Diseases of cultured marine fishes caused by Platyhelminthes (Monogenea, Digenea, Cestoda)

Ectoparasites of the Critically Endangered green sawfish Pristis zijsron and sympatric elasmobranchs in Western Australia

This study reports the metazoan ectoparasite fauna of juvenile Critically Endangered green sawfish, Pristis zijsron, and sympatric elasmobranchs in Western Australia. Five parasite taxa were found on 76 screened P. zijsron: Caligus furcisetifer (Copepoda: Caligidae), Dermopristis pterophila (Monogenea: Microbothriidae), Branchellion plicobranchus and Stibarobdella macrothela (Hirudinea: Piscicolidae), and praniza larvae of an unidentified gnathiid isopod. Only C. furcisetifer and D. pterophila were common, exhibiting discrepant site-specificity, with C. furcisetifer occurring mostly on the head and rostrum, and D. pterophila around the pectoral and pelvic fins. Intensity of infection for C. furcisetifer and D. pterophila increased with host total length and was influenced by host sex, but in opposite directions; intensity of C. furcisetifer was greater on female P. zijsron, whereas intensity of D. pterophila was greater on males. In the Ashburton River, likelihood of infection for C. furcisetifer and D. pterophila on P. zijsron increased with time since substantial freshwater discharge events, suggesting decreased salinity impacts both taxa. In addition to P. zijsron, five other sympatric elasmobranch species were opportunistically screened for ectoparasites in the study area: the giant shovelnose ray, Glaucostegus typus, the eyebrow wedgefish, Rhynchobatus palpebratus, the nervous shark, Carcharhinus cautus, the lemon shark, Negaprion acutidens, and the graceful shark, Carcharhinus amblyrhynchoides. Caligus furcisetifer was found on R. palpebratus; no other parasites of P. zijsron were found on other sympatric elasmobranch species. Conversely, Perissopus dentatus (Copepoda: Pandaridae) was found on all three carcharhinids but not on batoid rays (P. zijsron, G. typus or R. palpebratus).

Impact of transport conditions and underlying disease on post-stocking survival of juvenile Lates calcarifer

Diseases caused by pathogens commonly occurring in the aquatic environment or those that are non-host specific are prevalent and threaten the rapid growth of tropical aquaculture. This study investigates causes of mortality in 12 batches of newly stocked juvenile Lates calcarifer from three different hatcheries. Cytology based on Diff-Quik™-stained tissue and blood smears provides rapid diagnosis of possible causes of mortality, while histopathology and haematology provide a better understanding of how prolonged transport and fish with existing chronic disease are more likely to experience elevated mortality post-stocking. Our findings showed that accumulation of ammonia during prolonged transport causes extensive damage to epithelial barriers in gastrointestinal tracts and depressed immunity due to marked hypoglycaemia, predisposing fish to acute Streptococcosis. Lates calcarifer with chronic bacterial enteritis developed severe hypoglycaemia, had low circulating total plasma protein, and suffered high mortality within 24 hours post-stocking. Hypoglycaemia and low circulating blood proteins disrupt osmoregulation and exacerbate dehydration, which is fatal in fish in sea water. Dying L. calcarifer tested PCR positive for scale drop disease virus (SDDV) at 28 days post-stocking showed a 10-fold elevation of white blood cell counts, severe vasculitis, and obstruction of blood supply to major organs. Destruction of important immune organs such as spleen is a hallmark of SDDV infection that explains high incidences of opportunistic Vibrio harveyi infections in 61% of fish with SDDV. Overall, this study reiterates the importance of stocking disease-free fish and reducing transport stress.

Fish ectoparasite detection, collection and curation

Fish parasitology is a dynamic and internationally important discipline with numerous biological, ecological and practical applications. We reviewed optimal fish and parasite sampling methods for key ectoparasite phyla (i.e. Ciliophora, Platyhelminthes, Annelida and Arthropoda) as well as recent advances in molecular detection of ectoparasites in aquatic environments. Ideally, fish capture and anaesthesia as well as parasite recovery methods should be validated to eliminate potential sampling bias and inaccuracy in determining ectoparasite population parameters. There are considerable advantages to working with fresh samples and live parasites, when combined with appropriate fixation methods, as sampling using dead or decaying materials can lead to rapid decomposition of soft-bodied parasites and subsequent challenges for identification. Sampling methods differ between target phyla, and sometimes genera, with optimum techniques largely associated with identification of parasite microhabitat and the method of attachment. International advances in fish parasitology can be achieved through the accession of whole specimens and/or molecular voucher specimens (i.e. hologenophores) in curated collections for further study. This approach is now critical for data quality because of the increased application of environmental DNA (eDNA) for the detection and surveillance of parasites in aquatic environments where the whole organism may be unavailable. Optimal fish parasite sampling methods are emphasised to aid repeatability and reliability of parasitological studies that require accurate biodiversity and impact assessments, as well as precise surveillance and diagnostics.

Proteomic analysis of Neobenedenia sp. and Rhabdosynochus viridisi (Monogenea, Monopisthocotylea): Insights into potential vaccine targets and diagnostic markers for finfish aquaculture

Monogeneans are parasitic flatworms that represent a significant threat to the aquaculture industry. Species like Neobenedenia melleni (Capsalidae) and Rhabdosynochus viridisi (Diplectanidae) have been identified as causing diseases in farmed fish. In the past years, molecular research on monogeneans of the subclass Monopisthocotylea has focused on the generation of genomic and transcriptomic information and the identification in silico of some protein families of veterinary interest. Proteomic analysis has been suggested as a powerful tool to investigate proteins in parasites and identify potential targets for vaccine development and diagnosis. To date, the proteomic dataset for monogeneans has been restricted to a species of the subclass Polyopisthocotylea, while in monopisthocotyleans there is no proteomic data. In this study, we present the first proteomic data on two monopisthocotylean species, Neobenedenia sp. and R. viridisi, obtained from three distinct sample types: tissue, excretory-secretory products (ESPs), and eggs. A total of 1691 and 1846 expressed proteins were identified in Neobenedenia sp. and R. viridisi, respectively. The actin family was the largest protein family, followed by the tubulin family and the heat shock protein 70 (HSP70) family. We focused mainly on ESPs because they are important to modulate the host immune system. We identified proteins of the actin, tubulin, HSP70 and HSP90 families in both tissue and ESPs, which have been recognized for their antigenic activities in parasitic flatworms. Furthermore, our study uncovered the presence of proteins within ESPs, such as annexin, calcium-binding protein, fructose bisphosphate aldolase, glutamate dehydrogenase, myoferlin, and paramyosin, that are targets for immunodiagnostic and vaccine development and hold paramount relevance in veterinary medicine. This study expands our knowledge of monogeneans and identified proteins that, in other platyhelminths are potential targets for vaccines and drug discovery.

Understanding the host response of farmed fish to blood flukes (Trematoda: Aporocotylidae) for developing new treatment strategies

Aporocotylids (Trematoda: Digenea), also known as fish blood flukes infect the circulatory system of fish leading to serious health problems and mortality. Aporocotylids are a particular concern for farmed fish as infection intensity can increase within the farming environment and lead to mortalities. In the context of managing these infections, one of the most crucial aspects to consider is the host response of the infected fish against these blood flukes. Understanding the response is essential to improving current treatment strategies that are largely based on the use of anthelmintic praziquantel to manage infections in aquaculture. This review focuses on the current knowledge of farmed fish host responses against the different life stages of aporocotylids. New treatment strategies that are able to provide protection against reinfections should be a long-term goal and is not possible without understanding the fish response to infection and the interactions between host and parasite.

Evaluation of bromocriptine and plumbagin against the monogenean Rhabdosynochus viridisi: Computational drug repositioning and in vitro approaches

Monogeneans are parasitic platyhelminths that can harm the health of farmed fish. Few treatments are available against monogeneans, and the incentive to develop new antiparasitic agents is similar or even lower than the incentive for neglected parasitic diseases in humans. Considering that searching for and developing new antimonogenean compounds may require enormous investments of time, money, and animal sacrifice, the use of a computer-guided drug repositioning approach is a reasonable alternative. Under this context, this study aimed to evaluate the effectiveness of plumbagin and bromocriptine against adults and eggs of the monogenean Rhabdosynochus viridisi (Diplectanidae). Plumbagin is a phytochemical compound that has recently emerged as a potent antimonogenean; however, further investigation is required to determine its effects on different monogenean species. Bromocriptine was selected through a computational approach that included molecular docking analyses of 77 receptors of monogeneans (putative drug targets) and 77 ligands (putative inhibitors). In vitro experiments showed that bromocriptine does not exhibit mortality at concentrations of 0.1, 1, and 10 mg/L whereas plumbagin at 2 and 10 mg/L caused 100% monogenean mortality after 3 h and 30 min, respectively. The most effective concentration of plumbagin (10 mg/L) did not completely inhibit egg hatching. These findings underscore plumbagin as a highly effective agent against adult monogeneans and highlight the need for research to evaluate its effect(s) on fish. Although computational drug repositioning is useful for selecting candidates for experimental testing, it does not guarantee success due to the complexity of biological interactions, as observed here with bromocriptine. Therefore, it is crucial to examine the various compounds proposed by this method.

