Immunity to gastrointestinal microparasites of fish

Rodlet Cell Morpho-Numerical Alterations as Key Biomarkers of Fish Responses to Toxicants and Environmental Stressors

Rodlet cells (RCs) are specialised immune cells found in teleost fish, recognised for their unique morphology and potential roles in both immune responses and environmental adaptation. Herein, current knowledge on RCs is reviewed, focussing on their responsiveness to toxicants and environmental stressors. The historical context of RC research is discussed, including key milestones in the identification and characterisation of these cells. Recent studies highlight RCs' quantitative and qualitative changes in response to various pollutants, such as heavy metals, organic chemicals, and microplastics, underscoring their utility as biomarkers for environmental monitoring and assessment of ecological health. The underlying mechanisms that govern RC responses are explored, noting the limited research available at the molecular level, which hampers a comprehensive understanding of their functionality. Despite this, the consistent patterns of RC responses position them as valuable indicators of environmental health within the One Health framework, linking aquatic ecosystem integrity to broader human and animal health concerns. Additionally, the potential equivalence of RCs in other vertebrates is examined, which may provide insights into their evolutionary significance and functional roles across different species. The urgent need for further research is emphasised to enhance the understanding of RC biology and its applications in toxicology and environmental pathology.

White spots amidst the gold: ultrastructural and histological aspects of the chronic inflammatory response of goldfish with ichthyophthiriasis

Ichthyophthirius multifiliis, the causative agent of white spot disease, is a ciliated protozoan parasite that infects freshwater fish and induces high mortality. Outbreaks occur both in natural and production sites. The aim of the present study was to describe the lesions caused by chronic infection by I. multifiliis in goldfish (Carassius auratus) from an ornamental fish farm, highlighting important ultrastructural aspects of this protozoan. Damaged skin and gills, collected from fish with white or ulcerative skin lesions, were routinely processed for histological analysis and transmission electron microscopy. The parasitic forms present in the skin were associated with an inflammatory infiltrate consisting of macrophages, lymphocytes and other polymorphonuclear cells. The lesions associated with the presence of the parasite were organized in the form of granulomas, with macrophages in the layers closest to the parasites. A trophont-thickened membrane and induction of granulomatous inflammation were identified in this study as mechanisms for evasion of the immune response. We concluded that the presence of I. multifiliis trophonts resulted in the formation of granulomatous inflammation, whether associated or not with pathogen lysis, suggesting that the parasite can use an inflammatory response to evade the immune response.

Mixed Eimeria and Cryptosporidium infection and its effects on pathology and clinical outcomes in juvenile Asian seabass (Lates calcarifer) cultured in Thailand

Coccidiosis is an important disease in juvenile fish because of severe intestinal injury during infection. We first reported the mixed infection of intestinal coccidia and its association with health status and pathological findings in juvenile Asian seabass (Lates calcarifer) cultured in Thailand. Two groups of Asian seabass, 60-day fish and 90-day fish, were sampled to investigate prevalence and coccidian infection intensity using morphological characterization and PCR. Phylogenetic analysis of 18S rRNA gene amplified from the intestines revealed Eimeria sp. and Cryptosporidium sp. infection. The prevalence of Eimeria sp. and Cryptosporidium sp. in sampled fish was 100%. Clinical outcomes assessed, using health assessment index (HAI) scoring and semi-quantitative grading of intestinal lesions and inflammation, demonstrated that all fish developed variety of pathology and clinical illness; however, infection intensity in 60-day fish was significantly higher (p < .05) than 90-day fish. The HAI score of 60-day fish was poorer than 90-day fish, which correlated to a high infection intensity (r = .397), analysed by Pearson correlation coefficient. Overproduction of intestinal oxidants contributing to mucosal injury was examined by nitrotyrosine expression. The high production of reactive nitrogen species indicated severe inflammatory response, and intestinal injuries occurred mainly in the 60-day fish.

Infection of endemic chub Squalius tenellus with the intestinal tapeworm Caryophyllaeus brachycollis (Cestoda): histopathology and ultrastructural surveys

The endemic chub Squalius tenellus (Heckel, 1843) was introduced more than 100 years ago to Lake Blidinje (Bosnia-Herzegovina). Only 1 species of enteric helminth was found in a sample of 35 chubs, the tapeworm Caryophyllaeus brachycollis (Janiszewska, 1953). The paper includes histopathological investigation with identification of innate immune cells involved in host reaction and molecular data allowed correct designation of the cestode species. Of 35 specimens of chub examined, 21 (60%) harboured individuals of C. brachycollis and a total of 1619 tapeworms were counted, the intensity of infection ranged from 1 to 390 worms per fish (46.2 ± 15.3, mean ± s.e.). Histopathological and ultrastructural investigations showed strict contact between the worm's body and the epithelia and increase in the number of mucous cells, rodlet cells among the epithelial cells. Within the tunica propria-submucosa, beneath the site of scolex attachment, numerous neutrophils and mast cells were noticed. This is the first study of the occurrence of C. brachycollis in chub from Lake Blidinje and on the response of the innate immune cells of S. tenellus to this tapeworm. Interestingly, in 3 very heavily infected chubs, perforation of the intestinal wall was documented; this is uncommon among cestodes which use fish as a definitive host.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Distinctive location of piscine intestinal coccidiosis in Asian seabass fingerlings

Background and Aim:

Coccidian infection (coccidiosis) is one of the most important causes of illness and death in the fish population, including Asian sea bass. The fingerling developmental stage is sensitive to various infectious agents. Economic losses are sustained by the sea bass aquaculture industry due to coccidiosis annually. However, the related pathological changes in the Asian sea bass fingerlings’ three-part intestine remain unknown. This study aimed to investigate the Asian sea bass fingerlings’ infection rate, infection location and site, and specific pathological lesions in the small intestinal tissues in a marine cage farming operation.

