Trematode diversity in freshwater fishes of the Globe I: ‘Old World’

Prevalence and identification of cyathocotylid trematodes infecting African catfish in Egypt

The trematode family Cyathocotylidae infects various hosts worldwide, including birds, mammals, reptiles, and fish. However, the lack of molecular data from adult worms hinders phylogenetic, epidemiological, and host association studies. This study aims to identify the common cyathocotylid trematodes infecting African catfish in Egypt using morphological and molecular evidence. Out of 142 Clarias gariepinus, 123 fish (86.6%) harbored cyathocotylid metacercariae, with a mean metacercarial intensity of 201 ± 38.5/g. Cyathocotylid metacercariae prevalence gradually rose as host size increased. Although there was no significant difference between groups, larger fish had a higher mean metacercarial intensity. The prevalence and intensity were unrelated to the fish gender. Histopathological examination of metacercariae-infected catfish revealed varying degrees of degenerative changes, including intermuscular edema leading to muscle fiber dispersion and atrophy, involving 11% to over 81% of muscle sections. We identified three cyathocotylid metacercariae and eight cyathocotylid adult species from experimental infection using morphometric and molecular data, including internal transcribed spacer (ITS) and/or mitochondrial cytochrome c oxidase subunit 1 (cox1) sequences. We determined the phylogenetic position of these cyathocotylid samples. The ITS sequence analysis linked the isolated Cyathocotylidae sp. 1 and 2 metacercariae to Prohemistomum vivax adults. Mesostephanus appendiculatoides and Paracoenogonimus ovatus were reported for the first time in Egypt. These findings may provide valuable genetic data for future molecular epidemiological and phylogenetic studies of cyathocotylid trematodes.

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.

Selected Wildlife Trematodes

The trematodes are a species-rich group of parasites, with some estimates suggesting that there are more than 24,000 species. However, the complexities associated with their taxonomic status and nomenclature can hinder explorations of the biology of wildlife trematodes, including fundamental aspects such as host use, life cycle variation, pathology, and disease. In this chapter, we review work on selected trematodes of amphibians, birds, mammals, and their snail intermediate hosts, with the goal of providing a tool kit on how to study trematodes of wildlife. We provide a brief introduction to each group of wildlife trematodes, followed by some examples of the challenges each group of trematodes has relative to the goal of their identification and understanding of the biology and interactions these organisms have with their wildlife hosts.

Review of the metazoan parasites of the economically and ecologically important African sharptooth catfish Clarias gariepinus in Africa: Current status and novel records

One of the most widely distributed African freshwater fish is the African sharptooth catfish Clarias gariepinus (Burchell) that is naturally distributed in 8 of the 10 ichthyofaunal regions of this continent. Clarias gariepinus is a highly valued and cheap staple to local communities and an ideal aquaculture species. Consequently, interest in the parasitic communities of C. gariepinus has increased as parasites may accidentally be ingested by humans when eating uncooked fish or can be introduced into culture systems through fish stocks supplied from local rivers which affect yield, growth, and marketability. This review provides an overview of the ∼107 metazoan parasite species known to parasitise C. gariepinus in Africa and their general life cycles, morphology, paratenic and post-cyclic infections, and the biogeography and validity of records are discussed. A brief overview is included on the application of some of these parasites in environmental studies and their link to human health.

Diversity of European lissorchiid trematodes from fish and snail hosts with comments on the validity of the genus Parasymphylodora Szidat, 1943