Mucosal affairs: glycosylation and expression changes of gill goblet cells and mucins in a fish-polyopisthocotylidan interaction

Introduction

Secreted mucins are highly O-glycosylated glycoproteins produced by goblet cells in mucosal epithelia. They constitute the protective viscous gel layer overlying the epithelia and are involved in pathogen recognition, adhesion and expulsion. The gill polyopisthocotylidan ectoparasite Sparicotyle chrysophrii, feeds on gilthead seabream (Sparus aurata) blood eliciting severe anemia.

Methods

Control unexposed and recipient (R) gill samples of gilthead seabream experimentally infected with S. chrysophrii were obtained at six consecutive times (0, 11, 20, 32, 41, and 61 days post-exposure (dpe)). In histological samples, goblet cell numbers and their intensity of lectin labelling was registered. Expression of nine mucin genes (muc2, muc2a, muc2b, muc5a/c, muc4, muc13, muc18, muc19, imuc) and three regulatory factors involved in goblet cell differentiation (hes1, elf3, agr2) was studied by qPCR. In addition, differential expression of glycosyltransferases and glycosidases was analyzed in silico from previously obtained RNAseq datasets of S. chrysophrii-infected gilthead seabream gills with two different infection intensities.

Results and Discussion

Increased goblet cell differentiation (up-regulated elf3 and agr2) leading to neutral goblet cell hyperplasia on gill lamellae of R fish gills was found from 32 dpe on, when adult parasite stages were first detected. At this time point, acute increased expression of both secreted (muc2a, muc2b, muc5a/c) and membrane-bound mucins (imuc, muc4, muc18) occurred in R gills. Mucins did not acidify during the course of infection, but their glycosylation pattern varied towards more complex glycoconjugates with sialylated, fucosylated and branched structures, according to lectin labelling and the shift of glycosyltransferase expression patterns. Gilthead seabream gill mucosal response against S. chrysophrii involved neutral mucus hypersecretion, which could contribute to worm expulsion and facilitate gas exchange to counterbalance parasite-induced hypoxia. Stress induced by the sparicotylosis condition seems to lead to changes in glycosylation characteristic of more structurally complex mucins.

An inside out journey: biogenesis, ultrastructure and proteomic characterisation of the ectoparasitic flatworm Sparicotyle chrysophrii extracellular vesicles

BACKGROUND

Helminth extracellular vesicles (EVs) are known to have a three-way communication function among parasitic helminths, their host and the host-associated microbiota. They are considered biological containers that may carry virulence factors, being therefore appealing as therapeutic and prophylactic target candidates. This study aims to describe and characterise EVs secreted by Sparicotyle chrysophrii (Polyopisthocotyla: Microcotylidae), a blood-feeding gill parasite of gilthead seabream (Sparus aurata), causing significant economic losses in Mediterranean aquaculture.

METHODS

To identify proteins involved in extracellular vesicle biogenesis, genomic datasets from S. chrysophrii were mined in silico using known protein sequences from Clonorchis spp., Echinococcus spp., Fasciola spp., Fasciolopsis spp., Opisthorchis spp., Paragonimus spp. and Schistosoma spp. The location and ultrastructure of EVs were visualised by transmission electron microscopy after fixing adult S. chrysophrii specimens by high-pressure freezing and freeze substitution. EVs were isolated and purified from adult S. chrysophrii (n = 200) using a newly developed ultracentrifugation-size-exclusion chromatography protocol for Polyopisthocotyla, and EVs were characterised via nanoparticle tracking analysis and tandem mass spectrometry.

RESULTS

Fifty-nine proteins involved in EV biogenesis were identified in S. chrysophrii, and EVs compatible with ectosomes were observed in the syncytial layer of the haptoral region lining the clamps. The isolated and purified nanoparticles had a mean size of 251.8 nm and yielded 1.71 × 108 particles · mL-1. The protein composition analysis identified proteins related to peptide hydrolases, GTPases, EF-hand domain proteins, aerobic energy metabolism, anticoagulant/lipid-binding, haem detoxification, iron transport, EV biogenesis-related, vesicle-trafficking and other cytoskeletal-related proteins. Several identified proteins, such as leucyl and alanyl aminopeptidases, calpain, ferritin, dynein light chain, 14-3-3, heat shock protein 70, annexin, tubulin, glutathione S-transferase, superoxide dismutase, enolase and fructose-bisphosphate aldolase, have already been proposed as target candidates for therapeutic or prophylactic purposes.

CONCLUSIONS

We have unambiguously demonstrated for the first time to our knowledge the secretion of EVs by an ectoparasitic flatworm, inferring their biogenesis machinery at a genomic and transcriptomic level, and by identifying their location and protein composition. The identification of multiple therapeutic targets among EVs' protein repertoire provides opportunities for target-based drug discovery and vaccine development for the first time in Polyopisthocotyla (sensu Monogenea), and in a fish-ectoparasite model.

Other Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis (schistosomiasis) affecting more than 200 million people in tropical and subtropical countries, and infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The "Aporocotylidae" sensu lato are pathogenic in fish, "Spirorchiidae" sensu lato in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in mollusks and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Transcriptome Profile Analyses of Head Kidney in Roach (Rutilus rutilus), Common Bream (Abramis brama) and Their Hybrids: Does Infection by Monogenean Parasites in Freshwater Fish Reveal Differences in Fish Vigour among Parental Species and Their Hybrids?

Hybrid generations usually face either a heterosis advantage or a breakdown, that can be expressed by the level of parasite infection in hybrid hosts. Hybrids are less infected by parasites than parental species (especially F1 generations) or more infected than parental species (especially post-F1 generations). We performed the experiment with blood-feeding gill parasite Paradiplozoon homoion (Monogenea) infecting leuciscid species, Abramis brama and Rutilus rutilus, their F1 generation and two backcross generations. Backcross generations tended to be more parasitized than parental lines and the F1 generation. The number of differentially expressed genes (DEGs) was lower in F1 hybrids and higher in backcross hybrids when compared to each of the parental lines. The main groups of DEGs were shared among lines; however, A. brama and R. rutilus differed in some of the top gene ontology (GO) terms. DEG analyses revealed the role of heme binding and erythrocyte differentiation after infection by blood-feeding P. homoion. Two backcross generations shared some of the top GO terms, representing mostly downregulated genes associated with P. homoion infection. KEGG analysis revealed the importance of disease-associated pathways; the majority of them were shared by two backcross generations. Our study revealed the most pronounced DEGs associated with blood-feeding monogeneans in backcross hybrids, potentially (but not exclusively) explainable by hybrid breakdown. The lower DEGs reported in F1 hybrids being less parasitized than backcross hybrids is in line with the hybrid advantage.

Correlation of endemicity between monogenean trematode, Microcotyle sebastis, and parasitic copepods in Korean rockfish, Sebastes schlegelii, from a fish farm in Tongyeong-si, Gyeongsangnam-do, Korea

Infestations of parasites, particularly those caused by copepods and monogeneans, are a major hindrance to aquaculture and have a big negative economic impact. Sebastes schlegelii, a farmed Korean rockfish, is particularly prone to copepods and monogeneans. This study comprehended how parasitic copepods and rockfish from a farm in Tongyeong-si, Gyeongsangnam-do, Korea, are related to the monogenean trematode Microcotyle sebastis. Our research revealed that monogeneans predominated, with a rate of 98.4% and an average infection intensity of 7 per infected fish. With an average infection intensity of 4 per sick fish, the infection rate for parasitic copepods was equally considerable, coming in at 91.7%. The high co-infection rate of both parasite families, at 90.8%, is noteworthy. Furthermore, Lepeophtheirus elegans and Peniculus truncatus, 2 parasitic copepod species, were discovered to have M. sebastis eggs attached to their bodies. This demonstrates a connection between parasitic copepod infestations in farmed Korean rockfish and the monogenean trematode M. sebastis. These discoveries highlight the necessity for creating more potent parasite control methods for the aquaculture sector and shed light on the intricate relationships between various parasite species.

