National Science Centre, OPUS 17 “Mechanism of pathogenesis of ulcerative dermal necrosis – a fatal disease of salmonid fish” 20.02.2020-19.03.2025

Project Manager: Professor Michał Reichert, DVM, PhD, ScD,

Funding: PLN 1,267,228.00

Project Description: The aim of the study is to understand the mechanism of pathogenesis of Ulcerative Dermal Necrosis (UDN) – a fatal disease affecting the skin of salmonid fish, mainly Atlantic salmon (Salmo salar) and sea trout (Salmo trutta m. trutta). Both sea trout and Atlantic salmon (Salmo salar) are anadromous fish belonging to the same genus and are considered one of the most economically, biologically, and socially important fish species, supporting commercial and recreational fisheries along the Atlantic and Baltic coasts. Sea trout, like other anadromous fish, also plays an important role in maintaining the overall balance of ecosystems. The project focuses on the extremely important issue of biodiversity protection. In the Baltic region, sea trout naturally reproduce in 25 rivers, and some of these populations are supported by stocking. In Poland, sea trout are found in many Pomeranian rivers and the Odra and Vistula river basins. Sea trout catches (coastal, river, and marine) between 1972 and 1994 ranged within a small margin, reaching 50 to 200 tons. Only exceptionally, in 1990, it was about 500 tons. From 1995 onwards, catches steadily increased, reaching as much as 863 tons in 2002. After 2002, catches systematically declined. Among the causes of the continuous decline in sea trout catches observed since 2002, UDN can be seriously considered, as after several decades of relative calm, an outbreak of UDN was recorded in Pomeranian rivers in 2007. The severe losses of sea trout caused by UDN threaten the preservation of the genetic resources of this valuable species. The most severe course of the disease in Poland was observed in fish from the Słupia River, where 74.7% of individuals caught in 2007 showed clinical symptoms of UDN. Clinical symptoms suggest an infectious (fungal/bacterial and/or viral) etiology of the disease. However, what we usually observe, i.e., dramatically affected fish covered with ulcers and mold, is only the final stage of the disease. We do not know what initiates the pathological process. Solving this puzzle is key to explaining the mechanism of the disease’s pathogenesis. So far, no virus has been detected as a pathogenic agent associated with UDN. However, over 40 years ago, it was shown that filtrates from UDN-affected tissues could transmit the disease to healthy fish. Early attempts to search for a UDN-associated virus, based on transmission and scanning electron microscopy, were unsuccessful. The answer to the above questions and hypotheses and the explanation of the pathological nature of the disease syndrome require the broadest possible research. The comprehensive approach proposed in the project includes a wide range of methods, from classical histopathological studies, electron microscopy, and virological studies, to the application of innovative methods in the field of toxicology, proteomics, and genomics. To this end, we propose to study the infectious nature of the UDN syndrome (searching for bacteria, viruses, and/or fungi likely involved in the development of the disease). Virological studies will consist of two parts: i) in vivo experiments and ii) virus detection based on NGS (Next Generation Sequencing). The aim of in vivo experiments is to verify the possibility of pathogen transmission from UDN to healthy fish. NGS-based studies of nucleic acid samples from UDN-affected fish tissues will be performed using high-throughput sequencing on the Illumina platform, followed by bioinformatic analysis. The search for bacteria and fungi involved in UDN will be carried out using routine, accredited methods (API tests). We also plan histopathological, TEM, and SEM examination of pathologically altered tissues. The possible toxicological aspect (dioxins and related compounds) and the level of trace elements in sea trout tissues will also be investigated. Finally, proteomic analysis of UDN-affected fish tissues is planned. We believe that such a wide range of studies, unavailable in the 1970s and 1980s when many researchers tried to explain the pathogenesis of the disease, and now available, will lead to an explanation of the mechanism of the UDN syndrome.