Materials and Methods:

A cross-sectional study was conducted on 44 fingerling fishes. Major coccidia proportions were identified morphologically at both the macroscopic and microscopic levels. The infection number was determined based on coccidia presence at various intestinal locations and sites. All areas were assessed for pathological lesions using semi-quantitative grading. Analysis of variance was used to perform all data analyses using the SPSS software. Data were expressed as means ± standard deviation. p < 0.05 was considered statistically significant.

Results:

All Asian sea bass fingerlings studied were infected with coccidia. Enteritis and mucosal necrosis were distinct lesions found in the anterior intestine, which had the highest infection rate (49.94%), followed by the mid intestine (35.63%), and the posterior intestine (22.43%). The most common coccidian infection site was extracellular (subepithelial), followed by intracytoplasmic, and epicellular sites. Histopathological lesion determination revealed that intestinal tissue inflammation and epithelial injuries were predominantly seen in the anterior gut (p < 0.05).

Conclusion:

There was a high coccidian infection rate in Asian sea bass fingerlings from marine cage farming operations. Infection and intestinal damage at the anterior intestine, a major site, led to fingerling death. Disease prevention in the nursery should be intensive from the fingerling period to decrease the fatality rate caused by coccidia.

A NEW SPECIES OF MYXOBOLUS (CNIDARIA: MYXOSPOREA: MYXOBOLIDAE) FROM THE GILLS OF CREEK CHUB, SEMOTILUS ATROMACULATUS (CYPRINIFORMES: LEUCISCIDAE: PLAGIOPTERINAE), FROM THE OUACHITA DRAINAGE OF ARKANSAS

During October and November 2021, 33 creek chubs, Semotilus atromaculatus, were collected from 3 sites in Polk County, Arkansas (Ouachita River drainage), and their gills, gallbladder, fins, integument, musculature, and other major organs were examined for myxozoans. The gills of 9 (27%) were infected with a new myxozoan, Myxobolus fountainae n. sp. Qualitative and quantitative morphological data were from fresh and formalin-fixed preserved spores, while molecular data consisted of a 1918 base pair sequence of the partial small subunit ribosomal RNA gene. Phylogenetic analysis grouped M. fountainae n. sp. with the other leuciscid-infecting myxobolids from North America and within a larger clade of European myxozoans. In addition, histological information is provided on the infection. A previous record of Myxobolus muelleriBütschli, 1882, from the gills and ureters of S. atromaculatus is considered invalid and represents an unknown species. Myxobolus fountainae n. sp. is the only named myxozoan known to infect the gill filaments of S. atromaculatus, whereas Myxobolus pendula (Guilford, 1967) infects the gill arches.

Comparative Study of Physiological Changes in Turbot Scophthalmus maximus in Different Living Conditions

The aim of this study was to compare the organismal responses of farmed and wild-caught turbot, Scophthalmus maximus, based on analyses of biochemical plasma parameters, leukocyte profile, and histological tissue profile of gills, kidney, liver, intestine, and spleen, as well as gene expression of stress proteins in kidney and liver tissue. The results revealed significant differences in plasma triglycerides (TRIG), total protein (TP), albumin (ALB), globulin (GLOB), bilirubin (TBIL), creatinine (CRE) levels, creatine kinase (CK), and superoxide dismutase (SOD) activities that were higher, and A/G ratio, calcium (Ca) and phosphorus (P) concentrations, alkaline phosphatase (ALP), and glutathione peroxidase (GPx) activity, which were lower in farmed population. The neutrophil-leukocyte (N:L) ratio and gene expression of HSP70, HSP90, and WAP65-2 were increased in the wild-caught turbot. The wild-caught turbot were infested with the gill digenean parasite Dactylogyrus sp. and tapeworm Bothriocephalus scorpii. The obtained results provide valuable data for the assessment of the physiological responses of turbot for future comparative studies of the effects of various endogenous and exogenous factors on homeostasis of this species.

Rodlet cells, fish immune cells and a sentinel of parasitic harm in teleost organs

Rodlet cells (RCs) are the enigmatic and distinctive pear-shaped cells had found in many tissues of marine and freshwater teleosts. They have a distinctive fibrous capsule or the cell cortex that envelopes conspicuous inclusions called rodlets, basally situated nucleus, and poorly developed mitochondria. The contraction of the cell cortex results in the expulsion of the cell contents through an apical opening. One hundred and thirty years since rodlet cells were first reported, many questions remain about their origin and a function. This review will present new evidence regarding the relationship between RCs and metazoan parasites, and a protozoan infecting organs of different fish species, and update the state of knowledge about the origin, structure and the function of these intriguing fish cells.