Parasite Diversity in a Freshwater Ecosystem

Parasites are a significant component of biodiversity. They negatively affect fish appearance, growth, and reproduction. In this study, the prevalence of infection, diversity, and mean intensity of parasites were examined in 9 freshwater fish species (45 samples per fish species). Ecto-parasites were examined on the skin, gills, and fins with a hand lens. Wet mounts were prepared using mucosal scrapings from all the external and internal organs of the sampled fish. Microscopy, muscle compression, and the pepsin-HCL artificial digestion technique were also performed. In this study, 26 species of parasites were identified including three taxa belonging to 9 species of protozoan parasites, 11 treamtodes, and 6 monogenean parasites. The identified protozoan parasites were Entamoeba histolitica, Chilodonella sp., Coccidia sp., Costia sp., Cryptobia sp., Ichthyopthiris-multifilis, Microsporidia, Piscinoodinium sp., and Ichthyobodo necator. The identified trematode parasites were Fasciola gigantica, Echinostoma revolutum, Fasciola hepatica, Haplorchis pumilio, Brachylaima cribbi, Echinostoma cinetorchis, Neascus sp., Deropegus sp., Trematode Soldier, Centrocestus formosanus, and Clinostomum marginatum. The identified monogenean parasites were Dactylogyrus limipopoensis, Dactylogyrus anchoratus, Dactylogyrus myersi, Dactylogyrus vastator, Gyrodactylus salaris, and Ancyrocephalus. The diversity of parasites was maximum at the Okara site. The host's organs that were targeted for parasitic infection included the intestine, liver, gills, fins, skin, and kidneys. The majority of the parasites were identified in Labeo rohita followed by Hypophthalmichthys molitrix, Ctenopharyngodon idella, Oreochromis niloticus, Cyprinus carpio, and Wallagu attu. Two species appeared to be resistant species because none of the parasites were observed in Notopterus notopterus or Sperata seenghala. This study also concluded that the prevalence of parasites increased with increasing length, size, and age of fish.

An insight into the functional genomics and species classification of Eudiplozoon nipponicum (Monogenea, Diplozoidae), a haematophagous parasite of the common carp Cyprinus carpio

BACKGROUND

Monogenea (Platyhelminthes, Neodermata) are the most species-rich class within the Neodermata superclass of primarily fish parasites. Despite their economic and ecological importance, monogenean research tends to focus on their morphological, phylogenetic, and population characteristics, while comprehensive omics analyses aimed at describing functionally important molecules are few and far between. We present a molecular characterisation of monogenean representative Eudiplozoon nipponicum, an obligate haematophagous parasite infecting the gills of the common carp. We report its nuclear and mitochondrial genomes, present a functional annotation of protein molecules relevant to the molecular and biochemical aspect of physiological processes involved in interactions with the fish hosts, and re-examinate the taxonomic position of Eudiplozoon species within the Diplozoidae family.

RESULTS

We have generated 50.81 Gbp of raw sequencing data (Illumina and Oxford Nanopore reads), bioinformatically processed, and de novo assembled them into a genome draft 0.94 Gbp long, consisting of 21,044 contigs (N50 = 87 kbp). The final assembly represents 57% of the estimated total genome size (~ 1.64 Gbp), whereby repetitive and low-complexity regions account for ~ 64% of the assembled length. In total, 36,626 predicted genes encode 33,031 proteins and homology-based annotation of protein-coding genes (PCGs) and proteins characterises 14,785 (44.76%) molecules. We have detected significant representation of functional proteins and known molecular functions. The numbers of peptidases and inhibitors (579 proteins), characterised GO terms (16,016 unique assigned GO terms), and identified KEGG Orthology (4,315 proteins) acting in 378 KEGG pathways demonstrate the variety of mechanisms by which the parasite interacts with hosts on a macromolecular level (immunomodulation, feeding, and development). Comparison between the newly assembled E. nipponicum mitochondrial genome (length of 17,038 bp) and other diplozoid monogeneans confirms the existence of two distinct Eudiplozoon species infecting different fish hosts: Cyprinus carpio and Carassius spp.

CONCLUSIONS

Although the amount of sequencing data and characterised molecules of monogenean parasites has recently increased, a better insight into their molecular biology is needed. The E. nipponicum nuclear genome presented here, currently the largest described genome of any monogenean parasite, represents a milestone in the study of monogeneans and their molecules but further omics research is needed to understand these parasites' biological nature.

A New Species of Unibarra (Monogenoidea, Dactylogyridae) Parasite of Oxydoras niger From Juruá River, State of Acre, Brazil and New Data for U. paranoplatensis

Monogenoids are ectoparasites that have a simple one-host lifecycle, high species diversity, and a relatively high host specificity. During studies on the helminth fauna of fishes from the Jurua River, in Acre State, Brazil, a new species of the monotypic genus Unibarra Suriano & Incorvaia, 1995 was found parasitizing Oxydoras niger Valenciennes, 1821. Unibarra juruaensis n. sp. is allocated in the genus based on the presence of a single haptoral bar, marginal hooks similar in shape and size, gonads partially overlapping, and a conspicuous filament which connects the base of the male copulatory organ with the accessory piece. The new species differs from the only species of the genus by the smaller size of the body and of the structures, by the morphology of copulatory complex, with an accessory piece thinner than that of U. paranoplatensis Suriano & Incorvaia, 1995 and by the presence of two eyespots. The type species, U. paranoplatensis, is referred in a new host, Pimelodus blochii Valenciennes, 1840, with new morphological data. A table of measurements of the new species and previous and the present reports of U. paranoplatensis is presented.

De novo transcriptome assembly and identification of G-Protein-Coupled-Receptors (GPCRs) in two species of monogenean parasites of fish

Genomic resources for Platyhelminthes of the class Monogenea are scarce, despite the diversity of these parasites, some species of which are highly pathogenic to their fish hosts. This work aimed to generate de novo-assembled transcriptomes of two monogenean species, Scutogyrus longicornis (Dactylogyridae) and Rhabdosynochus viridisi (Diplectanidae), providing a protocol for cDNA library preparation with low input samples used in single cell transcriptomics. This allowed us to work with sub-microgram amounts of total RNA with success. These transcriptomes consist of 25,696 and 47,187 putative proteins, respectively, which were further annotated according to the Swiss-Prot, Pfam, GO, KEGG, and COG databases. The completeness values of these transcriptomes evaluated with BUSCO against Metazoa databases were 54.1% and 73%, respectively, which is in the range of other monogenean species. Among the annotations, a large number of terms related to G-protein-coupled receptors (GPCRs) were found. We identified 109 GPCR-like sequences in R. viridisi, and 102 in S. longicornis, including family members specific for Platyhelminthes. Rhodopsin was the largest family according to GRAFS classification. Two putative melatonin receptors found in S. longicornis represent the first record of this group of proteins in parasitic Platyhelminthes. Forty GPCRs of R. viridisi and 32 of S. longicornis that were absent in Vertebrata might be potential drug targets. The present study provides the first publicly available transcriptomes for monogeneans of the subclass Monopisthocotylea, which can serve as useful genomic datasets for functional genomic research of this important group of parasites.

From development to taxonomy: the case of Sciaenacotyle pancerii (Monogenea: Microcotylidae) in the Mediterranean meagre

The microcotylid Sciaenacotyle pancerii is a pathogenic monogenean infecting Argyrosomus regius, a candidate for species diversification in the Mediterranean aquaculture. Life-history stages of S. pancerii commonly co-occur in field infections, but to date, morphological data have only been provided for oncomiracidia and adults although identifying life-history stages can be useful in infection management. A total of 114 specimens of S. pancerii were analysed to characterize the developmental events and to assess morphological and morphometric variations before and after maturity. The post-larval development of S. pancerii is characterized by: expansion and bifurcation of the gut, loss of the larval haptor, protandrous development of the genitalia and vitellaria formation. The size variability of larval hooks, hamuli and germanium of S. pancerii is firstly reported and dimensional ranges of parasite body, haptor, testes, posteriormost clamps and eggs are widened. The size of most of the diagnostic features of S. pancerii significantly increases after parasite maturity and therefore, only those specimens with more than 116 clamps should be considered for minimising development-related variability in size. The high number of clamps, their fast development and the asymmetry in their size and arrangement suggest that S. pancerii may use a mixed attachment strategy between the closely related microcotylids and heteraxinids. This combination of features may be host related and linked to the gill structure of the sciaenid fish and the phylogenetic position of the genus Sciaenacotyle; distant from other microcotylids while close to heteraxinid species.

