Red mark syndrome in rainbow trout Oncorhynchus mykiss: investigation of immune responses in lesions using histology, immunohistochemistry and analysis of immune gene expression

Understanding the Pathogenesis of Red Mark Syndrome in Rainbow Trout (Oncorhynchus mykiss) through an Integrated Morphological and Molecular Approach

Red mark syndrome (RMS) is a widespread skin disorder of rainbow trout in freshwater aquaculture, believed to be caused by a Midichloria-like organism (MLO). Here, we aimed to study the pathologic mechanisms at the origin of RMS by analyzing field samples from a recent outbreak through gene expression, MLO PCR, quantitative PCR, and a histopathological scoring system proposed for RMS lesions. Statistical analyses included a One-Way Analysis of Variance (ANOVA) with a Dunnett's multiple comparisons test to assess differences among gene expression groups and a nonparametric Spearman correlation between various categories of skin lesions and PCR results. In short, the results confirmed the presence of a high quantity of 16S gene copy numbers of Midichloria-like organisms in diseased skin tissues. However, the number of Midichloria-like organisms detected was not correlated to the degree of severity of skin disease. Midichloria-like organism DNA was found in the spleen and head kidney. The spleen showed pathologic changes mainly of hyperplastic type, reflecting its direct involvement during infection. The most severe skin lesions were characterized by a high level of inflammatory cytokines sustaining and modulating the severe inflammatory process. IL-1 β, IL-6, IL-10, MHC-II, and TCR were upregulated in severe skin lesions, while IL-10 was highly expressed in moderate to severe ones. In the moderate form, the response was driven to produce immunoglobulins, which appeared crucial in controlling the skin disease's severity. Altogether our results illustrated a complex immune interaction between the host and Midichloria-like organism.

Transcriptomic characterisation of a common skin lesion in farmed chinook salmon

Little is known about host responses of farmed Chinook salmon with skin lesions, despite the lesions being associated with increased water temperatures and elevated mortality rates. To address this shortfall, a transcriptomic approach was used to characterise the molecular landscape of spot lesions, the most commonly reported lesion type in New Zealand Chinook salmon, versus healthy appearing skin in fish with and without spot lesions. Many biological (gene ontology) pathways were enriched in lesion adjacent tissue, relative to control skin tissue, including proteolysis, fin regeneration, calcium ion binding, mitochondrial transport, actin cytoskeleton organisation, epithelium development, and tissue development. In terms of specific transcripts of interest, pro-inflammatory cytokines (interleukin 1β and tumour necrosis factor), annexin A1, mucin 2, and calreticulin were upregulated, while cathepsin H, mucin 5AC, and perforin 1 were downregulated in lesion tissue. In some instances, changes in gene expression were consistent between lesion and healthy appearing skin from the same fish relative to lesion free fish, suggesting that host responses weren't limited to the site of the lesion. Goblet cell density in skin histological sections was not different between skin sample types. Collectively, these results provide insights into the physiological changes associated with common spot lesions in farmed Chinook salmon.

Red mark syndrome of trout (Oncorhynchus mykiss; Walbaum, 1792): Histopathological scoring and correlation with gross lesions

Red mark syndrome (RMS) is a skin disorder affecting rainbow trout (Oncorhynchus mykiss). The present work aimed to correlate the gross skin lesions affecting 46 fish sampled from farms surveyed for RMS with their microscopic features, identifying histological parameters that may be suggestive of disease progression. Skin lesions were grossly included in one of three categories (types I, II and III) according to the progressive degree of severity. Histological parameters and anti-proliferating cell nuclear antigen (PCNA) tissue immunoreactivity were semi-quantitatively assessed. In the dermis, PCNA-positive lymphocytes, fibroblasts and endothelial cells were indicative of active phlogosis. A significant increase in PCNA-immunoreactive lymphocytes, from gross type I to type III cases, was found only in the hypodermis. The histological parameters significantly associated with the gross lesion severity were progressive loss of the epithelium and scales, recruitment of inflammatory cells in the stratum compactum, loss of architecture of the stratum compactum, perivascular and perineural granulomatous inflammation and increase in lymphocyte infiltration of the muscular layer. In the type II and type III categories, inflammation in the hypodermis and muscle displayed a granulomatous pattern, reinforcing the hypothesis of an immunopathological mechanism. The morphological diagnosis of "deep chronic dermatitis associated to panniculitis and myositis, characterised by lympho-histiocytic and granulomatous reaction" is suggested.

