Quinqueserialis quinqueserialis (Trematoda: Notocotilidae) is a widespread species in the Holarctic that parasitizes the cecum of the muskrat. The results of micromorphological studies of parenchymal organs of the muskrat at a high intensity of invasion by Q. quinqueserialis are presented for the first time. In infected muskrats, activation of the B-cell and macrophage components of the immune system was established against the background of inhibition of granulocytic reactions in the spleen. Various types of dystrophies were detected in the liver, and, with the formation of functional maladjustment of the organ, inflammatory changes were recorded in the kidneys with signs of alterative, necrobiotic processes. The development of destructive pathomorphological processes indicates the participation of trematodes in the pathogenesis of this parasitosis. The obtained data provide new information for understanding the functions of the lymphoid and parenchymal organs in wild rodents with trematodosis.

In this work, the anti-inflammatory potential of secretory-excretory products (SEP) of gull- tapeworm Dibothriocephalus dendriticus and ligula Ligula interrupta plerocercoids was studied for the first time in an in vitro model of LPS-induced activation of macrophages. A monocyte cell line derived from a patient with acute monocytic leukemia, THP-1, was used as a macrophage model. The anti-inflammatory properties of SEP were determined by the content of tumor necrosis factor (TNF) and interleukin-6 cytokines in the incubation medium using commercial kits for enzyme immunoassay. The results of our study indicated that SEP from L. interrupta plerocercoids have a pronounced anti-inflammatory effect, while SEP from D. dendriticus plerocercoids did not have such an effect. Next, we investigated the anti- inflammatory properties of L. interrupta SEP in a carrageenan-induced air-sac inflammation model in mice. A significant decrease in the volume of inflammatory exudate under the influence of L. interrupta SEP was found, as well as an increase in the level of the interleukin-6 cytokine. At the same time, SEP of L. interrupta had no effect on the number of cells per 1 ml of exudate, as well as on the level of the pro-inflammatory cytokine TNF. The low molecular weight fraction of L. interrupta SEP also increased the level of the anti-inflammatory cytokine interleukin-10, which indicates a more pronounced anti-inflammatory effect compared to the high molecular weight fraction. The results obtained, in general, indicate the anti- inflammatory properties of the SEP of L. interrupta plerocercoids. However, the mechanism of anti-inflammatory action has not been elucidated and requires further research.

For the first time, an analysis of leukocyte changes in the organs of the immune system (pronephros, mesonephros, and spleen) of the Baikal omul Coregonus migratorius (Georgi, 1775) (Salmoniformes: Coregonidae) infected with Dibothriocephalus dendriticus (Nitsch, 1824) (syn. Diphyllobothrium dendriticum) (Cestoda: Diphyllobothriidea) is presented in natural habitat conditions. In the pronephros of infected fish, the number of poorly differentiated neutrophils (myelocytes and metamyelocytes) was significantly higher than in uninfected fish, which indicated inflammatory processes. D. dendriticus triggered the immunoregulatory processes in the body of infected fish, as evidenced by the decrease in the total number of lymphocytes and prolymphocytes (B-lymphocytes) in the pronephros and the increase in prolymphocytes in the spleen. The other effector elements (cells of the monocyte–macrophage lineage, basophils, and eosinophils) were barely involved in antiparasitic defense.

For the first time, new data have been presented on the spleen transcriptome of the Baikal cisco Coregonus migratorius Georgi, 1775 (Salmoniformes: Coregonidae), infected with parasites of different systematic groups. Transcriptomic libraries were sequenced on an Illumina NextSeq550 sequencer using the NextSeq® 550 High Output Kit v2. The de-novo transcriptome was assembled. Conserved domains and their associated Gene Ontology annotations were predicted with Blast2Go. The annotation results of the obtained transcripts found that transcripts were distributed in the spleen into the following categories: molecular functions, biological processes, and cellular components. Among the molecular functions, transcripts of enzyme binding (25.8%), transferase activity (24.7%), hydrolase activity (24.4%), catalytic activity affecting proteins (22.2%), and DNA binding (21%) predominated. Biological processes were dominated by transcripts of cellular processes (46.7%), metabolic processes (38.6%), biological regulation (38.2%), and regulation components of biological processes (36.6%). The category of cellular GO components identified terms, namely cytoplasmic vesicles (25.9%), cytoplasmic membranes (23.1%), nucleoplasm (22.1%), cytoskeletal part (18.6%), cytosol and cellular compounds (17%) in a greater extent.

