Morphological and molecular assessment of the diversity of trematode communities in freshwater gastropods and bivalves in Los Tuxtlas tropical rainforest

Diversity of trematodes (Platyhelminthes) in Mexico with an assessment of the availability of genetic data for their conservation

Trematodes are one of the most abundant and diverse groups of platyhelminths. They parasitize all major groups of vertebrates as definitive hosts and therefore play an important role in ecosystem composition. It is estimated that 18,000 to 25,000 species of trematodes exist worldwide, of which 685 have been reported in Mexico. Although this group is an integral part of ecosystems, there are still no studies that highlight the importance of parasites, especially in conservation approaches. Here, we recompiled information on the occurrence and available genetic data of trematodes in Mexico to estimate the specific richness of their representation across the Protected Areas (PAs) and provinces of Mexico. We consulted national and international databases (e.g., GBIF, CONABIO, CNHE-UNAM) and genetic repositories (e.g., GenBank) to generate curated datasets. We obtained 6,780 records that represent 99% of species reported in Mexico (680 species), of which only 10.2% are included in PAs. For genetic data, we found information from five nuclear regions (28S, 18S, ITS1, ITS2 and 5.8S) and two mitochondrial genes (COI and NAD1) for 118 species, of which only 3.5% were associated with PAs. With these results, we provide a spatial distribution of records (occurrence and genetic data) of trematodes present in Mexico and its PAs and identify poorly represented biogeographic provinces (e.g., Sierra Madre del Sur). We also highlight that this is the first study in Mexico to include this group in a conservation approach, and we record valuable information for future studies.

High prevalence of natural infection by the ruminant blood fluke Schistosoma spindale in the intermediate snail host Indoplanorbis exustus in Uttaradit, Northern Thailand

Background and Aim

Freshwater snails belonging to the family Planorbidae serve as the first intermediate hosts of many species of important parasitic flukes of animals and humans. Information regarding the occurrence of planorbid snail larval trematode infection is limited in Northern Thailand. Thus, this study aimed to estimate the prevalence of larval trematode infection of the freshwater snail Indoplanorbis exustus in Uttaradit, Thailand, and to identify trematode species based on their morphological and molecular characteristics.

Materials and Methods

Planorbid snail specimens were collected from a water reservoir in Uttaradit, Thailand, from June to August 2023. Snails were assessed for larval trematode infection through cercarial shedding and crushing methods. The released cercariae were preliminarily identified on the basis of their morphological characteristics. In addition, species identification of the detected cercariae was conducted using 28S ribosomal RNA and cytochrome c oxidase subunit 1 gene sequence analyses.

Results

The overall prevalence of cercarial infection was 61.5% (107/174) in planorbid snails in Uttaradit province. Two species of cercarial trematodes, Schistosoma spindale (106/174, 60.9%) and Artyfechinostomum malayanum (1/174, 0.6%), were identified using morphological and molecular analyses, of which S. spindale was the most abundant species. Our studied snails did not have mixed infection with more than two cercarial species.

Conclusion

Our findings reveal a remarkably high prevalence of S. spindale cercariae infecting planorbid snails in Uttaradit, indicating that humans and animals across the study area are at risk of infection. Our data may contribute to the development of effective strategies to control this zoonotic infectious disease.

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.

Let's restart formally naming 'larval' trematodes

Arguably the most unique biological features of trematode parasites involve their clonal parthenitae and cercariae. These life stages are biologically fascinating, medically and scientifically important, and often studied for years, lacking knowledge of their corresponding sexual adult stages. But sexual adults are the focus of trematode species-level taxonomy, partially explaining the relative neglect of documenting the diversity of parthenitae and cercariae and why researchers who do describe them give them only provisional names. Provisional names are unregulated, unstable, often ambiguous, and, I argue, often unnecessary. I suggest that we recommence formally naming parthenitae and cercariae using an improved naming scheme. The scheme should permit us to reap the benefits of formal nomenclature and thereby enhance research involving these diverse and important parasites.

