Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is a serious parasitic disease threatening both farmed and wild salmonid populations, but very little is currently known about the distribution of the parasite in the Baltic Sea region. In this study we (1) report the development of a novel multiplex PCR method for fast and reliable screening of T. bryosalmonae; (2) use this multiplex PCR method to show that the PKD agent T. bryosalmonae is widespread in natural brown trout Salmo trutta L. populations in Estonia; (3) evaluate monthly and yearly variation of T. bryosalmonae prevalence in juvenile trout; (4) assess T. bryosalmonae prevalence in different age-classes of fish (0+ vs. 1+ and older) and report the presence of the PKD agent in the kidneys of returning sea trout spawners; and (5) suggest the freshwater bryozoan Plumatella fungosa as a putative invertebrate host of T. bryo salmonae in Estonia. Our results demonstrate a highly heterogeneous distribution of T. bryosalmonae at the micro-geographic scale, indicating that PKD could have an important negative effect on recruitment in Estonian brown trout populations.KEY WORDS: Tetracapsuloides bryosalmonae · Salmo trutta · Salmonid fish · Natural population · Parasite · Host · Myxozoa · Bryozoa Resale or republication not permitted without written consent of the publisherDis Aquat Org 109: [139][140][141][142][143][144][145][146][147][148] 2014 While the effects of temperature on T. bryosalmonae proliferation and the renal pathological responses of the salmonid host are well documented, considerably less is known about the distribution of the parasite and the severity of PKD in wild salmonid populations (Sterud et al. 2007, Peeler et al. 2008, Kristmundsson et al. 2010, Okamura et al. 2011, Skovgaard & Buchmann 2012. For example, the prevalence of PKD is spatially variable within and among river systems (Feist et al. 2002, Sterud et al. 2007, Wahli et al. 2008, Skovgaard & Buchmann 2012. However, the potential effects of temporal variation, different life history strategies (e.g. migratory vs. resident fish), and juvenile age on T. bryosalmonae prevalence is not known. Yet, such knowledge is necessary for a better understanding of the life cycle and dispersal of T. bryosalmonae, and the epidemiology of PKD.The Baltic Sea catchment contains approximately 1000 anadromous brown trout Salmo trutta L. populations and numerous wild resident brown trout stocks (HELCOM 2011). Very little is currently known about the distribution of the PKD-causing parasite T. bryosalmonae and its bryozoan host(s) in the Baltic Sea region and its potential impact on wild trout populations (Skovgaard & Buchmann 2012). Earlier reports from Estonia documented the occurrence of T. bryosalmonae in Atlantic salmon and brown trout in a few rivers based on histopathological work (Kadakas et al. 2001), but no information is available on the prevalence of T. bryosalmonae among wild anadromous and resident brown trout populations in Eston...