Recent Advances with Fish Microsporidia

There have been several significant new findings regarding Microsporidia of fishes over the last decade. Here we provide an update on new taxa, new hosts and new diseases in captive and wild fishes since 2013. The importance of microsporidiosis continues to increase with the rapid growth of finfish aquaculture and the dramatic increase in the use of zebrafish as a model in biomedical research. In addition to reviewing new taxa and microsporidian diseases, we include discussions on advances with diagnostic methods, impacts of microsporidia on fish beyond morbidity and mortality, novel findings with transmission and invertebrate hosts, and a summary of the phylogenetics of fish microsporidia.

Molecular insights of a novel fish Toll-like receptor 9 homologue in Nibea albiflora to reveal its function as PRRs

Toll-like receptors (TLRs) are an important class of molecules involved in non-specific immunity, and they are also the bridge connecting between non-specific immunity and specific immunity. As a vital member of TLR family TLR9 can be activated by bacterial DNA and induce the production of inflammatory cytokines. In this study, a full length of TLR9 homologue of 3677 bp in Nibea albiflora (named as NaTLR9, GenBank accession no: MN125017.1) was characterized, and its ORF was 3180 bp encoding 1059 amino acid residues with a calculated molecular weight of 121.334 kDa (pI = 6.29). Several leucine-rich repeated sequences (LRR domain) and conservative TIR domain were found in NaTLR9, which was mainly expressed in dendritic cells and macrophages. The phylogenetic and synteny analysis further revealed high sequence identity of NaTLR9 with its counterparts of other teleost, confirming their correct nomenclature and conservative during evolution as an important pattern recognition receptor. The NaTLR9-TIR-pEGFP-N1 fusion protein showed green fluorescence and mainly distributed in the cytoplasm. After co-transfection of NaTLR9-TIR-pEGFP-N1 and NaMyD88-pDsRED-Monomer-N1, green fluorescence obviously overlapped with red and changed into yellowish-green, which suggested that there might be the interaction between homologous NaTLR9-TIR and MyD88. Based on this result the pCDNA3.1-NaTLR9-TIR-flag and pcMV-NaMyD88-TIR-Myc plasmids were co-transfected into 293T cells for the immunoprecipitation test. According to Western blot, TLR9 and MyD88 protein could interact with each other. Furthermore, NaTLR9 was ubiquitously expressed in all the investigated tissues, most abundantly in head kidney, followed by stomach, spleen, liver and gill, but lower in muscle. The vitro immune stimulation experiments revealed that Pseudomonas plecoglossicida and polyinosinic-polycytidylic acid [Poly (I:C)] induced higher levels of NaTLR9 mRNA expression with the peaks of 9.52 times at 2 h and 39.91 times at 24 h compared with the control group respectively. The functional domains (LRRs and TIR, named NaTLR9-TIR and NaTLR9-LRR respectively) of NaTLR9 were expressed and purified, the recombinant proteins both could bind three kinds of typical aquatic pathogenic bacteria (Vibrio. parahaemolyticus, Vibrio alginolyticus, and Vibrio harveyi), which showed that NaTLR9 could couple to bacteria by its function domains. The aforementioned results indicated that NaTLR9 played a significant role in the defense against pathogenic bacteria infection in innate immune response of sciaenidae fish, which may provide some further understandings of the regulatory mechanisms in the teleostean innate immune system.

Protection of Teleost Fish against Infectious Diseases through Oral Administration of Vaccines: Update 2021

Immersion and intraperitoneal injection are the two most common methods used for the vaccination of fish. Because both methods require that fish are handled and thereby stressed, oral administration of vaccines as feed supplements is desirable. In addition, in terms of revaccination (boosting) of adult fish held in net pens, oral administration of vaccines is probably the only feasible method to obtain proper protection against diseases over long periods of time. Oral vaccination is considered a suitable method for mass immunization of large and stress-sensitive fish populations. Moreover, oral vaccines may preferably induce mucosal immunity, which is especially important to fish. Experimental oral vaccine formulations include both non-encapsulated and encapsulated antigens, viruses and bacteria. To develop an effective oral vaccine, the desired antigens must be protected against the harsh environments in the stomach and gut so they can remain intact when they reach the lower gut/intestine where they normally are absorbed and transported to immune cells. The most commonly used encapsulation method is the use of alginate microspheres that can effectively deliver vaccines to the intestine without degradation. Other encapsulation methods include chitosan encapsulation, poly D,L-lactide-co-glycolic acid and liposome encapsulation. Only a few commercial oral vaccines are available on the market, including those against infectious pancreatic necrosis virus (IPNV), Spring viremia carp virus (SVCV), infectious salmon anaemia virus (ISAV) and Piscirickettsia salmonis. This review highlights recent developments of oral vaccination in teleost fish.

