temperatures and seasons previously considered to be non permissive for PKD infection. The exposure of naive rainbow trout Oncorhynchus mykiss (Walbaum) to PKD-infected water ranging from 8 to 13 degreesC during the A utumn, Winter and early Spring, resulted in the infection of kidney interst itium once the trout were transferred to 16 degreesC. In addition, cohabita tion studies were conducted with the bryozoan host Fredericella sultana col lected from a river at times of low seasonal temperatures because this bryo zoan species overwinters as living colonies. Cohabitation of trout with col onies of F sultana in parasite-free city water at 16 degreesC, also led to renal lymphoid tissue infection with the parasite and even to nephromegaly. Our results provide evidence that the infectious stages of T bryosalmonae for rainbow trout were present in the water throughout the entire year and that the impact of temperature on the development of PKD is primarily a res ult of the kinetics of Tetracapsula multiplication in bryozoan and fish hos ts.