Trematode Cercariae from Lymnaea gedrosiana in the Caspian Sea Littoral in Iran: a one health concern

INTRODUCTION

Lymnaea gedrosiana snails are hosts to a variety of trematode cercaria of public and veterinary health importance. In Guilan Province, Iran, a region with a high level of fish and bird farming and wetlands important for migratory birds, little is known about the trematode cercaria from L. gedrosiana.

METHODS

From April 2020 to October 2021, six freshwater sites in Guilan Province were sampled for Lymnaeidae snails three times per season (spring, summer, autumn and winter). Snails were exposed to light and heat to induce cercaria shedding and shredded cercaria were identified morphologically and molecularly.

RESULTS

In total, 5,712 Lymnaeidae snails were collected of which 3,288 (57.6%) were identified to be L. gedrosiana with 54.3% containing trematode cercaria. Snail and cercaria recovery were highest in the spring and summer. Trematode cercaria identified included Telorchis assula, Hypoderaeum conoideum, Apharyngostrigea pipientis, Sanguinicola cf. inermis, Opisthioglyphe ranae, Diplostomum pseudospathaceum, and Australapatemon burti.

DISCUSSION

The four trematodes D. pseudospathaceum, S. inermis, A. burti, and A. pipientis have not been previously reported in Iran; all four of these can infect migratory birds. The most common cercaria found, H. conoideum (18.3% of the snails) is of zoonotic importance. The third most common cercaria found, S. inermis (10.0% of the snails) is detrimental to fish production. Given the importance of the wetlands in the region for wildlife and migratory birds as well as the number of fish and bird farms in the area, efforts to control L. gedrosiana snails are needed to protect wildlife and human health. In addition, monitoring programs should be implemented to identify and prevent introductions of new trematode species.

First integrative study of the diversity and specificity of metacercariae of Posthodiplostomum Dubois, 1936 from native and introduced fishes in the Caribbean

Metacercariae of the genus Posthodiplostomum are often recorded in freshwater fish hosts. While the diversity and taxonomy of this genus are receiving increasing attention in molecular phylogenetic studies, available data remain geographically biased. Most molecular studies of Posthodiplostomum and morphologically similar (neascus) worms originate in North America and Europe and Asia (more than 60% of DNA sequences are from USA and Canada), with few data currently available from the Neotropics, where high host diversity suggests high and under-sampled parasite diversity. In this study, we report molecular and morphological data from metacercariae of Posthodiplostomum in fish in Puerto Rico, where only a single species has been previously reported. Partial sequences of cytochrome c oxidase subunit 1 from metacercariae from Dajaus monticola (native to Puerto Rico) and the introduced fishes Poecilia reticulata, Parachromis managuensis, Lepomis macrochirus and Micropterus salmoides revealed 7 genetically distinct species-level lineages, of which 4 were novel. We report novel molecular life-cycle linkages in Posthodiplostomum macrocotyle (metacercariae in muscle of the cichlid Pa. managuensis), a species previously known only from adults in birds from South America; and in Posthodiplostomum sp. 23 (metacercariae in poeciliids), which has recently been found in Ardea herodias in Georgia, USA. We also report the first molecular data from Posthodiplostomum sp. 8 in M. salmoides in the Caribbean. Metacercariae of most species were morphologically distinguished and all displayed narrow specificity for fish hosts, with no indication of parasite sharing among introduced and native fishes.

Morphological, molecular, and life cycle study of a new species of Oligogonotylus Watson, 1976 (Digenea, Cryptogonimidae) from Colombia

The present study describes Oligogonotylusandinus sp. nov. and its life cycle from a rural fish farm in Sopetrán, Antioquia, Colombia. The endemic species of snail Aroapyrguscolombiensis and the fishes Poeciliacaucana and Andinoacaralatifrons are identified as the first intermediate host, the second intermediate host and the definitive host, respectively. The new species was defined through an integrative approach, combining the traditional morphology of its developmental stages with molecular analyses of the markers ITS2 from ribosomal DNA and COI from mitochondrial DNA. This new species can be distinguished from its congeners by genetic divergence, the position of the vitelline fields, and the number of gonotyls. This work represents the first report of a species of this genus in South America.