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

Chestnut Shell Tannins: Effects on Intestinal Inflammation and Dysbiosis in Zebrafish

Simple Summary

With the increase in global population the production of animal proteins becomes increasingly crucial. Aquaculture is the first animal protein supply industry for human consumption. Intensive farming techniques are employed to increase productivity, but these may cause stressful conditions for fish, resulting in impaired growth and poor health conditions. Intestinal inflammation is one of the most common diseases of fish in intensive farming. Intestinal inflammation is usually accompanied by an alteration of the microbiota or dysbiosis. Inflammation and dysbiosis are so tightly intertwined that inflammation may contribute to or result from dysregulation of gut microbiota. Natural substances of plant origin rich in bioactive molecules or more simply phytochemicals, have been proved to be able to reduce inflammation and improve the general health status in various commercially relevant species. In this study, we evaluated the effect of tannins, a class of polyphenols, the most abundant phytochemicals, on intestinal inflammation and microbiota in zebrafish (Danio rerio), a small freshwater fish become an attractive biomedicine and aquaculture animal model during the last decades. The zebrafish has been employed in a vast array of studies aiming at investigating the essential processes underlying intestinal inflammation and injury due to its conservative gut morphology and functions. In this study, we administered a diet enriched with chestnut shell extract rich in tannins to a zebrafish model of intestinal inflammation. The treatment ameliorated the damaged intestinal morphophysiology and the microbiota asset. Our results sustain that products of natural origin with low environmental impact and low cost, such as tannins, may help to ease some of the critical issues affecting the aquaculture sector.

Abstract

The aim of the present study was to test the possible ameliorative efficacy of phytochemicals such as tannins on intestinal inflammation and dysbiosis. The effect of a chestnut shell (Castanea sativa) extract (CSE) rich in polyphenols, mainly represented by tannins, on k-carrageenan-induced intestinal inflammation in adult zebrafish (Danio rerio) was tested in a feeding trial. Intestinal inflammation was induced by 0.1% k-carrageenan added to the diet for 10 days. CSE was administered for 10 days after k-carrageenan induced inflammation. The intestinal morphology and histopathology, cytokine expression, and microbiota were analyzed. The k-carrageenan treatment led to gut lumen expansion, reduction of intestinal folds, and increase of the goblet cells number, accompanied by the upregulation of pro-inflammatory factors (TNFα, COX2) and alteration in the number and ratio of taxonomic groups of bacteria. CSE counteracted the inflammatory status enhancing the growth of health helpful bacteria (Enterobacteriaceae and Pseudomonas), decreasing the pro-inflammatory factors, and activating the anti-inflammatory cytokine IL-10. In conclusion, CSE acted as a prebiotic on zebrafish gut microbiota, sustaining the use of tannins as food additives to ameliorate the intestinal inflammation. Our results may be relevant for both aquaculture and medical clinic fields.

Survival of metazoan parasites in fish: Putting into context the protective immune responses of teleost fish

Defence mechanisms of fish can be divided into specific and non-specific that act in concert and are often interdependent. Most fish in both wild and cultured populations are vulnerable to metazoan parasites. Endoparasitic helminths include several species of digeneans, cestodes, nematodes, and acanthocephalans. Although they may occur in large numbers, helminth infections rarely result in fish mortality. Conversely, some ectoparasites cause mass mortality in farmed fish. Given the importance of fish innate immunity, this review addresses non-specific defence mechanisms of fish against metazoan parasites, with emphasis on granulocyte responses involving mast cells, neutrophils, macrophages, rodlet cells, and mucous cells. Metazoan parasites are important disease agents that affect wild and farmed fish and can induce high economic loss and, as pathogen organisms, deserve considerable attention. The paper will provide our light and transmission electron microscopy data on metazoan parasites-fish innate immune and neuroendocrine systems. Insights about the structure and functions of the cell types listed above and a brief account of the effects and harms of each metazoan taxon to specific fish apparati/organs will be presented.

Immunohistopathological response against anisakid nematode larvae and a coccidian in Micromesistius poutassou from NE Atlantic waters

A survey on Anisakis simplex (sensu stricto (s.s.)) from blue whiting, Micromesistius poutassou, in the north-eastern Atlantic Ocean revealed the occurrence of high infection levels of third larval stages in visceral organs and flesh. Larvae were genetically identified with a multilocus approach as A. simplex (s.s.). Histochemical, immunohistochemical and ultrastructural observations were conducted on 30 M. poutassou specimens. Gonads, pyloric caeca and flesh harboured encapsulated larvae of A. simplex (s.s.) but no intense host reaction was encountered around the parasite in the above organs. In the liver, the most infected organ, the larvae co-occurred with the coccidian Goussia sp. Within the granuloma around the A. simplex (s.s.) larvae, two concentric layers were recognized, an inner mostly comprising electron-dense epithelioid cells and an outer layer made of less electron-dense epithelioid cells. Macrophages and macrophage aggregates (MAs) were abundant out of the granulomas, scattered in parenchyma, and inside the MAs, the presence of engulfed Goussia sp. was frequent. In liver tissue co-infected with Goussia sp. and A. simplex (s.s.), hepatocytes showed cytoplasmic rarefaction and acute cell swelling. Results suggest that the host-induced encapsulation of A. simplex (s.s.) larvae is a strategic compromise to minimize collateral tissue damage around the larval infection sites, to facilitate the survival of both parasite and host.