Genetic markers, DNA sequences and karyotypes, of some European lissorchiid species from their intermediate and final hosts were obtained to clarify controversial data about their life cycles and taxonomy, and to reveal phylogenetic affinities. The life cycles of three species have been confirmed for the first time based on molecular data. Comparative analysis of internal transcribed spacer 2 (ITS2) and partial 28S rDNA sequences has undoubtedly proven that cercariaeum of type-species of the genus Asymphylodora, Asymphylodora tincae, develops in pulmonate snails, Anisus vortex and Stagnicola palustris, but not in the genus Bithynia. The faucet snail, Bithynia tentaculata, serves as the first intermediate host for Parasymphylodora (=Asymphylodora) markewitschi and Parasymphylodora parasquamosa; adults of both species were isolated from the common rudd, Scardinius erythrophthalmus. It has also been confirmed that B. tentaculata serves as the second intermediate host for P. parasquamosa. Phylogenetic analysis supports the validity of the genus Parasymphylodora. Two species, Parasymphylodora markewitschi and P. parasquamosa, with cercariaeum belonging to the Parasquamosum group, are closely related and are being recovered as a well-defined evolutionary lineage in phylogenetic trees. A significant divergence between Parasymphylodora spp. and Asymphylodora spp. was revealed. The diploid chromosome set of P. markewitschi is composed of 14 chromosomes and does not show similarities with karyotypes of other lissorchiid species. Asymphylodora progenetica and Asymphylodora tincae share the basal diploid value of the family, 2n = 20, and reveal very close morphology of the corresponding chromosome pairs. Karyotypic similarities of these species are in accordance with molecular phylogenetic data. Thus, the available molecular and cytogenetic data support the assignment of P. markewitschi and P. parasquamosa to a separate genus, meanwhile, the assignment of A. progenetica to the genus Parasymphylodora was not justified.

Component Endoparasite Communities Mirror Life-History Specialization in Syntopic Reed Frogs (Hyperolius spp.)

Most of our knowledge on the processes structuring parasite communities in amphibians originate from temperate-zone taxa, whereas Afrotropical communities have been neglected so far. We found evidence that ecological fitting of the hosts and, probably, differential immune response may influence the variation in parasite species richness, prevalence, and infestation intensity of East African frogs Hyperolius kivuensis and H. viridiflavus. The most closely related host species share the same macrohabitat (that implies the same pool of potential parasites), but differ in microhabitat preference, so that a comparative analyses of syntopic and allopatric populations is expedient to reveal ecological fitting. We detected 11 parasite species (one annelid, four nematodes, five trematodes, one cestode) and two endocommensal species (protozoans). The component parasite communities included 4–5 helminth species in H. kivuensis and 6–8 in the more aquatic H. viridiflavus, supporting the hypothesis that trematode diversity increases with the amount of time spent in water. Five parasite species (Orneoascaris chrysanthemoides, Clinostomum chabaudi, an undetermined echinostomatid) and two protozoans (Nyctotheroides sp., and Protoopalina sp.) are shared among the syntopic amphibian populations. This finding indicates a similar susceptibility of these amphibians to infestation from the local parasite pool. Yet, the low prevalence of single- and multi-species infestations in H. kivuensis indicates that parasite clearing by its immune response is probably more effective and prominent than in H. viridiflavus. Therefore, H. viridiflavus suffered from significantly reduced short-term survival due to the infection. Thus, we conclude that the processes structuring component parasite communities in amphibians do not differ generally between temperate-zone and Afrotropical host species, but they do in the magnitude of ecological fitting.

High diversity of metazoan parasites in carp gudgeons (Eleotridae: Hypseleotris spp.) from Eastern Australia

