A checklist of larval Digenea (Platyhelminthes: Trematoda) in molluscs from inland waters of Argentina: one hundred years of research

A checklist of larval digeneans parasitising molluscs from inland waters of Argentina is presented here. Based on the bibliographical survey of 113 scientific articles and nine theses published between 1930 and 2021, 213 digeneans were found, which were distributed within 13 superfamilies and 35 families. The parasites were identified in 102 locations, encompassing eight of the sixteen biogeographical provinces in Argentina. Digeneans were found in 34 mollusc species (31 gastropods and three bivalves) belonging to 17 genera. The available data are presented for each digenean species, and included host information, localities, prevalence values, type of habitat, life cycle information (natural or experimental host), and information on material and genetic sequences deposited in repositories. Only 21.1% of individuals were identified to species level, and 8.5% to genus level. In addition, the genetic sequences of only 10 species (4.7%) were available at GenBank. This survey constitutes the first checklist of parasitic helminths in molluscs from inland waters of Argentina.

Taxonomy of Echinostoma revolutum and 37-Collar-Spined Echinostoma spp.: A Historical Review

Echinostoma flukes armed with 37 collar spines on their head collar are called as 37-collar-spined Echinostoma spp. (group) or 'Echinostoma revolutum group'. At least 56 nominal species have been described in this group. However, many of them were morphologically close to and difficult to distinguish from the other, thus synonymized with the others. However, some of the synonymies were disagreed by other researchers, and taxonomic debates have been continued. Fortunately, recent development of molecular techniques, in particular, sequencing of the mitochondrial (nad1 and cox1) and nuclear genes (ITS region; ITS1-5.8S-ITS2), has enabled us to obtain highly useful data on phylogenetic relationships of these 37-collar-spined Echinostoma spp. Thus, 16 different species are currently acknowledged to be valid worldwide, which include E. revolutum, E. bolschewense, E. caproni, E. cinetorchis, E. deserticum, E. lindoense, E. luisreyi, E. mekongi, E. miyagawai, E. nasincovae, E. novaezealandense, E. paraensei, E. paraulum, E. robustum, E. trivolvis, and Echinostoma sp. IG of Georgieva et al., 2013. The validity of the other 10 species is retained until further evaluation, including molecular analyses; E. acuticauda, E. barbosai, E. chloephagae, E. echinatum, E. jurini, E. nudicaudatum, E. parvocirrus, E. pinnicaudatum, E. ralli, and E. rodriguesi. In this review, the history of discovery and taxonomic debates on these 26 valid or validity-retained species are briefly reviewed.

The diverse echinostomes from East Africa: With a focus on species that use Biomphalaria and Bulinus as intermediate hosts

Echinostomes are a diverse group of digenetic trematodes that are globally distributed. The diversity of echinostomes in Africa remains largely unknown, particularly in analyses using molecular markers. Therefore, we were interested in the composition and host usage patterns of African echinostomes, especially those that also use schistosome transmitting snails as intermediate hosts. We collected adults and larval stages of echinostomes from 19 different localities in East Africa (1 locality in Uganda and 18 in Kenya). In this study we provide locality information, host use, museum vouchers, and genetic data for two loci (28S and nad1) from 98 samples of echinostomes from East Africa. Combining morphological features, host use information, and phylogenetic analyses we found 17 clades of echinostomes in East Africa. Four clades were found to use more than one genus of freshwater snails as their first intermediate hosts. We also determined at least partial life cycles (2 of the 3) of four clades using molecular markers. Of the 17 clades, 13 use Biomphalaria or Bulinus as a first intermediate host. The overlap in host usage creates opportunities for competition, including against human schistosomes. Thus, our study can be used as a foundation for future studies to ascertain the interactions between schistosomes and echinostomes in their respective intermediate hosts.

