Recently, Poland has become a leading producer of sturgeon meat and caviar in Europe and is one of the largest in the world. The growing importance of this branch of aquaculture means that diseases of these fish, especially viral ones, are becoming the object of interest for ichthyopathologists. In recent years, there have been increasing reports of health problems in the dynamically developing sturgeon farming. The greatest risk appears to be emerging infectious diseases that are caused by viruses and that can become a serious threat to the development of the aquaculture industry and the success of sturgeon restitution programs undertaken in many European countries, including Poland. In this paper, an attempt was made to determine the spread of the two most important groups of viruses in Polish sturgeon farming: These include the herpesviruses and sturgeon nucleocytoplasmic large DNA viruses (sNCLDV), in particular, mimiviruses. In the years 2016–2020, 136 samples from nine farms were collected and tested by using the WSSK-1 cell line, PCR and Real Time PCR methods. All results were negative for herpesviruses. Out of the samples, 26% of the samples have been tested positive for mimiviruses. Sanger sequencing of mimiviruses demonstrated their affiliation with AciV-E. The sequence characterization confirmed the presence of both V1 and V2 lineages in Polish fish facilities, but variant V2 seems to be more widespread, as is observed in other European countries.

The mechanism of latency and the ability of the cyprinid herpesvirus 3 (CyHV-3) to establish life-long infections in carp remains poorly understood. To explain the role of miRNAs in this process we applied a range of molecular tools including high-throughput sequencing of RNA libraries constructed from the blood samples of infected fish followed by bioinformatic and functional analyses which show that CyHV-3 profoundly influences the expression of host miRNAs in vivo. We demonstrated the changed expression of 27 miRNAs in the clinical phase and 5 in the latent phase of infection. We also identified 23 novel, not previously reported sequences, from which 8 showed altered expressions in control phase, 10 in clinical phase and 5 in latent phase of infection. The results of our analysis expand the knowledge of common carp microRNAs engaged during CyHV-3 infection and provide a useful basis for the further study of the mechanism of CyHV-3 induced pathology.

The red-eared slider (RES) ranavirus (RESRV) was isolated from a free-ranging RES turtle that died with evidence of respiratory disease. The RESRV genome sequence (106,878 bp) was determined, and phylogenetic analysis revealed that it is a common midwife toad virus (CMTV) strain. This study is the first report of CMTV in RES.

Hepatospora eriocheir is a microsporidian parasite of the Chinese mitten crab Eriocheir sinensis, an invasive decapod now widely distributed in European inland and coastal waters. Although the host is common across much of Europe, confirmed European records of H. eriocheir have remained scarce and, until now, have not included the Baltic Sea region. In this study, 15 adult E. sinensis collected from the Vistula Lagoon, southern Baltic Sea, were examined using gross pathological assessment, wet-mount microscopy, histopathology, PCR amplification, Sanger sequencing, and phylogenetic analyses of parasite SSU rRNA and host COI sequences. Hepatopancreatic alterations were observed in several individuals, ranging from pale discolouration and friability to loss of normal tissue organisation. Spores were detected in fresh squash preparations from affected tissue, and histology revealed epithelial disruption, intratubular spore accumulation, and necrotic changes consistent with progressive microsporidian infection. Molecular screening confirmed H. eriocheir in 60.0% of crabs, with positive cases occurring in both females and males. The parasite sequences formed a single, well-supported clade and were highly similar to previously reported H. eriocheir sequences from the United Kingdom and China. Host COI sequences represented three mitochondrial haplotypes, indicating that the infection occurred across more than one host mitochondrial haplotype background. These findings constitute the first record of H. eriocheir in E. sinensis from the Baltic Sea and support the hypothesis that infected crabs reaching the Vistula Lagoon are connected with the wider North Sea invasion system rather than an isolated Baltic lineage.

Every year, ulcerative dermal necrosis (UDN) affects salmonids that spend most of their lives in the sea during their migration to the rivers of northern Poland to spawn. The clinical form of the disease manifests itself in ulcerative skin lesions, which lead to significant weakening of the fish and, in most cases, result in their death. This study was carried out on samples taken from sea trout in the Słupia River in northern Poland. In order to identify the pathogen, experiments on the transmission of the disease were carried out, and additional histopathological, microbiological and electron microscopic examinations were performed. As a result of these studies, it was possible to experimentally transfer the disease from sick to healthy fish. The results indicate a complex etiology of the disease (lack of a clearly defined pathogen), in which the change in the environment from salty to freshwater triggers the related changes in skin physiology, which are the main causes of increased susceptibility to the development of the disease.