Knowledge of the parasite fauna of Australian freshwater fish is fragmentary and incomplete. An understanding of fish hosts and their associated parasites is vital for the successful management of aquatic ecosystems. In this study, we surveyed the parasite fauna of carp gudgeons (Hypseleotris spp.), a complex of species of Australian freshwater fishes, using morphology and molecular data for the 18S and 28S ribosomal RNA genes. We examined 137 individuals of three different taxa in the carp gudgeon species complex and found 16 parasitic taxa of the Digenea, Cestoda, Nematoda and Arthropoda (five adults and 11 larvae). Eleven parasites are reported for the first time from the carp gudgeons (Pseudodactylogyrus sp., Gyrodactylus sp., Clinostomum sp., Paradilepis patriciae, P. cf. kempi, two unidentified species of Paradilepis, Dendrouterina sp., Parvitaenia sp., two lineages of Cyclophyllidea gen. sp., Procamallanus sp., larvae of a spirurine nematode and Lernaea sp.), in addition to Apatemon cf. hypseleotris Negm-Eldin & Davies, 2001 and the invasive tapeworm Schyzocotyle acheilognathi (Yamaguti, 1934), which were previously reported from these fish hosts. Parasite species richness was double in Lake's and Midgley's carp gudgeons relative to western carp gudgeon. These findings highlight the key role of carp gudgeons as intermediate hosts for multiple parasites with complex life cycles using native birds as definitive hosts and the usefulness of DNA data for the identification of parasite larvae.

The state of fish parasite discovery and taxonomy: a critical assessment and a look forward

Efforts to find and characterise new parasite species in fish hosts are crucial not just to complete our inventory of Earth's biodiversity, but also to monitor and mitigate disease threats in fisheries and aquaculture in the face of global climate change. Here, we review recent quantitative assessments of research efforts into fish parasite discovery and taxonomy. We address broad questions including: Are efforts aimed at finding new parasite species targeted at geographical hotspots of fish biodiversity, where there should be more parasite species to be found? Is the application of molecular tools to study parasite genetic diversity deployed strategically across regions of the world, or focused disproportionately on certain areas? How well coordinated is the search for new parasite species of fish among workers specialising on different higher helminth taxa? Are parasite discovery efforts in any geographical area consistent over time, or subject to idiosyncrasies due to the waxing and waning of highly prolific research careers? Is the quality of taxonomic descriptions of new species improving over time, with the incorporation of new tools to characterise species? Are taxonomic descriptions moving away from a focus on the adult stage only toward attempts to characterise the full life cycle of newly-discovered helminth species? By using empirical evidence to answer these questions, we assess the current state of research into fish parasite discovery and taxonomy. We also explore the far-reaching implications of recent research on parasite microbiomes for parasite taxonomy. We end with recommendations aimed at maximising the knowledge gained per fish sacrificed, and per dollar and time invested into research on fish parasite biodiversity.

Characterization of the complete mitochondrial genome of Plagiorchis maculosus (Digenea, Plagiorchiidae), Representative of a taxonomically complex digenean family

Despite the highly divergent morphology, pathogenicity and worldwide distribution of digenean parasites belonging to one of the largest families, the Plagiorchiidae, there are no complete mitochondrial (mt) genomes published to date for plagiorchiids. In this study, we obtained nuclear ribosomal DNA (ITS region and 28S rDNA) sequences and the complete mt genome sequences of Plagiorchis maculosus (Rudolphi 1802) Braun, 1902, and assessed its phylogenetic relationship with other xiphidiates, based on the mtDNA sequences. The obtained ITS and 28S rDNA sequences were identical to the corresponding sequences of P. maculosus available in GenBank. The complete mitochondrial genome of P. maculosus (14,124 bp) contained 36 genes (atp8 is absent) and a long non-coding region (NCR) with two sets of repeated sequences of 283 nucleotides each. The phylogenetic tree resulting from Bayesian inference (BI) analyses based on concatenated nucleotide sequences of all 36 genes of P. maculosus and other xiphidiates mitochondrial genomes, indicated that P. maculosus (and the Plagiorchiidae) is phylogenetically closest to the Brachycladiidae and Paragonimidae. The present study describes the first mitochondrial genome from the type genus of the family Plagiorchiidae. The overall gene arrangement, nucleotide composition, A + T contents, AT and GC skew and codon usage with relative synonymous codon usage (RSCU) for 12 PCGs are described. Characterization of mitochondrial genomes from additional plagiorchiid taxa is necessary to make further progress in phylogenetic and epidemiological studies of these digeneans as well as accurate diagnostics of these parasites including those parasitic in humans.

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.