Case of highly prevalent papilloma-like skin lesions in whitespotted char (Salvelinus leucomaenis) in Kamchatka (Russia)

In October 2014, a case of mass skin lesions was reported in whitespotted char (Salvelinus leucomaenis) caught in the Nalycheva River on the Kamchatka peninsula (Russia). The proportion of affected fish was approximately 50% of the catch. Lesions of varying severity were noted from small tubercles to large single and confluent formations and ulcers. Papilloma-like growths were found in the epidermal layer of the skin of the affected fish. Histopathological alterations included the following: submerged growth of epithelial cells, infiltration of inflammatory cells, oedema of scale pockets, resorption of scales, haemorrhages, local necrosis and an increased number of mucous cells. In the dermal layer of the skin, haemorrhages and extensive necrosis were recorded. Inflammation, necrosis and destruction of myocytes were found in the underlying musculature. This is the first documented case of papilloma-like skin lesions found in salmonids in Kamchatka. The affected whitespotted char were deemed unsuitable as a food product due to their unsatisfactory appearance and impermissible levels of bacterial contamination of the muscles.

Cranial Mandibular Fibrosis Syndrome in Adult Farmed Rainbow Trout Oncorhynchus mykiss

An unusual condition affecting market size rainbow trout was investigated. This condition was prevalent for several years at low levels but affected a large proportion of stock during 2018 and 2019. Chronic fibrosis affecting cranial tissues and the jaw was observed in samples collected in 2018. A larger sampling was then conducted in 2019 to investigate the presence of an infectious agent(s). An extensive inflammatory response in the mandibular region was the main finding, however infectious agents in the lesions were not identified through classical virology and bacteriology analysis. Tetracapsuloides bryosalmonae infection, calcinosis, and a Gram-positive bacterial infection of a single fish cardiac tissue was observed, however, a correlation of these pathologies and the cranial mandibular fibrosis (CMF) syndrome was not established. The gene expression of a panel of 16 immune-related genes was studied. Among these, tgf-b, sIgM, il11, hspa, and the antimicrobial peptides lys and cath1 were up-regulated in jaw sections of CMF-affected fish, showing a strong positive correlation with the severity of the lesions. Idiopathic chronic fibrosis with the activation of the Tfg-B pathway and local hyper-immunoglobulaemia was therefore diagnosed. Initiating factors and causative agent(s) (biotic or abiotic) of CMF remain, at present, unclear.

Effects of dietary anthocyanin on innate immune parameters, gene expression responses, and ammonia resistance of Nile tilapia (Oreochromis niloticus)

The present study investigated the effects of dietary anthocyanin on the growth performance, haematological, non-specific immune, and spleen gene expression responses of Nile tilapia, Oreochromis niloticus. Five experimental groups of fish with mean weights of 8.24 ± 0.64 g were used in the study; four of these were fed with diets incorporating anthocyanin (20 mg kg -¹, 40 mg kg^-1, 80 mg kg^-1 and 160 mg kg^-1), while the fifth was a control group without dietary anthocyanin. Growth performance and haematological parameters of tilapia were not affected by anthocyanin-supplemented diets (p > 0.05). Dietary anthocyanin significantly increased respiratory burst activity, phagocytic activity, phagocytic index, lysozyme activity, myeloperoxidase activity, serum total superoxide dismutase (T.SOD) activity, and serum catalase (CAT) activity (p < 0.05). The total immunoglobulin level was highest in the 80 mg kg^-1 group compared with the other groups (p < 0.05). In addition, with the anthocyanin-containing diets, the gene levels of interleukin 1, beta (IL-1β), interleukin 8 (IL-8), tumor necrosis factor (TNF-α), heat shock protein 70 (HSP70), and interferon gamma (IFN-γ) were increased in the fish spleen, and the gene levels of CAT, GPx, and SOD were also increased in fish liver (p < 0.05). At the end of the experiment, the fish were subjected to ammonia stress. The groups fed with 20 and 40 mg kg^-1 anthocyanin exhibited higher survival rates than the other groups. In summary, feeding Nile tilapia with anthocyanin-containing diets caused increases in the innate immune parameters, gene expression responses, and the survival rate of the fish subjected to ammonia stress.