Development of Fish Parasite Vaccines in the OMICs Era: Progress and Opportunities

Globally, parasites are increasingly being recognized as catastrophic agents in both aquaculture sector and in the wild aquatic habitats leading to an estimated annual loss between 1.05 billion and 9.58 billion USD. The currently available therapeutic and control measures are accompanied by many limitations. Hence, vaccines are recommended as the "only green and effective solution" to address these concerns and protect fish from pathogens. However, vaccine development warrants a better understanding of host-parasite interaction and parasite biology. Currently, only one commercial parasite vaccine is available against the ectoparasite sea lice. Additionally, only a few trials have reported potential vaccine candidates against endoparasites. Transcriptome, genome, and proteomic data at present are available only for a limited number of aquatic parasites. Omics-based interventions can be significant in the identification of suitable vaccine candidates, finally leading to the development of multivalent vaccines for significant protection against parasitic infections in fish. The present review highlights the progress in the immunobiology of pathogenic parasites and the prospects of vaccine development. Finally, an approach for developing a multivalent vaccine for parasitic diseases is presented. Data sources to prepare this review included Pubmed, google scholar, official reports, and websites.

Effects of inflammation and/or infection on the neuroendocrine control of fish intestinal motility: A review

Food is the largest expense in fish farms. On the other hand, the fish health and wellbeing are determining factors in aquaculture production where nutrition is a vital process for growing animals. In fact, it is important to remember that digestion and nutrition are crucial for animals' physiology. However, digestion is a very complex process in which food is processed to obtain necessary nutrients and central mechanisms of this process require both endocrine and neuronal regulation. In this context, intestinal motility is essential for the absorption of the nutrients (digestive process determining nutrition). An imbalance in the intestinal motility due to an inadequate diet or an infectious process could result in a lower use of the food and inefficiency in obtaining nutrients from food. Very frequently, farmed fish are infected with different pathogenic microorganism and this situation could alter gastrointestinal physiology and, indirectly reduce fish growth. For these reasons, the present review focuses on analysing how different inflammatory molecules or infections can alter conventional modulators of fish intestinal motility.

Challenge for macrophages and mast cells of Chelon ramada to counter an intestinal microparasite, Myxobolus mugchelo (Myxozoa)

Thinlip mullet Chelon ramada is the most abundant mullet species found in the Comacchio lagoons (northern Adriatic Sea, Italy). Histological and ultrastructural sections of the intestine of C. ramada showed that over 83% of 48 mullets were infected with the intestinal parasite Myxobolus mugchelo (Myxozoa). In histological sections, plasmodia of M. mugchelo containing mature spores were situated closer to mucosal folds and were surrounded by numerous mast cells (MCs). Mature spores, generally oval in shape, were observed in the paracellular space among the enterocytes or within them. Near the infected epithelial cells, several MCs, rodlet cells and few neutrophils occurred. In intestinal epithelium, large cells resembling macrophages, some with spores of M. mugchelo inside, were observed. These macrophage-like cells were foamy and possessed elongate striated granules. The number of MCs and macrophages in the intestinal epithelium was significantly higher in parasitized fish. In some parasitized intestines, portions of epithelium were displaced by spores, or the spores were observed inside the damaged enterocytes. Immunohistochemical analysis of C. ramada infected or uninfected intestinal tissue revealed the presence of histamine, serotonin (5-HT), leu-enkephalin and inducible-nitric oxide synthase in epithelial macrophages. Several epithelial cells positive to proliferating cell-nuclear antigen were also observed in the proximity of the macrophages. The current study is the first to record the occurrence of intraepithelial macrophages which engulf myxozoan spores. A hypothesis on migration of spores from pancreas via intestinal wall to gut lumen is presented.

Composition of the microbial communities in the gastrointestinal tract of perch (Perca fluviatilis L. 1758) and cestodes parasitizing the perch digestive tract

Using the approach of sequencing the V3-V4 region of the 16S rRNA gene, we have analysed the bacterial diversity associated with the distinct compartments of the gastrointestinal tract of perch (Perca fluviatilis) and cestodes (Proteocephalus sp.) parasitizing their digestive tract. The dominant microbiota associated with cestodes (Proteocephalus sp.) was represented by bacteria from the genera Serratia, Pseudomonas and Mycoplasma. By comparing the associated microbiota of perch and cestodes, a clear difference in bacterial composition and diversity was revealed between the community from the stomach content and other parts of the gastrointestinal tract of fish. Microbiota associated with cestodes was not significantly different in comparison with microbiota of different subcompartments of perch (mucosa and content of intestine and pyloric caeca) (ADONIS, p > .05) excluding microbiota of stomach content (ADONIS, p ≤ .05). PICRUSt-based functional assessments of the microbial communities of perch and cestodes indicated that they mainly linked in terms of metabolism and environmental information processing and could play an important role in the nutrition and health of host.

RNA-seq analysis of local tissue of Carassius auratus gibelio with pharyngeal myxobolosis: Insights into the pharyngeal mucosal immune response in a fish-parasite dialogue