Acute toxicity of praziquantel to fish Danio rerio and planktonic crustacean Daphnia magna

This study evaluated the toxicity of the pyrazino isoquinoline anthelmintic praziquantel (PZQ) to the Danio rerio zebrafish and Daphnia magna water flea. The estimated 24 h and 96 h LC50 of PZQ to the zebrafish was 39.9 mg/l and 30.4 mg/l, respectively. The highest 24 h and 96 h non-lethal concentration (LC0) was 21.7 mg/l and 21.2 mg/l, respectively. The mobility inhibition test of the juvenile Daphnia magna revealed a 48 h EC50 of 42.7 mg/l.

A systematic review and meta-analysis of the relationship between farm management, water quality and pathogen outbreaks in tilapia culture

A systematic review and meta-analysis of on farm management and water quality as risk factors triggering pathogen outbreaks in tilapia culture is presented. A total of 121 papers from 28 countries, 37 devoted to management and 84 to water quality associated with the presence of pathogens in tilapia culture were analysed. Most of research has been made on ponds, focusing on bacteria and metazoans. Meta-analysis reveals the proportion of research showing statistical inferences between management (34%), water quality (38% for bacteria and 16.2% for metazoans) and pathogen outbreaks. The most cited parameters related to pathogens outbreaks were temperature (>30°C), dissolved oxygen (<5.0 mg/L), pH (<8.0) and ammonia (1.0 >mg/L) 65, 38, 32 and 34 papers, respectively. However, statistical inference was <20%. Meta-analysis reveals tendencies between high temperatures (30-35°C), low dissolved oxygen (<5 mg/L) and high NH₃ (1-5 mg/L) with pathogen outbreaks. Despite the knowledge about the relationship between management and water quality as factors for pathogen effects, most information is descriptive and empirical. Future research on tilapia culture outbreaks should be focused on the effects of multiple stressors affecting tilapia pathogen outbreaks, thereby generating strategies to prevent diseases and financial losses.

A World of Viruses Nested within Parasites: Unraveling Viral Diversity within Parasitic Flatworms (Platyhelminthes)

Because parasites have an inextricable relationship with their host, they have the potential to serve as viral reservoirs or facilitate virus host shifts. And yet, little is known about viruses infecting parasitic hosts except for blood-feeding arthropods that are well-known vectors of zoonotic viruses. Herein, we uncovered viruses of flatworms (phylum Platyhelminthes, group Neodermata) that specialize in parasitizing vertebrates and their ancestral free-living relatives. We discovered 115 novel viral sequences, including 1 in Macrostomorpha, 5 in Polycladida, 44 in Tricladida, 1 in Monogenea, 15 in Cestoda, and 49 in Trematoda, through data mining. The majority of newly identified viruses constitute novel families or genera. Phylogenetic analyses show that the virome of flatworms changed dramatically during the transition of neodermatans to a parasitic lifestyle. Most Neodermata viruses seem to codiversify with their host, with the exception of rhabdoviruses, which may switch hosts more often, based on phylogenetic relationships. Neodermata rhabdoviruses also have a position ancestral to vertebrate-associated rhabdo viruses, including lyssaviruses, suggesting that vertebrate-associated rhabdoviruses emerged from a flatworm rhabdovirus in a parasitized host. This study reveals an extensive diversity of viruses in Platyhelminthes and highlights the need to evaluate the role of viral infection in flatworm-associated diseases.

IMPORTANCE Little is known about the diversity of parasite-associated viruses and how these viruses may impact parasite fitness, parasite-host interactions, and virus evolution. The discovery of over a hundred viruses associated with a range of free-living and parasitic flatworms, including parasites of economic and clinical relevance, allowed us to compare the viromes of flatworms with contrasting lifestyles. The results suggest that flatworms acquired novel viruses after their transition to a parasitic lifestyle and highlight the possibility that they acquired viruses from their hosts and vice versa. An interesting example is the discovery of flatworm rhabdoviruses that have a position ancestral to rabies viruses and other vertebrate-associated rhabdoviruses, demonstrating that flatworm-associated viruses have emerged in a vertebrate host at least once in history. Therefore, parasitic flatworms may play a role in virus diversity and emergence. The roles that parasite-infecting viruses play in parasite-associated diseases remain to be investigated.

Parasite community analysis of the gray snapper Lutjanus griseus (Perciformes,Lutjanidae) in a tropical region of the Southern Gulf of Mexico

The gray snapper Lutjanus griseus is a commercially important fish species along its distribution range in the western Atlantic Ocean. However, despite its importance, there is still little knowledge about its parasitic fauna for the Mexican coasts of the Gulf of Mexico. The aims of this research were to generate a list of the parasitic fauna present in juvenile gray snapper L. griseus from a coastal lagoon located in southeastern Mexico, to evaluate the infection levels of parasites and to determine the relationship between the abundance of parasites and the fish size and condition factor. Samples of L. griseus (12 – 29.2 mm) were obtained in two periods of the year (dry and rainy seasons) to examine the intra-annual variability of its parasitic fauna. A total of 17 parasite species were recorded belonging to six taxonomic groups (Myxozoa, Monogenea, Digenea, Cestoda, Nematoda and Acanthocephala). The highest levels of infection (abundance, prevalence and intensity of infection) were found for the monogeneans Euryhaliotrema griseus and Euryhaliotrema fastigatum. There were no significant correlations between the total abundance of parasites and the fish condition and size (total length) in not any of the two seasons studied, suggesting that the body size and the biological condition index of the host did not directly influence the abundance of parasites in early life stages of L. griseus. Moreover, the species of parasites found that could be zoonotic for humans through the consumption of raw or inadequately cooked fish were the nematodes Contracaecum sp. type 1, Contracaecum sp. type 2, Cucullanus pargi and Pseudoterranova sp. The presence of the monogeneans E. griseus and E. fastigatum was also highlighted because these ectoparasite species are known to cause harm to fish under culture systems. All the parasite species found in this study, except nematodes, were new records of geographic distribution.

Praziquantel use in aquaculture - Current status and emerging issues

Parasitic diseases are major constraints in fish mariculture. The anthelmintic praziquantel (PZQ) can effectively treat a range of flatworm parasites in a variety of fish species and has potential for broader application than its current use in the global aquaculture industry. In this review we report on PZQ's current use in the aquaculture industry and discuss its efficacy against various flatworm parasites of fish. Routes of PZQ administration are evaluated, along with issues related to palatability, pharmacokinetics and toxicity in fish, while PZQ's effects on non-target species, environmental impacts, and the development of drug-resistance are discussed.

In silico identification of tetraspanins in monopisthocotylean (Platyhelminthes: Monogenea) parasites of fish

Tetraspanins are a superfamily of transmembrane proteins that in flatworms have structural roles in the development, maturation or stability of the tegument. Several tetraspanins are considered as potential candidates for vaccines or drugs against helminths. Monopisthocotylean monogeneans are ectoparasites of fish that are health hazards for farmed fish. The aim of this study was to identify in silico putative tetraspanins in the genomic datasets of four monopisthocotylean species. The analysis predicted and classified 40 tetraspanins in Rhabdosynochus viridisi, 39 in Scutogyrus longicornis, 22 in Gyrodactylus salaris and 13 in Neobenedenia melleni, belonging to 13 orthologous groups. The high divergence of tetraspanins made it difficult to annotate their function. However, a conserved group was identified in different metazoan taxa. According to this study, metazoan tetraspanins can be divided into 17 monophyletic groups. Of the 114 monogenean tetraspanins, only seven were phylogenetically close to tetraspanins from non-platyhelminth metazoans, which suggests that this group of proteins shows rapid sequence divergence. The similarity of the monopisthocotylean tetraspanins was highest with trematodes, followed by cestodes and then free-living platyhelminths. In total, 27 monopisthocotylean-specific and 34 flatworm-specific tetraspanins were identified. Four monogenean tetraspanins were orthologous to TSP-1, which is a candidate for the development of vaccines and a potential pharmacological target in trematodes and cestodes. Although studies of tetraspanins in parasitic flatworms are scarce, this is an interesting group of proteins for the development of new methods to control monogeneans.

