Selected pathological, immunohistochemical and ultrastructural changes associated with an infection by Diphyllobothrium dendriticum (Nitzsch, 1824) (Cestoda) plerocercoids in Coregonus lavaretus (L.) (Coregonidae)

Invasive parasites and global change: Evidence for the recent and rapid spillover of a potential pathogen of tilapias with a complex, three-host life cycle

Biological invasions pose a serious threat to local flora and fauna and have negative impacts on ecosystems. Invasive parasites can also cause severe losses in aquaculture. In this article, we provide evidence of the recent spillover of an African parasite with a complex, three-host life cycle that has rapidly and successfully established itself in the Middle East, most likely due to the recent migration of its final hosts (great cormorant) from Africa. This case of parasite introduction into a country with intensive aquaculture is also important from an economic point of view, since large (up to 2 cm long) larvae of this parasite, the cyclophyllidean tapeworm Amirthalingamia macracantha (Cestoda) localised in the liver, can be pathogenic to their fish hosts, including farmed and wild fish, as shown by our histopathological examination of heavily infected fish. Since its first detection in Israel in November 2020, the parasite has spread rapidly and is currently found in both migratory (great cormorant, Phalacrocorax carbo) and non-migratory birds (pygmy cormorant, Microcarbo pygmaeus), as well as in fish intermediate hosts, including farmed tilapia in several farms in Israel and wild cichlids. There are numerous examples of the spillover of introduced parasites, including those that parasitise fish of commercial importance, but have a direct life cycle or use only a single intermediate host. Tilapines are the second most important group of farmed fish in the world after carps and are produced mainly in Southeast Asia, Central and South America. The global spread of great cormorants and the early evidence that pygmy cormorant may also harbour A. macracantha pose the risk of further spread of this invasive parasite to other countries and areas. In addition, global warming and reductions in foraging and resting areas near these waters may allow the parasite to complete its life cycle in new hosts.

Parasites and the neuroendocrine control of fish intestinal function: an ancient struggle between pathogens and host

Most individual fish in wild and farmed populations can be infected with parasites. Fish intestines can harbour protozoans, myxozoans and helminths, which include several species of digeneans, cestodes, nematodes and acanthocephalans. Enteric parasites often induce inflammation of the intestine; the pathogen provokes changes in the host physiology, which will be genetically selected for if they benefit the parasite. The host response to intestinal parasites involves neural, endocrine and immune systems and interaction among these systems is coordinated by hormones, chemokines, cytokines and neurotransmitters including peptides. Intestinal fish parasites have effects on the components of the enteric nervous and endocrine systems; mechanical/chemical changes impair the activity of these systems, including gut motility and digestion. Investigations on the role of the neuroendocrine system in response to fish intestinal parasites are very few. This paper provides immunohistochemical and ultrastructural data on effects of parasites on the enteric nervous system and the enteric endocrine system in several fish–parasite systems. Emphasis is on the occurrence of 21 molecules including cholecystokinin-8, neuropeptide Y, enkephalins, galanin, vasoactive intestinal peptide and serotonin in infected tissues.

Tapeworms as pathogens of fish: A review

Tapeworms (Cestoda) represents a species rich (about 5000 species) group of flatworms (Neodermata) parasitizing all groups of vertebrates including humans, with about 1000 species parasitizing elasmobranchs and almost 500 occurring in teleosts as adults. They are common parasites of cultured fish, both as adults and larvae (metacestodes), but only few adult tapeworms are actually pathogenic for their fish hosts. In contrast, cestode larvae can be harmful for fish, especially plerocercoids migrating throughout their tissue and internal organs. Current knowledge of host-parasite relationships, including immune response of fish infected with tapeworms, is still insufficient to enable adequate control of cestodoses, and most data available were obtained several decades ago. Treatment of fish infected with adult tapeworms is effective, especially with praziquantel, whereas the treatment of metacestodes is problematic. Control measures include interruption of the complex life cycle and prevention of transport of uninspected fish to new region.

Experimental study of ultrastructural mechanisms and kinetics of tegumental secretion in cestodes parasitizing fish (Cestoda: Diphyllobothriidea)

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.

Stages of Granulomatous Response Against Histozoic Metazoan Parasites in Mullets (Osteichthyes: Mugilidae)

Simple Summary

Parasitic diseases represent a common issue in fish and, when histozoic forms are present, this elicits a chronic inflammatory reaction leading to granuloma formation. Despite the large knowledge of granuloma formation due to parasites in visceral organs, little is known about the development and the evolutive stages of granulomas in naturally infected fish. Mullets (Osteichthyes: Mugilidae) are a widespread euryhaline fish species that harbor different parasites, thus representing a suitable model for the study of parasite-induced granulomas. Combining histopathology and immunohistochemical tools, we identified three developmental granuloma stages (pre-granuloma, intermediate, and late stage), that ranged from an intact parasite with mild signs of tissue reaction to the formation of a structured granuloma. The identified histological patterns could be reliable tools in the staging of the granulomatous response associated with histozoic parasites and are an attempt to broaden the knowledge of the inflammatory response in different host–parasite systems.