The lack of practical control measures for pharyngeal myxobolosis is becoming an important limiting factor for the sustainable development of the gibel carp (Carassius auratus gibelio) culture industry in China. Myxobolus honghuensis has been identified as the causative agent of this pandemic disease, which exclusively infects the pharynx of gibel carp, a potential important mucosal lymphoid-associated tissue (MLAT). Myxozoa generally initiate invasion through the mucosal tissues of fish, where some of them also complete their sporogonial stages. However, the pharynx-associated immune responses of teleost against myxosporeans infection remain unknown. Here, a de novo transcriptome assembly of the pharynx of gibel carp naturally infected with M. honghuensis was performed for the first time, using RNA-seq. Comparative analysis of severely infected and mildly infected pharyngeal tissues (SI group and MI group) from the same fish individuals and control pharyngeal tissues (C group) from the uninfected fish was carried out to investigate the potential mucosal immune function of the fish pharynx, and characterize the panoramic picture of pharynx local mucosal immune responses of gibel carp against the M. honghuensis infection. A total of 242,341 unigenes were obtained and pairwise comparison resulted in 13,009 differentially-expressed genes (DEGs) in the SI/C group comparison, 6014 DEGs in the MI/C group comparison, and 9031 DEGs in the SI/MI group comparison. Comprehensive analysis showed that M. honghuensis infection elicited a significant parasite load-dependent alteration of the expression of numerous innate and adaptive immune-related genes in the local lesion tissue. Innate immune molecules, including mucins, toll-like receptors, C-type lectin, serum amyloid A, cathepsins and complement components were significantly up-regulated in the SI group compared with the C group. Up-regulation of genes involved in apoptosis signaling pathway and the IFN-mediated immune system were found in the SI group, suggesting these two pathways played a crucial role in innate immune response to M. honghuensis infection. Up-regulation of chemokines and chemokine receptors and the induction of the leukocyte trans-endothelial migration pathways in the severely and mildly infected pharynx suggested that many leucocytes were recruited to the local infected sites to mount a strong mucosal immune responses against the myxosporean infection. Up-regulation of CD3D, CD22, CD276, IL4/13A, GATA3, arginase 2, IgM, IgT and pIgR transcripts provided strong evidences for the presence of T/B cells and specific mucosal immune responses at local sites with M. honghuensis infection. Our results firstly demonstrated the mucosal function of the teleost pharynx and provided evidences of intensive local immune defense responses against this mucosa-infecting myxosporean in the gibel carp pharynx. Pharyngeal myxobolosis was shaped by a prevailing anti-inflammatory response pattern during the advanced infection stages. Further understanding of the functional roles of fish immune molecules involved in the initial invasion and/or final sporogony site may facilitate future development of control strategies for this myxobolosis.

Histopathological changes induced by the digenean intestinal parasite Masenia nkomatiensis Dumbo, Dos Santos, & Avenant-Oldewage, 2019 of the catfish Clarias gariepinus (Burchell) from Incomati Basin, Mozambique

The intestines of 154 Clarias gariepinus were examined of which 29 were naturally infected with Masenia nkomatiensis, and of these, seven (intensity ranging from 8 to 231) were examined for pathology. Destruction of the epithelium covering the villi, detachment of epithelial cells and parts of villi were observed. Excessive mucus secretion occurred in the vicinity of the worm and catarrh was observed, indicative of an inflammatory response. The number of mucous and mast cells was higher at the attachment site than at an area 5,000 µm away and in uninfected individuals, suggesting that the parasite triggered a localized innate immune response. The number of neutrophils, basophils and lymphocytes in infected tissue was not significantly different from uninfected tissue confirming that no acquired immune response was produced against the maseniid. The caecae in the anterior part of the parasites' intestine consisted of convoluted epithelium forming invaginations or "crypts." Contraction of the thick layer of circular muscle fibres of the caeca facilitates the movement of digested material. Observation of digested host cells and cell debris within the caecae provides further evidence that M. nkomatiensis is consuming host cells.

Dietary phytogenics and galactomannan oligosaccharides in low fish meal and fish oil-based diets for European sea bass (Dicentrarchus labrax) juveniles: Effects on gut health and implications on in vivo gut bacterial translocation

European sea bass were fed four low FM/FO (10%/6%) diets containing galactomannan oligosaccharides (GMOS), a mixture of garlic oil and labiatae plants oils (PHYTO), or a combination of both functional products (GMOSPHYTO) for 63 days before exposing the fish to an intestinal Vibrio anguillarum infection combined with crowding stress. In order to evaluate functional diets efficacy in terms of gut health maintenance, structural, cellular, and immune intestinal status were evaluated by optical and electron microscopy and gene expression analyses. A semi-automated software was adapted to determine variations in goblet cell area and mucosal mucus coverage during the challenge test. Feeding with functional diets did not affect growth performance; however, PHYTO and GMOS dietary inclusion reduced European sea bass susceptibility to V. anguillarum after 7 days of challenge testing. Rectum (post-ileorectal valve) showed longer (p = 0.001) folds than posterior gut (pre-ileorectal valve), whereas posterior gut had thicker submucosa (p = 0.001) and higher mucus coverage as a result of an increased cell density than rectum. Functional diets did not affect mucosal fold length or the grade of granulocytes and lymphocytes infiltration in either intestinal segment. However, the posterior gut fold area covered by goblet cells was smaller in fish fed GMOS (F = 14.53; p = 0.001) and PHYTO (F = 5.52; p = 0.019) than for the other diets. PHYTO (F = 3.95; p = 0.049) reduced posterior gut goblet cell size and increased rodlet cell density (F = 3.604; p = 0.068). Dietary GMOS reduced submucosal thickness (F = 51.31; p = 0.001) and increased rodlet cell density (F = 3.604; p = 0.068) in rectum. Structural TEM analyses revealed a normal intestinal morphological pattern, but the use of GMOS increased rectum microvilli length, whereas the use of PHYTO increased (p≤0.10) Ocln, N-Cad and Cad-17 posterior gut gene expression. After bacterial intestinal inoculation, posterior gut of fish fed PHYTO responded in a more controlled and belated way in terms of goblet cell size and mucus coverage in comparison to other treatments. For rectum, the pattern of response was similar for all dietary treatments, however fish fed GMOS maintained goblet cell size along the challenge test.