Trichlorfon is effective against Dawestrema cycloancistrium and does not alter the physiological parameters of arapaima (Arapaima gigas): A large Neotropical fish from the Amazon

This study investigated the in vitro and in vivo efficacy of trichlorfon against Dawestrema cycloancistrium, as well as its physiological effects on arapaima. Naturally parasitized arapaima gill arches were exposed in vitro to 100, 250, 500 and 750 mg/L of trichlorfon and a control group (only distilled water), in triplicate. Parasites were monitored, and mortality was used to determine the median effective concentration (EC₅₀ ). The 750 mg/L concentration demonstrated 100% in vitro efficacy against D. cycloancistrium after 60 min, while the intermediate (500 mg/L) and the lowest (100 and 250 mg/L) tested concentrations were completely efficient after 90 and 130 min, respectively. The EC_50-1h of trichlorfon for D. cycloancistrium was determined at 171.73 mg/L. Parasitized arapaima juveniles were exposed to a control group and 150 mg/L of trichlorfon in triplicate. Fish were exposed to two therapeutic baths for 60 min with 24-h intervals between treatments. Therapeutic baths with 150 mg/L of trichlorfon were 92.99% effective against D. cycloancistrium and did not bring about haematological alterations (erythrogram, white blood cell count, thrombogram, plasma glucose and total proteins). Therefore, 150 mg/L of trichlorfon can be used in therapeutic baths to control and treat D. cycloancistrium infestations with no physiological impairments for arapaima.

Antiparasitic Efficacy of Crude Plant Extracts and Compounds Purified from Plants against the Fish Monogenean Neobenedenia girellae

Neobenedenia girellae is a pathogenic ectoparasite of many marine fishes, and it causes major epidemics in marine aquaculture. In this study, the efficacy of ethanol extracts of huangqi Astragalus membranaceus (known as milkvetch in North America), guanzhong Dryopteris setosa (known as beaded wood fern in North America), gancao Glycyrrhiza uralensis (known as Chinese licorice in North America), danshen Salvia miltiorrhiza (known as red sage in North America), and pomegranate Punica granatum, as well as seven phytochemicals (10-gingerol, curcumin, cynatratoside-C, emodin, kuwanon-G, kuwanon-O, and sophoraflavanone-G), against adult N. girellae was investigated. In vitro results indicated that pomegranate extract killed all adult N. girellae at a 62.5-mg/L concentration with an 8-h exposure, but gancao extract did not cause 100% mortality until a 1,000-mg/L concentration was used. Additionally, all adult N. girellae died after an 8-h exposure to cynatratoside-C, kuwanon-G, kuwanon-O, or sophoraflavanone-G at a concentration of 125 mg/L. Curcumin, emodin, and 10-gingerol at a concentration of 1,000 mg/L did not kill all parasites after an 8-h exposure. These findings demonstrate that plant extracts and active phytochemicals are potential sources of botanical drugs for controlling N. girellae infection in aquaculture.

First Report of Blood Fluke Pathogens with Potential Risk for Emerging Yellowtail Kingfish (Seriola lalandi) Aquaculture on the Chilean Coast, with Descriptions of Two New Species of Paradeontacylix (Aporocotylidae)

Blood flukes are digeneans that infect wild and farmed fish that can cause a severe and potentially lethal disease in farmed fish. These parasites are undetectable in the larval stage based on macroscopic observations in the definitive host with the infection becoming evident when eggs accumulate in the branchial vessels. There are nine known species of the genus Paradeontacylix and seven exclusively parasitize Seriola spp. from several geographical areas. Seriola lalandi aquaculture farms are emerging at various localities in northern Chile. Here, we report, for the first time, two blood fluke species parasitizing S. lalandi in the Southeastern Pacific (Chile). In the laboratory, the gills and heart of fish were removed. The retained blood flukes were separated according to the infection site, fixed in 70% or 95% ethanol for taxonomic and molecular analysis, respectively. Morphometrical differences among the fluke species were evaluated with a principal component analysis (PCA) using proportional body measurements. Phylogenetic trees were constructed based on 28S rDNA, cox1 mDNA using Bayesian inference (BI), and maximum likelihood (ML). Based on morphology, morphometry, and molecular analyses, two new species are proposed: P. humboldti n. sp. from the gills and P. olivai n. sp. from the heart of S. lalandi. Both were clearly distinguished from other species of Paradeontacylix by a combination of morphologic features (posterior tegumental spines, testes arrangement, body size). The genetic distance (based on cox1) among species was >10%. P. humboldti n. sp. and P. olivai n. sp. are sister species (with a common ancestor) independent of P. godfreyi from S. lalandi in Australia. The newly identified parasites may pose a risk to farmed S. lalandi as aporocotylids have been the cause of diseases in farmed fish from other geographical areas. In addition, some cages of S. lalandi are currently maintained in an open circulating system, which could favor the transmission of these parasites (if involved hosts are present in the environment).

Parasite Fauna of the White-Streaked Grouper (Epinephelus ongus) from the Thousand Islands, Java, Indonesia

PURPOSE

Fish parasites can cause diseases in humans and lead to commercial losses in fisheries and aquaculture. The objectives of this study were to analyze E. ongus's parasite fauna regarding food safety and parasite transmission risk between Epinephelus species and test whether E.ongus populations can be distinguished by their parasite community.

METHODS

We studied the metazoan parasite fauna of 30 white-streaked groupers Epinephelus ongus from the Thousand Islands, Java Sea, Indonesia, and compared the parasite community with specimens from Karimunjawa archipelago, Java Sea, from a former study. We used common fish parasitological methods for fish examination and parasite calculations.

RESULTS

We found 12 metazoan parasite species, establishing five new host and five new locality records, increasing the known parasite fauna of E. ongus by 21%. No anisakid worms infected E. ongus. All but one (trematode Gyliauchen cf. nahaensis) species have been previously reported from Epinephelus. Parasite abundance of E. ongus differed significantly between the two regions.

CONCLUSIONS

Due to a certain degree of host specificity to groupers, there is potential risk of parasite transmission from E. ongus into groupers in mariculture or surrounding fishes, which increases (sea) food security related health risks from zoonotic parasites and calls for better monitoring and management plans for E. ongus. The regional separation of the Thousand Islands and Karimunjawa with different food availability and fish ecology causes different parasite abundances, distinguishing two separate E. ongus populations by their parasite fauna.

Blood Analyte Changes of Wild-Caught Adult Almaco Jack in Response to Acclimation to Recirculating Aquaculture Systems and Hyposalinity Treatment

Despite the utility of blood analyte evaluation as a diagnostic tool to assist in monitoring the health of marine fishes, baseline data are often lacking for many commercially important finfish species. The objective of this study was to compare hematology and plasma chemistry data for adult wild-caught Almaco Jack Seriola rivoliana at time of capture and again following a period of acclimation to a recirculating aquaculture system and hyposalinity treatment. A total of 30 clinically healthy adult fish were caught via hook and line in the eastern Gulf of Mexico, approximately 120 mi offshore from Madeira Beach, Florida. Blood was collected from a subset of these fish (n = 13) immediately after capture and again at 16 weeks postcapture from another subset (n = 12) following a 45-d antiparasitic hyposalinity treatment. A 19% increase in fish body weight was observed during the study period (16 weeks) and no overt health issues or mortality were noted. Compared to fish that were sampled immediately following capture, several significant differences (P < 0.05) were observed. Absolute white blood cells were lower in captive held fish, suggesting biological variation, antigenic stimulation in wild fish, and/or immunosuppression associated with stress in captive held fish. Lower sodium, chloride, and calculated osmolality indicate osmoregulatory adjustments following the hyposalinity treatment by 16 weeks postcapture. Other observed plasma biochemical differences presumptively reflect dietary and/or environmental changes, or physiological variation following acclimation to captive culture conditions. This study reports baseline blood analyte data of wild-caught Almaco Jack and documents hematological and plasma biochemical responses to their new environment as captive broodstock. Baseline hematological and plasma biochemistry data obtained during this study are the first reported for this species.