Abstract

Histozoic parasite–fish host interaction is a dynamic process that leads to the formation of a granuloma, a specific chronic inflammatory response with discernible histological features. Mullets (Osteichthyes: Mugilidae) represent a suitable model concerning the development of such lesions in the host–parasite interface. The present work aimed to identify granuloma developmental stages from the early to the late phase of the infection and to characterize the immune cells and non-inflammatory components of the granuloma in different stages. For this purpose, 239 mullets were collected from 4 Sardinian lagoons, and several organs were examined by combining histopathological, bacteriological, and immunohistochemical methods. Granulomas associated with trematode metacercariae and myxozoan parasites were classified into three developmental stages: (1) pre-granuloma stage, characterized by intact encysted parasite and with no or mild tissue reaction; (2) intermediate stage, with partially degenerated parasites, necrosis, and a moderate number of epithelioid cells (ECs); and (3) late stage, with a necrotic core and no detectable parasite with a high number of ECs and fibroblasts. The three-tier staging and the proposed morphological diagnosis make it conceivable that histopathology could be an essential tool to evaluate the granulomas associated with histozoic parasitic infection in fish.

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.

Pike intestinal reaction to Acanthocephalus lucii (Acanthocephala): immunohistochemical and ultrastructural surveys

Background

The Northern pike, Esox lucius, is a large, long-lived, top-predator fish species and occupies a broad range of aquatic environments. This species is on its way to becoming an important model organism and has the potential to contribute new knowledge and a better understanding of ecology and evolutionary biology. Very few studies have been done on the intestinal pathology of pike infected with helminths. The present study details the first Italian record of adult Acanthocephalus lucii reported in the intestine of E. lucius.

Results

A total of 22 pike from Lake Piediluco (Central Italy) were examined, of which 16 (72.7%) were infected with A. lucii. The most affected areas of gastrointestinal tract were the medium and distal intestine. The intensity of infection ranged from 1 to 18 parasites per host. Acanthocephalus lucii penetrated mucosal and submucosal layers which had a high number of mast cells (MCs) with an intense degranulation. The cellular elements involved in the immune response within the intestine of pike were assessed by ultrastructural techniques and immunohistochemistry using antibodies against met-enkephalin, immunoglobulin E (IgE)-like receptor (FCεRIγ), histamine, interleukin-6, interleukin-1β, substance P, lysozyme, serotonin, inducible-nitric oxide synthase (i-NOS), tumor necrosis factor-α (TNF-α) and the antimicrobial peptide piscidin 3 (P3). In intestines of the pike, several MCs were immunopositive to 9 out of the 11 aforementioned antibodies and infected fish had a higher number of positive MCs when compared to uninfected fish.

Conclusions

Pike intestinal tissue response to A. lucii was documented. Numerous MCs were seen throughout the mucosa and submucosal layers. In infected and uninfected intestines of pike, MCs were the dominant immune cell type encountered; they are the most common granulocyte type involved in several fish-helminth systems. Immunopositivity of MCs to 9 out of 11 antibodies is of great interest and these cells could play an important key role in the host response to an enteric helminth. This is the first report of A. lucii in an Italian population of E. lucius and the first account on positivity of MCs to piscidin 3 and histamine in a non-perciform fish.

Liver of the fish Gymnotus inaequilabiatus and nematode larvae infection: Histochemical features and expression of proliferative cell nuclear antigen

Histopathological lesions due to third-larval stage of nematode Brevimulticaecum sp. within the liver of a subpopulation of 31 Gymnotus inaequilabiatus from the Pantanal Region (Brazil) were studied with histochemical and immunohistochemical methods. In 93.5% of fish, livers harboured nematode larvae and the intensity of infection ranged from 8 to 293. In livers with highest number of larvae, the hepatic tissue was occupied primarily by the nematodes. Each larva was encircled by focal inflammatory granulomatous reaction. Within the thickness of the granuloma, three concentric layers were recognized: an inner layer of densely packed epithelioid cells, a middle layer of mast cells (MCs) entrapped in a thin fibroblast-connective mesh and an outer layer of fibrous connective tissue with fibroblasts. Epithelioid cells and fibroblasts within the thickness of the granuloma wall were positive for proliferative cell nuclear antigen (PCNA). Moreover, several hepatocytes in infected liver were immunoreactive to PCNA. Occurrence of rodlet cells and MCs in parenchyma, in close proximity to the encysted nematode larvae and near the blood vessel of infected liver, was observed. Macrophage aggregates (MAs) were numerous within the granulomas and scattered in parenchyma of the infected liver. High quantity of haemosiderin was encountered in MAs and hepatocytes of infected liver.

