Patterns of occurrence and 18S rDNA sequence variation of PKX (Tetracapsula bryosalmonae), the causative agent of salmonid proliferative kidney disease

Recent progress in understanding the etiology of proliferative kidney disea se (PKD) includes the identification of freshwater bryozoans as the natural hosts of the myxozoan parasite that causes the disease in salmonid fish an d formal description of the parasite as Tetracapsula bryosalmonae. This pap er presents data on patterns of occurrence of T. bryosalmonae and sequence variation among isolates. T. bryosalmonae infects bryozoans that range from primitive to more derived genera within the Phylactolaemata and that diffe r in growth form and habits. Infected bryozoans have been collected in dive rse habitats including cold, clear streams and warm, eutrophic lakes. Tempo ral surveys reveal intra- and interannual variation in infection levels, an d spatial variation in incidence of infection is implicit by the apparent a bsence of T. bryosalmonae from many bryozoan populations. The significance of minor variation in partial sequences of 18S rDNA requires further invest igation. The information presented here provides the first significant insi ghts into the ecology of T. bryosalmonae.