In silico identification of excretory/secretory proteins and drug targets in monogenean parasites

The Excretory/Secretory (ES) proteins of parasites are involved in invasion and colonization of their hosts. In addition, since ES proteins circulate in the extracellular space, they can be more accessible to drugs than other proteins, which makes ES proteins optimal targets for the development of new and better pharmacological strategies. Monogeneans are a group of parasitic Platyhelminthes that includes some pathogenic species problematic for finfish aquaculture. In the present study, 8297 putative ES proteins from four monogenean species which genomic resources are publicly available were identified and functionally annotated by bioinformatic tools. Additionally, for comparative purposes, ES proteins in other parasitic and free-living platyhelminths were identified. Based on data from the monogenean Gyrodactylus salaris, 15 ES proteins are considered potential drug targets. One of them showed homology to 10 cathepsins with known 3D structure. A docking molecular analysis uncovered that the anthelmintic emodepside shows good affinity to these cathepsins suggesting that emodepside can be experimentally tested as a monogenean's cathepsin inhibitor.

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.

Cardicola mediterraneus n. sp. (Trematoda, Aporocotylidae): a new species infecting the gilthead seabream, Sparus aurata L., from the Western Mediterranean Sea

The genus Cardicola Short, 1953 has the highest number of species within the family Aporocotylidae (Trematoda: Digenea). Five Cardicola species have been reported to date in the Mediterranean Sea, one of them in the gilthead seabream Sparus aurata L. Analyses of infected S. aurata recovered from cultured fish off Sardinia (Italy) and from wild and cultured fish off the Levantine coast (Southeastern Spain) have revealed the presence of two species identified as Cardicola aurata Holzer, Montero, Repullés, Nolan, Sitjà-Bobadilla, Álvarez-Pellitero, Zarza and Raga, 2008 and Cardicola mediterraneus n. sp.. New morphological and molecular data are provided for both species. Features of C. aurata specimens differ slightly from those of the original description of the species, the most important differences being the longer extension of the metraterm and the central and posterior position of the female genital pore. Cardicola mediterraneus n. sp. can be easily distinguished from other Cardicola species by two unique specific characters: (i) the very unequal posterior caeca length and (ii) the shape of the testis, deeply notched at the anterior extremity. Cardicola spp. from sparids occupy a basal phylogenetic position respect the other congeneric species. The genus Cardicola has a complex taxonomy and shows high intrageneric differences for both 28S and ITS2 rDNAs, similar to the intergeneric differences among other aporocotylid genera, suggesting that it could be split. The presence of two Cardicola species could hamper treatment design and application; thus, data discriminating species herein reported can improve the infection management in fish farms.

Mitochondrial genome of Rhabdosynochus viridisi (Monogenea: Diplectanidae), a parasite of Pacific white snook Centropomus viridis

We report the nearly complete mitochondrial genome of Rhabdosynochus viridisi - the first for this genus - achieved by combining shotgun sequencing of genomic and cDNA libraries prepared using low-input protocols. This integration of genomic information leads us to correct the annotation of the gene features. The mitochondrial genome consists of 13,863 bp. Annotation resulted in the identification of 12 protein-encoding genes, 22 tRNA genes and two rRNA genes. Three non-coding regions, delimited by three tRNAs, were found between the genes nad5 and cox3. A phylogenetic analysis grouped R. viridisi with three other species of diplectanid monogeneans for which mitochondrial genomes are available.

Eudiplozoon nipponicum (Monogenea, Diplozoidae) and its adaptation to haematophagy as revealed by transcriptome and secretome profiling

Background

Ectoparasites from the family Diplozoidae (Platyhelminthes, Monogenea) belong to obligate haematophagous helminths of cyprinid fish. Current knowledge of these worms is for the most part limited to their morphological, phylogenetic, and population features. Information concerning the biochemical and molecular nature of physiological processes involved in host–parasite interaction, such as evasion of the immune system and its regulation, digestion of macromolecules, suppression of blood coagulation and inflammation, and effect on host tissue and physiology, is lacking. In this study, we report for the first time a comprehensive transcriptomic/secretome description of expressed genes and proteins secreted by the adult stage of Eudiplozoon nipponicum (Goto, 1891) Khotenovsky, 1985, an obligate sanguivorous monogenean which parasitises the gills of the common carp (Cyprinus carpio).

Results

RNA-seq raw reads (324,941 Roche 454 and 149,697,864 Illumina) were generated, de novo assembled, and filtered into 37,062 protein-coding transcripts. For 19,644 (53.0%) of them, we determined their sequential homologues. In silico functional analysis of E. nipponicum RNA-seq data revealed numerous transcripts, pathways, and GO terms responsible for immunomodulation (inhibitors of proteolytic enzymes, CD59-like proteins, fatty acid binding proteins), feeding (proteolytic enzymes cathepsins B, D, L1, and L3), and development (fructose 1,6-bisphosphatase, ferritin, and annexin). LC-MS/MS spectrometry analysis identified 721 proteins secreted by E. nipponicum with predominantly immunomodulatory and anti-inflammatory functions (peptidyl-prolyl cis-trans isomerase, homolog to SmKK7, tetraspanin) and ability to digest host macromolecules (cathepsins B, D, L1).

Conclusions

In this study, we integrated two high-throughput sequencing techniques, mass spectrometry analysis, and comprehensive bioinformatics approach in order to arrive at the first comprehensive description of monogenean transcriptome and secretome. Exploration of E. nipponicum transcriptome-related nucleotide sequences and translated and secreted proteins offer a better understanding of molecular biology and biochemistry of these, often neglected, organisms. It enabled us to report the essential physiological pathways and protein molecules involved in their interactions with the fish hosts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07589-z.

Ultrastructural observations on the oncomiracidium epidermis and adult tegument of Discocotyle sagittata, a monogenean gill parasite of salmonids

During their different life stages, parasites undergo remarkable morphological, physiological, and behavioral "metamorphoses" to meet the needs of their changing habitats. This is even true for ectoparasites, such as the monogeneans, which typically have a free-swimming larval stage (oncomiracidium) that seeks out and attaches to the external surfaces of fish where they mature. Before any obvious changes occur, there are ultrastructural differences in the oncomiracidium's outer surface that prepare it for a parasitic existence. The present findings suggest a distinct variation in timing of the switch from oncomiracidia epidermis to the syncytial structure of the adult tegument and so, to date, there are three such categories within the Monogenea: (1) Nuclei of both ciliated cells and interciliary cytoplasm are shed from the surface layer and the epidermis becomes a syncytial layer during the later stages of embryogenesis; (2) nuclei of both ciliated cells and interciliary syncytium remain distinct and the switch occurs later after the oncomiracidia hatch (as in the present study); and (3) the nuclei remain distinct in the ciliated epidermis but those of the interciliary epidermis are lost during embryonic development. Here we describe how the epidermis of the oncomiracidium of Discocotyle sagittata is differentiated into two regions, a ciliated cell layer and an interciliary, syncytial cytoplasm, both of which are nucleated. The interciliary syncytium extends in-between and underneath the ciliated cells and sometimes covers part of their apical surfaces, possibly the start of their shedding process. The presence of membranous whorls and pyknotic nuclei over the surface are indicative of membrane turnover suggesting that the switch in epidermis morphology is already initiated at this stage. The body tegument and associated putative sensory receptors of subadult and adult D. sagittata are similar to those in other monogeneans.

The Efficacy of Geraniol and ß-Citronellol against Freshwater and Marine Monogeneans

Monogeneans are parasitic flatworms that may be a threat for finfish aquaculture. In this study, the anthelmintic activity of two terpenes, geraniol and β-citronellol, was tested in vitro against ancyrocephalin and diplectanid monogeneans. Experiments were performed in both water and a culture medium. We observed that monogeneans in culture medium may be more tolerant to treatments compared with bioassays performed only in water. Concentrations of 300 mg/L of both compounds were required to kill 100% of monogeneans at 1 h postexposure. The toxicity of β-citronellol to fish was not evaluated. However, geraniol at 300 mg/L and 150 mg/L killed juvenile Nile Tilapia Oreochromis niloticus and White Snook Centropomus viridis, respectively, after a few minutes. Therefore, the present work suggests that other alternatives should be studied for use against monogeneans in aquaculture.