New data were presented for the first time on the intestinal transcriptome of the intestines (posterior section) of the Baikal omul Coregonus migratorius Georgi, 1775 (Salmoniformes: Coregonidae) belonging to the deep water bottom morphological group (Lake Baikal) infected with parasites of various systematic groups: cestodes Dibothriocephalus dendriticus, Proteocephalus longicollis, Eubothrium crassum, and nematode Contracaecum osculatum baicalensis. Sequencing of the transcriptomics libraries was performed on an Illumina NextSeq550 sequencer using the NextSeq® 550 High Output Kit v2. Based on the obtained data, de-novo transcriptome assembly was performed. Conserved domains and their associated Gene Ontology annotations were predicted using Blast2Go. As a result, it was found that in the posterior section of the intestine enriched with lymphoid tissue (GALT), expression of functional proteins was observed that were primarily associated with enzymatic activity, with the development of specialized tissues, with cellular, metabolic and secretory processes. It should be noted that a significant proportion of gene ontology terms is associated with the functioning of the immune system, and with the cellular response to stress, which, under conditions of sensitization by the metabolic end products of helminths and their traumatic and antigenic effects, is quite understandable. The identified transcriptome may provide new information for understanding the functions of lymphoid organs in salmonids with parasitosis.

Tapeworms of the genus Dibothriocephalus are widely distributed throughout the world, some of which are agents of human diphyllobothriasis, one of the most important fish-borne zoonoses caused by a cestode parasite. Genomic and transcriptomic data can be used to develop future diagnostic tools and epidemiological studies. The present work focuses on a comparative analysis of the transcriptomes of adult and plerocercoid D. dendriticus and the identification of their differentially expressed genes (DEGs). Transcriptome assembly and analysis yielded and annotated 35,129 unigenes, noting that 16,568 (47%) unigenes were not annotated in known databases, which may indicate a unique set of expressed transcripts for D. dendriticus. A total of 8022 differentially expressed transcripts were identified, including 3225 upregulated and 4797 downregulated differentially expressed transcripts from the plerocercoid and adult animals. The analysis of DEGs has shown that among the most differentially expressed genes, there are important genes characteristic of each stage. Thus, several genes are characteristic of D. dendriticus plerocercoids, including fatty acid-binding protein and ferritin. Among the most highly expressed DEGs of the adult stage of D. dendriticus is the Kunitz-type serine protease inhibitor, in two putative isoforms. The analyses of GO and KEGG metabolic pathways revealed that a large number of the DEGs of D. dendriticus are associated with the biosynthesis of various substances such as arginine and folate, as well as with various metabolic pathways such as galactose metabolism, selenocompound metabolism, and phosphonate and phosphinate metabolism. This will contribute to further research aimed at identifying targets for new generation drugs and the development of specific vaccines.

The cytomorphological parameters of hemopoietic organs (pronephros and spleen) of crucian carp ( Carassius auratus L., 1758) infected with Ligula ( Digramma ) interrupta (Logan, 2004) are studied for the first time. The biological material has been obtained from fish in Dolgoe and Cheremukhovoe lakes (the Lake Baikal basin, Buryatia) with an unfavorable digrammosis situation. Infection with plerocercoids causes a hematopoietic transformation: the compensatory activation of erythropoiesis in the pronephros against the background of a low number of mature erythrocytes and a decrease in the proliferative activity of erythroid elements in the spleen.