Fish innate immunity against intestinal helminths

Most individual fish in farmed and wild populations are infected with parasites. Upon dissection of fish, helminths from gut are often easily visible. Enteric helminths include several species of digeneans, cestodes, acanthocephalans and nematodes. Some insights into biology, morphology and histopathological effects of the main fish enteric helminths taxa will be described here. The immune system of fish, as that of other vertebrates, can be subdivided into specific and aspecific types, which in vivo act in concert with each other and indeed are interdependent in many ways. Beyond the small number of well-described models that exist, research focusing on innate immunity in fish against parasitic infections is lacking. Enteric helminths frequently cause inflammation of the digestive tract, resulting in a series of chemical and morphological changes in the affected tissues and inducing leukocyte migration to the site of infection. This review provides an overview on the aspecific defence mechanisms of fish intestine against helminths. Emphasis will be placed on the immune cellular response involving mast cells, neutrophils, macrophages, rodlet cells and mucous cells against enteric helminths. Given the relative importance of innate immunity in fish, and the magnitude of economic loss in aquaculture as a consequence of disease, this area deserves considerable attention and support.

Effects of Enteromyxum scophthalmi experimental infection on the neuroendocrine system of turbot, Scophthalmus maximus (L.)

Enteromyxum scophthalmi is an intestinal myxosporean parasite responsible for serious outbreaks in turbot Scophthalmus maximus (L.) culture, in North-western Spain. The disease affects the digestive tract, provokes severe catarrhal enteritis, emaciation and high rates of mortality. The digestive parasitization triggers a response with the coordinate participation of immune and neuroendocrine systems through the action of peptides released by enteroendocrine cells and present in nervous elements, acting as neuro-immune modulators. The present study was designed to assess the response of the turbot neuroendocrine system against E. scophthalmi infection. Immunohistochemical tests were applied to sections of the gastrointestinal tract of uninfected and E. scophthalmi-infected turbot to characterize the presence of bombesin (BOM), glucagon (GLUC), somatostatin (SOM), leu-enkephalin (LEU) and met-enkephalin (MET). The occurrence of E. scophthalmi in the turbot gastrointestinal tract increased the number of enteroendocrine cells immunoreactive to SOM, LEU and MET. On the other hand, BOM and GLUC immunoreactive cells were less numerous in the gastrointestinal tract of the parasitized turbot. Scarce immunoreactivity to BOM, GLUC and SOM was observed in nerve fibres and neurons of the myenteric plexus of control and infected fish. The results indicate that E. scophthalmi infection in turbot induced changes in the neuroendocrine system, with the diminution of the anorexigenic peptides BOM and GLUC; the increase of enkephalins, related to pro-inflammatory processes; and the increase of SOM, which may cause inhibitory effects on the immune response, constituting a compensatory mechanism to the exacerbated response observed in E. scophthalmi-infected turbot.

Larval gryporhynchid tapeworms (Cestoda: Cyclophyllidea) of British freshwater fish, with a description of the pathology caused by Paradilepis scolecina

Larvae of the cyclophyllidean tapeworms Paradilepis scolecina (Rudolphi, 1819), Neogryporhynchus cheilancristrotus (Wedl, 1855) and Valipora campylancristrota (Wedl, 1855), are described from British freshwater fish. The morphometrics of the rostellar hooks, infection characteristics and host ranges of these parasites from fisheries in England and Wales are presented. Difficulties in the detection, handling and identification of these tapeworms are highlighted, and may in part explain the paucity of records from Britain. Tissue digestion was shown to be a useful technique for the examination of these parasites, providing clear and consistent preparations of the rostellar hooks for measurement. The pathological changes caused by P. scolecina to the liver of wild tench, Tinca tinca, are detailed for the first time. Tapeworms located in the hepatic parenchyma and pancreatic tissues caused little pathological damage and invoked only mild inflammatory responses. The small size of these tapeworms and their encapsulation within host tissues appear to limit the severity of pathology, compared with parasites that insert their rostellum during attachment.

Intestinal inflammatory response of powan Coregonus lavaretus (Pisces) to the presence of acanthocephalan infections

Immunopathological and ultrastructural studies were carried out on the gut of 30 specimens of powan Coregonus lavaretus (L.) from Lake Piediluco, Italy. The digestive tracts of 10 (33·3%) of the powan were found to harbour an acanthocephalan Dentitruncus truttae (Sinzar 1955). The numerous trunk spines of D. truttae reduced the number of mucosal folds near the parasite site of infection. The acanthocephalan induced hyperplasia and hypertrophy of the intestinal mucous cells and many worms were surrounded with an adherent mucous gel. Near the site of acanthocephalan attachment, the number of mucous cells was significantly higher (P<0·01) in comparison to those found in uninfected intestines. Rodlet cells (RCs) were present in the epithelial layer in both infected and uninfected fish, with no significant difference in the numbers observed (P>0·05). In infected intestine, mast cells were more abundant than in uninfected gut (P<0·01). Migration of the mast cells and their intense degranulation at the site of infection were suggested. Immunohistochemical tests applied to sections of intestinal tissue of both infected and uninfected powan revealed that the parasitized C. lavaretus had a larger number of mast cells positive for met-enkephalin and serotonin antisera.