Acquired protective immune response in a fish-myxozoan model encompasses specific antibodies and inflammation resolution

The myxozoan parasite Enteromyxum leei causes chronic enteritis in gilthead sea bream (GSB, Sparus aurata) leading to intestinal dysfunction. Two trials were performed in which GSB that had survived a previous infection with E. leei (SUR), and naïve GSB (NAI), were exposed to water effluent containing parasite stages. Humoral factors (total IgM and IgT, specific anti-E. leei IgM, total serum peroxidases), histopathology and gene expression were analysed. Results showed that SUR maintained high levels of specific anti-E. leei IgM (up to 16 months), expressed high levels of immunoglobulins at the intestinal mucosa, particularly the soluble forms, and were resistant to re-infection. Their acquired-type response was complemented by other immune effectors locally and systemically, like cell cytotoxicity (high granzyme A expression), complement activity (high c3 and fucolectin expression), and serum peroxidases. In contrast to NAI, SUR displayed a post-inflammatory phenotype in the intestine and head kidney, characteristic of inflammation resolution (low il1β, high il10 and low hsp90α expression).

Detection of the intranuclear microsporidian Enterospora nucleophila in gilthead sea bream by in situ hybridization

Enterospora nucleophila is an intranuclear microsporidian responsible for emaciative microsporidiosis of gilthead sea bream (GSB). Its minute size and cryptic nature make it easily misdiagnosed. An in situ hybridization (ISH) technique based on antisense oligonucleotide probes specific for the parasite was developed and used in clinically infected GSB in combination with calcofluor white stain (CW) and other histopathological techniques. The ISH method was found to label very conspicuously the cells containing parasite stages, with the signal concentrating in merogonial and sporogonial plasmodia within the infected cell nuclei. Comparison with CW demonstrated limited ISH signal in cells containing mature spores, which was attributed mostly to the scarcity of probe targets present in these stages. Although spores were detected in other organs of the digestive system as well as in the peripheral blood, proliferative stages or parasite reservoirs were not found in this work outside the intestines. The study demonstrated a frequent disassociation between the presence of abundant spores and the intensity of the infections as determined by the parasite activity. The ISH allows confirmatory diagnosis of GSB microsporidiosis and estimation of infection intensity and will be a valuable tool for a more precise determination of parasite dissemination pathways and pathogeny mechanisms.

Transcriptome and metabolome analyses of Coilia nasus in response to Anisakidae parasite infection

Parasites from the family Anisakidae are capable of infecting a range of marine fish species worldwide. Coilia nasus, which usually feeds and overwinters in coastal waters and spawns in freshwater, is highly susceptible to infection by Anisakidae. In this study, we used scanning electron microscopes to show that C. nasus infected by Anisakidae exhibited damage and fibrosis of the liver tissue. To better understand host immune reaction and metabolic changes to Anisakidae infection, we used a combination of transcriptomic and metabolomic method to characterize the key genes and metabolites, and the signaling pathway regulation of C. nasus infected by Anisakidae. We generated 62,604 unigenes from liver tissue and identified 391 compounds from serum. Of these, Anisakidae infection resulted in significant up-regulation of 545 genes and 28 metabolites, and significant down-regulation of 416 genes and 37 metabolites. Seventy-four of the 961 differentially expressed genes were linked to immune response, and 1, 2-Diacylglycerol, an important immune-related metabolite, was significantly up-regulated after infection. Our results show activation of antigen processing and presentation, initiation of the T cell receptor signaling pathway, disruption of the TCA cycle, and changes to the amino acid and Glycerolipid metabolisms, which indicate perturbations to the host immune system and metabolism following infection. This is the first study describing the immune responses and metabolic changes in C. nasus to Anisakidae infection, and thus improves our understanding of the interaction mechanisms between C. nasus and Anisakidae. Our findings will be useful for future research on the population ecology of C. nasus.

Characterization of TLR5 and TLR9 from silver pomfret (Pampus argenteus) and expression profiling in response to bacterial components

Toll like receptor (TLR) 5 and 9 are important members of the TLR family that play key roles in innate immunity in all vertebrates. In this study, paTLR5 and paTLR9 were identified in silver pomfret (Pampus argenteus), a marine teleost of great economic value. Open reading frames (ORFs) of paTLR5 and paTLR9 are 2646 and 3225 bp, encoding polypeptides of 881 and 1074 amino acids, respectively. Sequence analysis revealed several conserved characteristic features, including signal peptides, leucine-rich repeat (LRR) motifs, and a Toll/interleukin-I receptor (TIR) domain. Sequence, phylogenetic and synteny analysis revealed high sequence identity with counterparts in other teleosts, confirming their correct nomenclature and conservation during evolution. Quantitative real-time PCR revealed that the that both TLRs were ubiquitously expressed in all investigated tissues, most abundantly in liver, kidney, spleen, intestine and gill, but lower in muscle and skin. In vitro immunostimulation experiments revealed that Aeromonas hydrophila lipopolysaccharide (LPS) and Vibrio anguillarum flagellin induced higher levels of paTLR9 and paTLR5 mRNA expression in isolated fish intestinal epithelial cells (FIECs) than Lactobacillus plantarum lipoteichoic acid (LTA), but all increased the secretion of IL-6 and TNF-α and induced cell apoptosis and necrosis. Together, these results indicate that paTLR5 and paTLR9 may function in the response to bacterial pathogens. Our findings enhance our understanding of the function of TLRs in the innate immune system of silver pomfret and other teleosts.