Hematopoiesis is a mechanism of multifunctional adaptation in vertebrates, including fish. Adaptive capabilities of populations are studied for a species of genus Salvelinus with high ecological plasticity. Char Salvelinus malma is represented in Kamchatka by the anadromous, river, brook, and several lake forms. Peripheral blood cells are investigated for this species juveniles and adults from the Raduga River in the lower Kamchatka River basin. The fish sampled in August 2013 were divided into two groups: sexually mature individuals (10 specimens) and juveniles (10 specimens). The body length (AC) of mature chars (mean ± standard deviation, range in brackets) was 338.90 ± 12.64 (304.0–380.0) mm, the body weight — 437.40 ± 57.74 (235.0–735.0) g, their sex ratio was close to 1 : 1. The length of juvenile chars was 150.50 ± 9.14 (104.0–195.0) mm, the weight — 33.90 ± 6.19 (10.1–68.8) g. Blood was sampled from the caudal vessel of alive fish immediately after catching, the blood smears were dried and fixed with May-Grunwald dye and then stained with azure-eosin. Composition of leukocytes and erythrocytes was determined on the stained blood smears. The blood cells were classified according to Ivanova (1983) and Zhiteneva et al. (1989). Statistical significance of the blood cells composition differences between fish groups was assessed using the Mann-Whitney U-test, after checking for normal distribution of characteristics (Kolmogorov-Smirnov test). The erythroid row of S. malma was represented by proliferating (erythroblasts, basophilic and polychromatophilic normoblasts), maturing (oxyphilic normoblasts) and mature erythrocytes, the leukocyte row — by blast cells, agranulocytes and granulocytes. The juveniles were characterized by intense proliferative processes (great number of immature erythrocytes, large and medium lymphocytes, presence of blast cells) and appearance of poikilocytes; the sexually mature chars were distinguished by greater number of granulocytes, mainly due to stab and segmented neutrophils (p < 0.01). Parasitological analysis showed different infestation of the juvenile and adult chars by pathogenic helminths (Anisakis simplex, Eubothrium salvelini, Neoechinorhynchus salmonis) that influenced on their blood. Broad adaptive capabilities of S. malma hematopoiesis provided by wide morphological and functional diversity of blood cells are concluded that maintains homeostasis of the species. Blood indices for char from the Raduga River can be used to form reference intervals for monitoring physiological state of this species.

The structural response and plasticity of the cestode tegument in response to the influence of the host organism is not yet well understood. The main aims of our in vitro study were to analyse the ultrastructural mechanisms and kinetics of tegumental secretion in two cestode species, Dibothriocephalus dendriticus and Ligula interrupta, in response to the influence of fish host blood serum. The incubation of plerocercoids in the culture medium, which contained fish host blood serum, resulted in an increased number of secretory products on the tegumental surface. Our study is the first to experimentally demonstrate the formation of plerocercoid protective layers influenced by the host's internal environment factors. The mechanism of the generation of the protective layer included the following: the intensive formation of organelles in the tegumental cytons and their transfer to the distal cytoplasm of the tegument; increases in extracellular vesicles and vacuoles released on the tegumental surface; arrangement of secretory products and fine-dispersed extracellular matrix in layers; and formation of the protective layer. The structural tegumental response included increases in the glycocalyx layer and structural changes. Our study revealed that the universal mechanism of protective layer formation was intrinsic to different tapeworms. We hypothesize that plerocercoids of cestodes parasitizing fish may use tegumental secretion in the formation of a protective layer and in the release of immunoregulator molecules to evade the host's immune response.

This study determined for the first time the changes in leukocyte composition of the pronephros (head kidney) of sand sculpin Leocottus kesslerii, (Sideleva, 2001) (Lake Gusinoye, Lake Baikal basin, Eastern Siberia, Russia), infected and uninfected with hemoflaggellates of the genus Trypanosoma. The results indicated that the trypanosomes modulated the development of the immune response of their host (suppression of B cell immunity and granulocytic reactions, activation of the leucopoiesis and monocytopoiesis). Selective suppression of immune response in host with trypanosomes promotes the survival and development of parasites.

The change in the composition of the protein fraction of Diphyllobothrium dendriticum (Cestoda) plerocercoids during incubation in a medium containing blood serum of their host—the Baikal omul Coregonus migratorius (Coregonidae)—was investigated. The change in the composition of the protein fraction of the incubation medium also was studied. On the one hand, the appearance of new high-molecular fractions with a molecular weight (MW) of 193 kDa after 3, 12, and 24 h of incubation and 88 kDa after 24 h was detected, which was associated at plerocercoids with the appearance of a new 189 kDa fraction after 12 h and the disappearance of the 80 kDa fraction. On the other hand, a decrease or the disappearance of two low-molecular fractions 57 and 42 kDa in the incubation medium after 3, 12, and 24 h of incubation were observed.

The cytomorphological composition of the blood and mesonephros of goldfish infected with Ligula interrupta has been studied under the conditions of the native range. A decrease in the total number of leukocytes and neutrophils in the blood at all stages of development and deregulation of the proliferation and differentiation of lymphoid, pseudo-eosinophilic, and pseudo-basophilic elements in the studied organs are established. Infiltration of neutrophilic mesonephros by low-differentiated forms is observed. It has been shown that, in this parasitic system, the partner relations cause the launch by L. interrupta of mechanisms that regulate the host immune response by inhibiting the acute inflammatory response and the development of mild chronic inflammation. B-cell activation in the immune response is accompanied by processes of cellular degeneration and immunosuppression, factors that are actively involved in cytotoxic reactions.