Emergence of Rickettsial Infection in Rainbow Trout (Oncorhynchus mykiss) Fry Displaying the Appearance of Red Mark Syndrome in Korea

Red mark syndrome (RMS) is a fish disease caused by the infection of Rickettsial agents, especially affecting rainbow trout (Oncorhynchus mykiss). The disease is prevalent in many countries in Europe (France, Switzerland, Italy, and Slovenia), South America (Chile), North America (USA), and even Asia (Japan). However, it has not been reported in Korean aquaculture. In February 2019, rainbow trout presenting red spot lesions with swollen features on the lateral side of their body were observed at a hatchery in Korea. Fishes showing those clinical signs were fry weighing 25 ± 5 g. Moreover, the fish showing the red spot lesions were found dead, which suggests an outbreak of a mortality-causing disease. The symptoms were similar to those of RMS, and we identified the presence of Rickettsia-like organisms associated with this disease using polymerase chain reaction (PCR), sequencing, histopathologic examination, and transmission electron microscopy. The distinct features of this infection, compared to that in previous reports, were that RMS occurred in small-sized fish and accompanied mortality. Additionally, the presence of the Rickettsia agent was accompanied with outbreak of the disease. Therefore, this is the first report of RMS outbreak in rainbow trout fisheries in Korea.

Skin immune response of rainbow trout (Oncorhynchus mykiss) experimentally exposed to the disease Red Mark Syndrome

Red Mark Syndrome (RMS) is a skin disease reported from farmed rainbow trout. Since the turn of the millennium it has been spreading through Europe. RMS is probably a bacterial disease caused by a Midichloria-like organism (MLO). It is non-lethal and causes little obvious changes in appetite or behavior but results in red hyperaemic skin lesions, which may lead to economic losses due to downgrading. Here we transfer RMS to naïve specific pathogen free (SPF) fish by cohabitation with RMS-affected seeder fish. During disease development we characterize local cellular immune responses and regulations of immunologically relevant genes in skin of the cohabitants by immunohistochemistry and qPCR. Skin samples from SPF controls and cohabitants (areas with and without lesions) were taken at 18, 61, 82 and 97 days post-cohabitation. Gene expression results showed that lesions had a Th1-type profile, but with concurrent high expression levels of all three classes of immunoglobulins (IgD, IgM and IgT). The marked local infiltration of IgD + cells in the skin lesions as well as a highly up-regulated expression of the genes encoding sIgD and mIgD indicate that this immunoglobulin class plays an important role in skin immunity in general and in RMS pathology in particular. The co-occurrence of an apparent B cell dominated immune reaction with a Th1-type profile suggests that the local production of antibodies is independent of the classical Th2 pathway.

Local regulation of immune genes in rainbow trout (Oncorhynchus mykiss) naturally infected with Flavobacterium psychrophilum

Flavobacterium psychrophilum is the etiological agent of bacterial cold water disease (BCWD), also referred to as rainbow trout fry syndrome (RTFS), a disease with great economic impact in salmonid aquaculture. Despite this, to date, not many studies have analyzed in depth how the immune system is regulated during the course of the disease. In the current study, we have studied the transcription of several immune genes related to T and B cell activity in the skin of rainbow trout (Oncorhynchus mykiss) naturally infected with F. psychrophilum in a farm located in Lake Titicaca (Peru). The levels of expression of these genes were tested and compared to those obtained in asymptomatic and apparently healthy rainbow trout. In the case of symptomatic fish, skin samples containing characteristic ulcerative lesions were taken, as well as skin samples with no lesions. Our results pointed to a significant local up-regulation of IgD, CD4, CD8, perforin and IFNγ within the ulcerative lesions. On the other hand, no differences between the levels of expression of these genes were visible in the spleen. To confirm these results, the distribution of IgD⁺ and CD3⁺ cells was studied through immunohistochemical techniques in the ulcerative lesions. Our results demonstrate a strong local response to F. psychrophilum in rainbow trout in which IgD and T cells seem to play a major role.