Bromodeoxyuridine DNA labelling reveals host and parasite proliferation in a fish-myxozoan model

Enteromyxum leei is a myxozoan parasite responsible for enteritis in gilthead sea bream (Sparus aurata). The parasite proliferates in the paracellular space of the intestinal epithelium and induces an inflammatory reaction. To assess intestinal cell turnover and parasite proliferation, fish were infected with the parasite by anal intubation; after 17 and 64 days, the cell proliferative marker bromodeoxyuridine (BrdU) was administered; and after 24 hr, tissue samples were taken for immunohistochemical detection. Parasite exposure induced increased epithelial and immune cell proliferation in all intestinal segments at all time points, even before parasite establishment. This increased turnover was triggered early after intubation and mainly at a local level, as shown by an increased proliferating cell nuclear antigen (pcna) gene expression only at the posterior intestine after 17 days (not found in lymphohaematopoietic organs). Incorporation of BrdU in parasite secondary and tertiary daughter cells indicated that parasite endogeny is not by schizogonial division, which uses de novo synthesis pathway of pyrimidines. Altogether, BrdU immunolabelling and pcna gene expression showed the rapid proliferative response of the fish intestines upon a myxozoan infection and how this response is effectively triggered even before the parasite reaches or establishes in the site.

Histopathological and ultrastructural assessment of two mugilid species infected with myxozoans and helminths

The histopathology and ultrastructure of the intestine of mullets, Liza ramada and Liza saliens, from Comacchio lagoons (northern Italy) naturally infected with myxozoans and helminths were investigated and described. Sixty-two (80.5%) of 77 mullets harboured one or more of the following parasites species: Myxobolus mugchelo (Myxozoa), Neoechinorhynchus agilis (Acanthocephala), Haplosplanchnus pachysomus and Dicrogaster contractus (Digenea). Co-occurrence of helminths with myxozoans was common. The main damage caused by digeneans was destruction of the mucosal epithelium of the villi, necrosis and degeneration of intestinal epithelial cells. More severe intestinal damage was caused by acanthocephalans which reach the submucosa layer with their proboscis. At the site of helminths infection, several mast cells (MCs), rodlet cells (RCs), mucous cells and few neutrophils and macrophages were observed in the epithelium. RCs and mucous cells exhibited discharge activity in close vicinity to the worm's tegument. M. mugchelo conspicuous plasmodia were encysted mainly in muscle and submucosa layers of the intestine. Indeed, spores of M. mugchelo were documented within the epithelial cells of host intestine and in proximity to MCs. Degranulation of the MCs near the myxozoans was very frequent.

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Immune response of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata

This study evaluated the pathophysiological, biochemical, and immunological status of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata. Flounder fish collected from Kudoa-infected and uninfected farms were confirmed by microscopic and TaqMan probe-based quantitative PCR screening. Morphological, biochemical, histological, and immune gene expression analyses were performed on uninfected and infected hosts to assess the effect of K. septempunctata. Histological studies confirmed the presence of Kudoa myxospores in the trunk muscles of infected flounder fish. Serum biochemical parameters, including the levels of myeloperoxidase activity, superoxide dismutase activity, alanine aminotransferase, alkaline phosphatase, amylase, bilirubin, total protein, cholesterol, calcium, potassium, sodium, phosphorus, glucose, and galactose, were found to exhibit no significant variations (p > 0.05) between uninfected and infected flounder fish. However, immune-related genes such as Mx, lysozyme, signal transducer and activator of transcription 1, interferon-γ, interferon regulatory factor, and tumour necrosis factor showed significantly elevated expression (p < 0.05) in the trunk muscles of infected flounder fish while no significant differences were noted in uninfected fish trunk muscle and head-kidney of infected and uninfected flounder fish.

Protective responses of intestinal mucous cells in a range of fish-helminth systems

Histopathological, immunofluorescence and ultrastructural studies were conducted on the intestines of four fish species infected with different taxa of enteric helminths. Brown trout (Salmo trutta trutta), eel (Anguilla anguilla) and tench (Tinca tinca) obtained from Lake Piediluco (central Italy) were examined. Brown trout and eel were infected with two species of acanthocephalans, and tench was parasitized with a tapeworm species. In addition to the above site, specimens of chub (Squalius cephalus) and brown trout infected with an acanthocephalan were examined from the River Brenta (north Italy). Moreover, eels were examined from a brackish water, Comacchio lagoons (north Italy), where one digenean species was the predominant enteric worm. All the helminths species induced a similar response, the hyperplasia of the intestinal mucous cells, particularly of those secreting acid mucins. Local endocrine signals seemed to affect the production and secretion of mucus in the parasitized fish, as worms often were surrounded by an adherent mucus layer or blanket. This is the first quantitative report of enteric worm effects on the density of various mucous cell types and on the mucus composition in intestine of infected/uninfected conspecifics. We provide a global comparison between the several fish-helminth systems examined.