In vivo cultivation of tuna blood fluke Cardicola orientalis in terebellid intermediate hosts

Some fish blood flukes of the genus Cardicola (Aporocotylidae) are considered important pathogens of farmed/ranched tuna, Thunnus spp. Infections with Cardicola spp. might obstruct the blood flow in the gills via massive accumulations of eggs and often lead to mass mortalities in captive tuna. At present, oral administration of an anthelminthic drug, praziquantel is the most effective treatment, but the tuna farming industries are seeking non-drug control measures. Development of prophylactic and holistic measures have been difficult, owing to a lack of basic knowledge about these parasites. Unlike other trematodes which use molluscs, blood flukes of marine actinopterygian fish use terebellid polychaetes as intermediate hosts. However, information about the development of Cardicola spp. within intermediate hosts is very limited. Recent success in Cardicola opisthorchis sporocyst transplantation into the host polychaete has opened possibilities for the cultivation of Cardicola in the laboratory. Here, we conducted several transplantation trials with another tuna blood fluke, Cardicol orientalis, into its natural and surrogate polychaete hosts. Cardicola orientalis sporocysts were injected into a total of 195 Nicolea gracilibranchis, the natural host, and clear sporocyst development and reproduction was observed in 32 recipients (overall success rate 16.4%). The production of daughter sporocysts in the transplanted polychaete occurred within 14 days post injection, and one sporocystogenous cycle took approximately 4 weeks. Serial passage culture via transplantation of in vivo-cultured sporocysts was also achieved, but with limited sporocyst reproduction. In addition, sporocysts were successfully retrieved from six and one individuals of the surrogate hosts, Thelepus setosus (n = 10) and Thelepus japonicus (n = 5), respectively. These results indicate that the in vivo cultivation of C. orientalis sporocysts is possible, not only in its natural host but also in other terebellids, although the problems of high mortality and inconsistency in successful transplantation need to be resolved.

Immune response of teleost fish to helminth parasite infection

Abstract Fish immune systems have become the subject of several studies due to the growing development of aquaculture and fisheries, and the demand for healthy produce for human consumption. Among the parasites responsible for diseases in fish farming, helminths stand out because they cause infections in farmed fish and decrease food conversion, zootechnical performance and meat quality. In the present review, the components that participate in the innate and adaptive immune responses of teleost fish that have so far been described are presented in order to summarize the defenses that these hosts have recourse to, in combating different groups of helminth parasites.

Cytogenetics of Eudiplozoon nipponicum (Monogenea, Diplozoidae): Karyotype, spermatocyte division and 18S rDNA location

Ectoparasitic monogeneans of the family Diplozoidae have direct and monoxenous life cycle. The cytogenetics of monogeneans in general and diplozoids in particular, is a relatively underexplored area. This is why each new detailed description of a karyotype provides significant information about the evolution of monogenean chromosomes and contributes to a better understanding of phylogenetic relationships within this group. This study offers new data on the chromosomes of Eudiplozoon nipponicum, an invasive parasite of the common carp. This species' karyotype consists of seven pairs of telocentric chromosomes (2n = 14 t). After DAPI staining, we marked heterochromatin blocks on all chromosomes in the pericentromeric region. Silver staining (AgNO₃) and staining with fluorescent dye YOYO-1 revealed the presence of one large active nucleolus. Fluorescent in situ hybridization with an 18S rDNA probe revealed one cluster of ribosomal genes at the terminal part of the long arms of chromosome pair No. 7. We compared our results with studies on the phylogenetic relationships of diplozoids which applied a combination of molecular methods and classical morphological characterization and found that the results of our cytogenetic analysis are consistent with the hypothesis that E. nipponicum is more basal member of the family Diplozoidae.

Genome-wide identification of ABC transporters in monogeneans

ATP-Binding Cassette (ABC) transporters are proteins that actively mediate the transport of a wide variety of molecules, including drugs. Thus, in parasitology, ABC transporters have gained attention as potential targets for therapeutic drugs. Among the parasitic Platyhelminthes, ABC transporters have been identified and classified in a few species of Trematoda and Cestoda but not in Monogenea. Monogeneans are mainly ectoparasites of marine and freshwater fish, although they can also be found on other aquatic organisms. Severe epizootics caused by monogeneans have been reported around the world, mainly in confined and/or overcrowded fish. The purpose of this study was to identify the ABC transporters in four species of monogeneans (Gyrodactylus salaris, Protopolystoma xenopodis, Eudiplozoon nipponicum and Neobenedenia melleni) for which genomic resources are publicly available. For comparative purposes, ABC transporters were also identified in endoparasitic (Schistosoma mansoni and Echinococcus granulosus) and free-living (Macrostomun lignano and Schmidtea mediterranea) platyhelminths. Thirty-two putative ABC transporters were identified in the genome of G. salaris, 40 in the genome of P. xenopodis, 46 in the transcriptome of E. nipponicum and 9 in a rather limited ESTs set available for N. melleni. Of the eight ABC subfamilies (A-H) known in metazoans, subfamily H was the only one not found in any monogenean species. In contrast, ABCC was the best represented subfamily. Phylogenetic analyses showed a few cases of one-to-one orthologous relationships, which agree with results from other metazoan species. We found some monogenean ABC members related to subfamilies B, C and G involved in drug resistance in humans. This information may be useful for future functional studies on ABC transporters in monogeneans.

Galactosomum otepotiense n. sp. (Trematoda: Heterophyidae) infecting four different species of fish-eating birds in New Zealand: genetically identical but morphologically variable

Trematodes of the genus Galactosomum are cosmopolitan parasites that infect the intestines of fish-eating birds and mammals. Adults of named Galactosomum species have not been recorded from bird hosts in New Zealand, despite their cercarial stage being known from various studies of the first intermediate host, Zeacumantus subcarinatus. Here we describe a new species of Galactosomum infecting four different piscivorous birds in New Zealand: Caspian terns, red-billed and black-backed gulls and little blue penguins. Specimens from each of these hosts are genetically identical in the genes sequenced, but show considerable morphological variability. Galactosomum otepotiense n. sp. is distinguished from most other members of the ‘bearupi-group’ in having a single circle of spines on the ventral sucker, and spines, as opposed to scales, over most of the body. It is most similar to G. bearupi and G. angelae, both from Caspian terns in Australia, but differs in the relative sizes of the reproductive organs and in the possession of a very long forebody. Molecular data confirm that G. otepotiense is not conspecific with G. bearupi, but 28S and ITS2 phylogenies show its close relationship to G. bearupi and other Australian species. We use the cox1 sequence to confirm identity with the larval stage infecting Z. subcarinatus, as previously described in the literature. We discuss briefly the relationships between Australian and New Zealand Galactosomum spp. and their hosts, variability between genetically identical specimens found in different hosts and their potential for harm to mariculture economy.

Three previous recorded species of Dactylogyrus Diesing, 1850 (Monogenea: Dactylogyridae) infecting cultured Carassius auratus in southern Brazil

The present study aimed to contribute to the monogenean dactylogyrids that occur in C. auratus from an ornamental fish farm in southern Brazil. Samples of goldfish were obtained from a fish farm and the gills were removed and analysed to identify the species of Dactylogyrus to determine the levels of infestation. Three species of Dactylogyrus were found in the gills: Dactylogyrus anchoratus, Dactylogyrus baueri and Dactylogyrus formosus. The most prevalent species was D. baueri [prevalence (P) = 70%, mean intensity (MI) = 2.14 ± 1.21, mean abundance (MA) = 1.5 ± 1.43], followed by D. anchoratus (P = 60%, MI = 1.67 ± 0.81, MA = 1.00 ± 1.05) and D. formosus (P = 30%, MI = 1.66 ± 1.15, MA = 0.5 ± 1.64). Dactylogyrus anchoratus shares morphological characteristics with D. arcuatus by the similarity in shape of the haptor parts. Dactylogyrus baueri shows similarity to D. dulkeiti in relation to the pair of anchors and accessory piece of the male copulatory complex. Dactylogyrus formosus presents an accessory piece with branch and total length of the anchors smaller than D. anchoratus. This study reports the occurrence of D. anchoratus, D. baueri and D. formosus in goldfish cultured in southern Brazil. These monogenean species have been described in cyprinid fishes that have a wide geographic distribution with the result of the dissemination of the host in the world.

In vitro efficacy of two terpenes against ancyrocephalid monogeneans from Nile tilapia

Terpenes are naturally produced compounds with a broad range of biological activities. Currently, there is limited information regarding the anthelminthic effect of terpenes against monogenean parasites of fish. The aim of this work was to evaluate the in vitro efficacy of two terpenes [α-terpinene and (+)-limonene oxide] against ancyrocephalid monogeneans found on farmed Nile tilapia (Oreochromis niloticus). (+)-Limonene oxide was more effective in killing these parasites than α-terpinene, with 86 and 90% mortality at concentrations of 36 and 55.4 mg/L, respectively, with a 5-h treatment. The estimated 5-h EC₅₀ of (+)-limonene oxide was 4.8 mg/L. Even though this compound has the potential to be used as an anthelmintic compound in finfish aquaculture, before in vivo experiments are performed, additional studies are needed to find a more effective concentration, as well as to evaluate other terpenic compounds.

