Tetracapsuloides bryosalmonae and PKD in juvenile wild salmonids in Denmark

The myxozoan Tetracapsuloides bryosalmonae is the causative agent of proliferative kidney disease (PKD), a widespread and serious condition in salmonid fishes in Europe and North America. In Europe, PKD is primarily reported affecting farmed rainbow trout Oncorhynchus mykiss, but limited information exists on the occurrence and effects of T. bryosalmonae in wild salmonids. We investigated the presence of T. bryosalmonae in salmonids in Denmark and found that the parasite is common in the dominant wild Danish salmonid, brown trout Salmo trutta, and that it also appears in wild Atlantic salmon S. salar. Clinical signs of PKD were present in some brown trout, but in most cases the parasite was found through histology and/or PCR investigations of kidney tissue in fish that showed no signs of infection. Even though there was high similarity between internal transcribed spacer 1 (ITS1) sequences of T. bryosalmonae from wild brown trout, Atlantic salmon and farmed rainbow trout, a geographic pattern was indicated among T. bryosalmonae ITS1 phylotypes. None of the investigated streams were found free of T. bryosalmonae, but prevalence of the parasite was highly variable.