Effects of dietary blackberry syrup supplement on growth performance, antioxidant, and immunological responses, and resistance of Nile tilapia, Oreochromis niloticus to Plesiomonas shigelloides

The present study investigated the effects of dietary blackberry syrup on growth performance, haematological, non-specific immune and spleen gene expression responses of Nile tilapia, Oreochromis niloticus. Five experimental groups of fish with mean weights of 26.75 ± 2.67 g were used in the study; three of them were fed with blackberry syrup incorporated diets (7.5 g kg^-1- BBRY7.5, 15 g kg^-1- BBRY15, 30 g kg^-1- BBRY30), whereas an additive free basal diet served as the control. Additionally, the fifth group was an antibiotic medicated diet (0.02 g kg^-1- ABTC), prepared with the florfenicol. Dietary blackberry syrup especially at 15 g kg^-1 significantly increased growth performance, respiratory burst activity, potential killing activity, phagocytic activity, phagocytic index, lysozyme activity, myeloperoxidase activity, total immunoglobulin levels, serum SOD activity and serum CAT activity (p < 0.05). Furthermore, dietary blackberry syrup increased the expression levels of immune [heat shock protein 70 (HSP70), interleukin 1, beta (IL-1β), tumor necrosis factor (TNF-α), interferon gamma (IFN-γ), immunoglobulin M (IgM)] and antioxidant [glutathione peroxidase (GPx)] related genes in the spleen of fish fed with especially 15 g kg^-1 blackberry syrup (p < 0.05). At the end of the 20-day challenge period the survival rates were significantly higher in the BBRY15 and ABTC groups compared to all other treatment groups (p < 0.05). As a result, feeding Nile tilapia with a diet containing 15 g kg^-1 blackberry syrup over a period of 90 days might be adequate to improve growth performance, fish immune parameters, antioxidant status, as well as survival rate against P. shigelloides, similar to antibiotic treatment. Hence, blackberry syrup can be used as an antibiotics replacer for controlling P. shigelloides in tilapia feed.

Trans-cinnamic acid application for rainbow trout (Oncorhynchus mykiss): I. Effects on haematological, serum biochemical, non-specific immune and head kidney gene expression responses

The present study investigated the effects of dietary trans-cinnamic acid (CA) on pre- and post-challenge haematological, serum biochemical, non-specific immune and head kidney gene expression responses of rainbow trout, Oncorhynchus mykiss juveniles. In this regard, fish with an average weight of 17.01 ± 0.05 g were divided into five groups, and fed daily with an additive free basal diet (control); 250, 500, 750 or 1500 mg kg^-1 CA for a 60-day period. Fish were sampled every 20 days during the experiment. On days 20, 40 and 60 (the pre-challenge period), the dietary CA especially at 250 and/or 500 mg kg^-1 significantly increased blood granulocyte percentage, and serum total protein, globulin, lysozyme and total immunoglobulin values. Furthermore, dietary CA increased activities of phagocytic activity, respiratory burst and potential killing, and increased the expression levels of immune related genes [serum amyloid A (SAA), interleukin 8 (IL-8), interleukin 1, beta (IL-1β), transforming growth factor beta (TGF-β), tumor necrosis factor (TNF-α), and immunoglobulin T (IgT)] in the head kidney of fish fed with 250 and/or 500 mg kg^-1 CA. Following 60 days of feeding, fish were challenged with Yersinia ruckeri and mortality was recorded for 20 days. Highest percentage survival (%) rate was found in the 250 and/or 500 mg kg^-1 CA-supplemented feeding groups. During the post-challenge period, red blood cell (RBC) count, hematocrit (%), respiratory burst activity, and total antiprotease activity increased in fish fed with feed containing 500 mg kg^-1 content. Moreover, markedly up-regulated the expression of related genes (SAA, IL-8, IL-1β, TGF-β, TNF-α, IFN-γ and IgM) in fish fed 250, 500 and/or 750 mg kg^-1 CA. Therefore, feeding O. mykiss for 60 days with dietary CA at 250-500 mg kg^-1 CA incorporation levels can be suggested as optimal to enhance the immunity and disease resistance against Y. ruckeri.