Preliminary Monitoring of Praziquantel in Water and Sediments at a Japanese Amberjack (Seriola quinqueradiata) Aquaculture Site

Praziquantel (PZQ), an active compound against Platyhelminthes, is an essential anthelmintic for the aquaculture industry. However, there are few reports of the environmental risks of PZQ use in aquaculture. In this study, we monitored PZQ in water and sediment at an aquaculture site of Japanese amberjack, also called yellowtail (Seriola quinqueradiata). Although PZQ was detected in water during treatment and 3 days post-treatment, PZQ levels were below the detectable limit in water 60 m from the net pen during the treatment, and in all sediment samples. In this preliminary study, we could not detect residue of PZQ from sediments in the aquaculture site, and no evidence about environmental effect of PZQ administration was obtained.

Acting locally - affecting globally: RNA sequencing of gilthead sea bream with a mild Sparicotyle chrysophrii infection reveals effects on apoptosis, immune and hypoxia related genes

BACKGROUND

Monogenean flatworms are the main fish ectoparasites inflicting serious economic losses in aquaculture. The polyopisthocotylean Sparicotyle chrysophrii parasitizes the gills of gilthead sea bream (GSB, Sparus aurata) causing anaemia, lamellae fusion and sloughing of epithelial cells, with the consequent hypoxia, emaciation, lethargy and mortality. Currently no preventive or curative measures against this disease exist and therefore information on the host-parasite interaction is crucial to find mitigation solutions for sparicotylosis. The knowledge about gene regulation in monogenean-host models mostly comes from freshwater monopysthocotyleans and almost nothing is known about polyopisthocotyleans. The current study aims to decipher the host response at local (gills) and systemic (spleen, liver) levels in farmed GSB with a mild natural S. chrysophrii infection by transcriptomic analysis.

RESULTS

Using Illumina RNA sequencing and transcriptomic analysis, a total of 2581 differentially expressed transcripts were identified in infected fish when compared to uninfected controls. Gill tissues in contact with the parasite (P gills) displayed regulation of fewer genes (700) than gill portions not in contact with the parasite (NP gills) (1235), most likely due to a local silencing effect of the parasite. The systemic reaction in the spleen was much higher than that at the parasite attachment site (local) (1240), and higher than in liver (334). NP gills displayed a strong enrichment of genes mainly related to immune response and apoptosis. Processes such as apoptosis, inflammation and cell proliferation dominated gills, whereas inhibition of apoptosis, autophagy, platelet activation, signalling and aggregation, and inflammasome were observed in spleen. Proteasome markers were increased in all tissues, whereas hypoxia-related genes were down-regulated in gills and spleen.

CONCLUSIONS

Contrasting forces seem to be acting at local and systemic levels. The splenic down-regulation could be part of a hypometabolic response, to counteract the hypoxia induced by the parasite damage to the gills and to concentrate the energy on defence and repair responses. Alternatively, it can be also interpreted as the often observed action of helminths to modify host immunity in its own interest. These results provide the first toolkit for future studies towards understanding and management of this parasitosis.

Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis affecting more than 200 million people in tropical and subtropical countries, infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The Aporocotylidae are pathogenic in fish, Spirorchiidae in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in molluscs and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive, but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Mucosal immunoglobulins protect the olfactory organ of teleost fish against parasitic infection

The olfactory organ of vertebrates receives chemical cues present in the air or water and, at the same time, they are exposed to invading pathogens. Nasal-associated lymphoid tissue (NALT), which serves as a mucosal inductive site for humoral immune responses against antigen stimulation in mammals, is present also in teleosts. IgT in teleosts is responsible for similar functions to those carried out by IgA in mammals. Moreover, teleost NALT is known to contain B-cells and teleost nasal mucus contains immunoglobulins (Igs). Yet, whether nasal B cells and Igs respond to infection remains unknown. We hypothesized that water-borne parasites can invade the nasal cavity of fish and elicit local specific immune responses. To address this hypothesis, we developed a model of bath infection with the Ichthyophthirius multifiliis (Ich) parasite in rainbow trout, Oncorhynchus mykiss, an ancient bony fish, and investigated the nasal adaptive immune response against this parasite. Critically, we found that Ich parasites in water could reach the nasal cavity and successfully invade the nasal mucosa. Moreover, strong parasite-specific IgT responses were detected in the nasal mucus, and the accumulation of IgT⁺ B-cells was noted in the nasal epidermis after Ich infection. Strikingly, local IgT⁺ B-cell proliferation and parasite-specific IgT generation were found in the trout olfactory organ, providing new evidence that nasal-specific immune responses were induced locally by a parasitic challenge. Overall, our findings suggest that nasal mucosal adaptive immune responses are similar to those reported in other fish mucosal sites and that an antibody system with a dedicated mucosal Ig performs evolutionary conserved functions across vertebrate mucosal surfaces.

The olfactory organ is a vitally important chemosensory organ in vertebrates but it is also continuously stimulated by pathogenic microorganisms in the external environment. In mammals and birds, nasopharynx-associated lymphoid tissue (NALT) is considered one of the first lines of immune defense against inhaled antigens and in bony fish, protecting against water-borne infections. However, although B-cells and immunoglobulins (Igs) have been found in teleost NALT, the defensive mechanisms of parasite-specific immune responses after pathogen challenge in the olfactory organ of teleost fish remain poorly understood. Considering that the NALT of all vertebrates has been subjected to similar evolutionary forces, we hypothesize that mucosal Igs play a critical role in the defense of olfactory systems against parasites. To confirm this hypothesis, we show the local proliferation of IgT⁺ B-cells and production of pathogen-specific IgT within the nasal mucosa upon parasite infection, indicating that parasite-specific IgT is the main Ig isotype specialized for nasal-adaptive immune responses. From an evolutionary perspective, our findings contribute to expanding our view of nasal immune systems and determining the fate of the host–pathogen interaction.

Cysteine peptidases of Eudiplozoon nipponicum: a broad repertoire of structurally assorted cathepsins L in contrast to the scarcity of cathepsins B in an invasive species of haematophagous monogenean of common carp

Background

Cysteine peptidases of clan CA, family C1 account for a major part of proteolytic activity in the haematophagous monogenean Eudiplozoon nipponicum. The full spectrum of cysteine cathepsins is, however, unknown and their particular biochemical properties, tissue localisation, and involvement in parasite-host relationships are yet to be explored.

Methods

Sequences of cathepsins L and B (EnCL and EnCB) were mined from E. nipponicum transcriptome and analysed bioinformatically. Genes encoding two EnCLs and one EnCB were cloned and recombinant proteins produced in vitro. The enzymes were purified by chromatography and their activity towards selected substrates was characterised. Antibodies and specific RNA probes were employed for localisation of the enzymes/transcripts in tissues of E. nipponicum adults.

Results

Transcriptomic analysis revealed a set of ten distinct transcripts that encode EnCLs. The enzymes are significantly variable in their active sites, specifically the S2 subsites responsible for interaction with substrates. Some of them display unusual structural features that resemble cathepsins B and S. Two recombinant EnCLs had different pH activity profiles against both synthetic and macromolecular substrates, and were able to hydrolyse blood proteins and collagen I. They were localised in the haematin cells of the worm’s digestive tract and in gut lumen. The EnCB showed similarity with cathepsin B2 of Schistosoma mansoni. It displays molecular features typical of cathepsins B, including an occluding loop responsible for its exopeptidase activity. Although the EnCB hydrolysed haemoglobin in vitro, it was localised in the vitelline cells of the parasite and not the digestive tract.

Conclusions

To our knowledge, this study represents the first complex bioinformatic and biochemical characterisation of cysteine peptidases in a monogenean. Eudiplozoon nipponicum adults express a variety of CLs, which are the most abundant peptidases in the worms. The properties and localisation of the two heterologously expressed EnCLs indicate a central role in the (partially extracellular?) digestion of host blood proteins. High variability of substrate-binding sites in the set of EnCLs suggests specific adaptation to a range of biological processes that require proteolysis. Surprisingly, a single cathepsin B is expressed by the parasite and it is not involved in digestion, but probably in vitellogenesis.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2666-2) contains supplementary material, which is available to authorized users.