Larval digenean communities in Biomphalaria species in two contrasting wetlands from South America

A key factor in the colonization probabilities of parasites is the host's characteristic habitat. Considering that rice fields are simple habitats compared with natural wetlands, we comparatively analyzed the composition of the cercariae communities in Biomphalaria snails in an agricultural and a nonagricultural habitat of Corrientes Province, Argentina. We collected 2998 Biomphalaria snails from November 2011 to May 2012: B. tenagophila, B. occidentalis, B. peregrina, and B. orbignyi in the nonagricultural habitat and B. straminea in the agricultural habitat. Nine cercaria species were found in the nonagricultural habitat and 12 in the agricultural habitat, with overall parasite prevalence of 9.27 and 7.69%, respectively. The overall prevalence and mean species richness of cercariae showed no significant differences between habitats. The cercariae communities of the 2 habitats showed low similarity (38%), sharing 4 species. The mean species diversity of cercariae was higher in the nonagricultural habitat. Our results suggest that rice fields provide conditions for the hosts and cercariae that are somewhat similar to those of nonagricultural wetlands, although the contrasting characteristics of habitats can influence the presence of different intermediate and definitive hosts and consequently impact parasite diversity.

Freshwater Snail Diversity in Mae Lao Agricultural Basin (Chiang Rai, Thailand) with a Focus on Larval Trematode Infections

The aim of this study was to conduct a freshwater snail survey in Mae Lao agricultural basin to assess the diversity with a focus on habitat types and their larval trematode infections. Snails were collected and examined in 14 sites of Mae Lao agricultural basin from August 2016 to October 2017. A total of 1,688 snail individuals were collected and classified into 7 families, 8 genera, and 12 species. Snail diversity and habitat types were higher in rice paddies than irrigation canals and streams. The most abundant species was Bithynia siamensis siamensis, representing 54.6% of the sample. Three species of snails act as first intermediate host were found with cercarial infections. They were Filopaludina sumatrensis polygramma, B. s. siamensis, and Melanoides tuberculata. The cercariae were categorized into 7 types; echinostome, monostome, gymnocephalous, virgulate, parapleurolophocercous, pleurolophocercous and megalurous cercariae. Parapleurolophocercous cercariae constituted the most common type of cercariae recovered, contributing 41.2% of all infections in snails. Echinostome metacercariae infections were found in 6 snail species with 7.6% prevalence. In addition, the metacercaria of avian trematode, Thapariella sp. were found in Filopaludina spp. snails and B. funiculata with a prevalence of 0.5%. This is the first report for Thapariella metacercariae in the snail host, B. funiculata, and also confirmed that viviparid and bithyniid snails act as the second intermediate hosts of this trematode. This work will provide new information on the distribution and intermediate host of trematode in this area.

New larval trematodes in Biomphalaria species (Planorbidae) from Northeastern Argentina

Larval trematodes infecting Biomphalaria tenagophila and B. occidentalis were surveyed in a suburban and semipermanent pond of Corrientes province, Northeastern Argentina. A total of 1,409 snails were examined between spring 2011 to winter 2013, and 8 different larval trematodes were studied morphologically. Three of these species-Echinocercaria sp. IV, Ribeiroia sp. and Echinocercaria sp. XIV-have been previously found in Corrientes province. Six other trematodes belonging to Strigeidae (Furcocercaria sp. III), Clinostomidae (Cercaria Clinostomidae sp.), Spirorchiidae (Cercaria Spirorchiidae sp.) and Echinostomatidae (Echinocercaria sp. 1, Echinocercaria sp. 2, Echinocercaria sp. 3) are new species parasitizing Biomphalaria snails. Cercaria Spirorchiidae sp. is the third larval trematode related to Spirorchiidae recorded in South America and the first one for Argentina. Cercaria Clinostomidae sp. is the first one related to Clinostomidae in northeastern Argentina. The prevalence of larval trematodes infecting B. tenagophila and B. occidentalis in the environment studied was low (<5%) with the echinostome group better represented in terms of prevalence and species richness. Drought periods could affect the dynamics of parasitic transmission due to the absence of trematodes in the autumn and winter of the first seasonal cycle. However, in humid periods parasite transmission can occur throughout the year due to the presence of larvae in all seasons of the second seasonal cycle, although the less-warm seasons showed higher prevalence than the summer period probably related to the subtropical climate of Corrientes province.