| Summary: | Proliferative kidney disease (PKD) is a serious disease of salmonids caused by a myxozoan parasite, Tetracapsuloides bryosalmonae (T.b.). PKD is temperature dependent and only develops when water temperature exceeds approximately 12-14°C for several weeks. If these criteria are reached the disease causes extensive renal swelling and can cause heavy mortalities in affected fish. If not, the parasite infects the fish without clinical symptoms developing. The parasite has a complex life cycle where its alternate hosts are freshwater bryozoans. The disease was first identified in Iceland in Lake Elliðavatn in 2008. Since then, studies on the geographic distribution of T.b. and the effect of the disease on wild populations of salmonids in Iceland have been ongoing. The main objective of this study was to increase our knowledge of the distribution and prevalence of T.b. in Icelandic fresh water and its effects on salmonid population in Iceland, especially contextually to rising temperature in Iceland. The results are based on statistical-, histological- and molecular methods. A diagnostic PCR and histological slides were used to screen for the parasite. Three staining methods were used, a general HE staining (haematoxylin and eosin), immunohistochemistry and in situ hybridization. The causative agent of PKD, T.b., had not been found in Iceland prior to the first identification of the disease. In the present study, the parasite was observed in kidney samples from the 1990s of Arctic charr and brown trout, which confirms the existence of the parasite long before its first finding. However, compared to more recent studies, it seems that its distribution has increased over the last two decades. Paralleled with increasing water temperature in Lake Elliðavatn, over the last decades, populations of Arctic charr have severely declined. Environmental conditions (temperature) for the disease to emerge have presumably been present since the 1990s. Rising temperatures in Lake Elliðavatn have been considered to be the most likely ...
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