The skin immune response of rainbow trout, Oncorhynchus mykiss (Walbaum), associated with puffy skin disease (PSD)

Puffy skin disease (PSD) is an emerging skin condition which affects rainbow trout, Oncorhynchus mykiss (Walbaum). The transmission pattern of PSD suggests an infectious aetiology, however, the actual causative infectious agent(s) remain(s) unknown. In the present study, the rainbow trout epidermal immune response to PSD was characterised. Skin samples from infected fish were analysed and classified as mild, moderate or severe PSD by gross pathology and histological assessment. The level of expression of 26 immune-associated genes including cytokines, immunoglobulins and cell markers were examined by TaqMan qPCR assays. A significant up-regulation of the gene expression of C3, lysozyme, IL-1β and T-bet and down-regulation of TGFβ and TLR3 was observed in PSD fish compared to control fish. MHCI gene expression was up-regulated only in severe PSD lesions. Histological examinations of the epidermis showed a significant increase in the number of eosinophil cells and dendritic melanocytes in PSD fish. In severe lesions, mild diffuse lymphocyte infiltration was observed. IgT and CD8 positive cells were detected locally in the skin of PSD fish by in situ hybridisation (ISH), however, the gene expression of those genes was not different from control fish. Total IgM in serum of diseased animals was not different from control fish, measured by a sandwich ELISA, nor was significant up regulation of IgM gene expression in PSD lesions observed. Taken together, these results show activation of the complement pathway, up-regulation of a Th17 type response and eosinophilia during PSD. This is typical of a response to extracellular pathogens (i.e. bacteria and parasites) and allergens, commonly associated with acute dermatitis.

Ulcerative dermatitis in wild dusky grouper Epinephelus marginatus (Lowe) from Libyan waters

In the period 2013-2015, wild dusky grouper, Epinephelus marginatus (Lowe), caught in Libyan coastal waters and ranging in size from 42 to 92 cm in total length, were observed to have distinctive skin lesions of unknown aetiology. Histopathologically, the lesions comprised a multifocal, unilateral or bilateral dermatitis, involving the epidermis, superficial dermis and scale pockets, and sometimes, in severe cases, the hypodermis. Severe lesions had marked epidermal spongiosis progressing to ulceration. Healing was observed in some fish. Bacteria and fungi could be isolated from severe lesions, although they were not seen histopathologically in early-stage lesions. By contrast, metazoan parasite eggs were observed in the dermis and epidermis of some fish with mild and moderate dermatitis. Unidentified gravid digenean trematode parasites carrying similar eggs were also seen within the blood vessels of the deep and superficial dermis. The cause of this distinctive condition, termed dusky grouper dermatitis (DGD), and its potential impact upon already threatened Mediterranean wild dusky grouper populations and upon cultured grouper more widely have yet to be established.

Case Report: Strawberry Disease in Farmed Chilean Rainbow Trout

Strawberry disease is a chronic, nonlethal skin condition that affects Rainbow Trout Oncorhynchus mykiss in the United States and several European countries, where it is also known as red-mark syndrome. We provide the first identification and characterization of three strawberry disease outbreaks occurring at two aquaculture farms in southern Chile. Clinically affected fish weighing an average of 400 g presented multiple bright-red, usually raised, skin lesions on the flank, ventral surface, and dorsal surface. A PCR using Rickettsia-like-organism (RLO)-specific primers was performed on nine affected fish, and all skin samples were positive for the RLO 16S ribosomal RNA sequence. All PCR results for Flavobacterium psychrophilum and other bacterial and viral pathogens were negative. Histopathological examination of the skin lesions revealed extensive dermatitis, with severe lymphocytic infiltration in advanced cases. This report is the first to describe strawberry disease in farmed Chilean Rainbow Trout. Additional studies are needed to evaluate the risk for Rainbow Trout culture; fish challenge experiments should be performed to fulfill Koch's postulates and to demonstrate that RLO is the cause of this disease. Received December 27, 2014; accepted October 23, 2015.

