Tetracapsuloides bryosalmonae and proliferative kidney disease in Icelandic salmonids – Comparative data from two different time periods

Tetracapsuloides bryosalmonae is a myxozoan parasite and the causative agent of proliferative kidney disease (PKD), a serious, temperature-dependent and emerging disease affecting salmonid fish. It was first identified in Iceland in 2008, from Arctic charr inhabiting a shallow lowland lake. The aim of this study was to investigate the distribution and prevalence of macroscopic and subclinical T. bryosalmonae infections in Icelandic salmonids and compare different time periods, in context with depths, volumes, altitudes and areas of the lakes and fish age. Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) from 34 lakes, sampled between 1994–1998 and 2009–2017, were examined for macroscopic signs of PKD (n = 2,151) and the presence of T. bryosalmonae infections (n = 1,424). In the earlier period, 43% of lakes (10/23) harboured T. bryosalmonae -infected fish. The mean prevalence in those lakes was 62.1%, being most common in shallow lowland lakes whilst deeper lakes at high altitudes were all free from infection. Only a single fish from one lake showed macroscopic signs of PKD, a shallow lowland lake in southwestern Iceland. In the latter period, T. bryosalmonae was found in 16/18 lakes studied (89%), with a mean prevalence of 78–79% (excluding T.b. free lakes), being most common in the smaller, shallower lakes at lower alttudes. Macroscopic signs of PKD were observed in 11 of 18 of the lakes studied (61%) with prevalences up to 67%, most common in younger fish inhabiting small shallow lowland lakes. The results indicate that the distribution of T. bryosalmonae and the presence of PKD in Iceland have increased over the last few decades. The disease was almost non-existent in the 1990s but has become very common during the last decade or two. With further water temperature increases, as predicted by climate models, PKD is likely to increasingly affect wild salmonid populations in Iceland.