Immunolocalization of tumor necrosis factor alpha in turbot (Scophthalmus maximus, L.) tissues

Tumor necrosis factor alpha (TNFα) is a cytokine involved in a broad spectrum of cellular and organismal responses. Its main function, as a potent pro-inflammatory mediator, has been demonstrated in numerous teleost species and there are many reports on the modulation of TNFα gene expression under pathological conditions. Nevertheless, there is still scarce knowledge about the tissue distribution and type of cells that express this cytokine in fish species, which would help to further investigate its biological activities. These studies are hampered by the lack of molecular markers for teleost that hinder the development of morphological techniques, like immunohistochemistry. The aim of this work was to develop an immunohistochemical technique for the detection of TNFα in paraffin-embedded organs from healthy turbot (Scophthalmus maximus), an economically-important marine fish species. A commercial anti-human TNFα antibody, whose specificity was confirmed by western blot analysis, was used. Immunoreactive cells were observed in higher numbers in the lymphohematopoietic organs, kidney, spleen and thymus, although TNFα-positive cells were also present in the digestive tract, liver, heart, gills and skin. Similarly to non-fish species, monocytes/macrophages appeared to be the main producers of this cytokine; nevertheless, the presence of immunoreactive rodlet cells in different tissues was also reported. The nature and distribution of the labeled cells appeared to be related with a strategic localization for defense response to antigenic challenge. The relative abundance of TNFα-positive cells in the lymphohematopoietic organs also suggests that this cytokine may have a broader role in the normal physiology of those organs. The immunohistochemical technique allowed the in-situ characterization of TNFα expression, representing a valid tool to investigate the immune response of turbot.

Histological, ultrastructural and heat shock protein 70 (HSP70) responses to heat stress in the sea cucumber Apostichopus japonicus

The aquaculture industry for Apostichopus japonicus has suffered severe economic and resource losses due to high temperature in recent summers. There is increasing concern about the effect of high temperature on this species. Histological, ultrastructural and HSP70 responses to heat stress were investigated in the intestine of A. japonicus. Tissue degradation was observed in muscular, submucosal and mucosal layers, with significant decrease in plicae circulares of the mucosal layer. Ultrastructural damage intensified with increasing stress time, and indicators of cell apoptosis were evident after 192 h heat stress. Immunostaining showed HSP70 mainly in mucosa and serosa, with faint staining in non-stressed individuals (the control group) and denser staining under stress (the 6, 48 and 192 h groups). Western blot detection confirmed ocurrence of HSP70 in all groups and significant up-regulation under stress. The rapid and persistent response of HSP70 implies its critical role in the heat shock response of A. japonicus.

Puffy skin disease (PSD) in rainbow trout, Oncorhynchus mykiss (Walbaum): a case definition

Puffy skin disease (PSD) is a disease that causes skin pathology in rainbow trout, Oncorhynchus mykiss (Walbaum). Incidence of PSD in UK fish farms and fisheries has increased sharply in the last decade, with growing concern from both industry sectors. This paper provides the first comprehensive case definition of PSD, combining clinical and pathological observations of diseased rainbow trout from both fish farms and fisheries. The defining features of PSD, as summarized in the case definition, were focal lateral flank skin lesions that appeared as cutaneous swelling with pigment loss and petechiae. These were associated with lethargy, poor body condition, inappetance and low level mortality. Epidermal hyperplasia and spongiosis, oedema of the dermis stratum spongiosum and a mild diffuse inflammatory cellularity were typical in histopathology of skin. A specific pathogen or aetiology was not identified. Prevalence and severity of skin lesions was greatest during late summer and autumn, with the highest prevalence being 95%. Atypical lesions seen in winter and spring were suggestive of clinical resolution. PSD holds important implications for both trout aquaculture and still water trout fisheries. This case definition will aid future diagnosis, help avoid confusion with other skin conditions and promote prompt and consistent reporting.