Taken to the limit — Is desiccation stress causing precocious encystment of trematode parasites in snails?

Stressed snails release Angiostrongylus cantonensis (rat lungworm) larvae in their slime

This study investigated the influence of stress on release of Angiostrongylus cantonensis larvae from a snail host, Parmarion martensi. We subjected 140 infected, wild-caught P. martensi to three stress-inducing treatments (heat, molluscicide, physical disturbance) and an unstressed control treatment for 24 h, after which larval presence and abundance in the slime were quantified by qPCR targeting the ITS1 region of the parasite's DNA, and compared among treatments. The significance of stress and host infection load on larval release was determined by generalized linear mixed models and permutation tests. The results indicated that stress significantly increased the probability of larval presence in slime and the number of larvae released, and highly infected snails were also more likely to release larvae. Among stressed snails, 13.3% released larvae into slime, the number of larvae present in the slime ranging from 45.5 to 4216. Unstressed controls released no larvae. This study offers a partial explanation for conflicting results from prior studies regarding A. cantonensis presence in snail slime and sheds light on the broader One Health implications. Stress-induced larval release highlights the potential role of slime as a medium for pathogen transmission to accidental, paratenic, definitive and other intermediate hosts. These findings emphasize the importance of considering stress-mediated interactions in host-parasite systems and their implications for zoonotic disease emergence. As stressors continue to escalate because of anthropogenic activities and climate change, understanding the role of stress in pathogen shedding and transmission becomes increasingly important for safeguarding human and wildlife health within the One Health framework.

Effects of droughts on the larval digenean assemblage that parasitizes the snail Biomphalaria peregrina (Planorbidae): a 4-year study in a temporary pond from Argentina

Temporary ponds are ecologically valuable habitats and useful sites for studies of diversity, ecology, evolution, and climate change. Organisms inhabiting these environments have developed strategies to ensure their survival. However, little is known about the temporal dynamics and strategies of parasites in these habitats. A 4-year study was carried out in a temporary pond to analyze the larval digenean assemblage in Biomphalaria peregrina, a potential vector of Schistosoma mansoni, a zoonosis of global importance. This environment had intermittent and irregular hydroperiods, and the overall prevalence showed differences throughout the years. After drought seasons, eight out of a total of nine species of digeneans were observed, and almost half of the snails were parasitized. In addition, six of the nine species of digeneans showed some type of abbreviation of their life cycles. These results suggest that digeneans have certain degree of plasticity in their life cycles in response to environmental changes in this pond, and the abbreviation of the digenean life cycle could be regarded as a parasite strategy to resist prolonged periods of desiccation.

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.

Non-host organisms impact transmission at two different life stages in a marine parasite

The potential for local biodiversity to 'dilute' infection risk has been shown to be particularly important in aquatic trematodes, where non-host organisms can feed on free-living infective stages (cercariae) and reduce transmission rates to target hosts. Non-host organisms could also impact transmission during other stages of the trematode life cycle. In Philophthalmus spp., cercariae encyst as metacercariae on external surfaces, where they remain exposed to the adverse effects of non-host organisms. In laboratory experiments, we tested the potential for a range of non-host organisms to (i) prey on cercariae, (ii) induce early (i.e., faster) encystment and (iii) prey on or destroy metacercariae. Our results show that intertidal anemones, and to a lesser extent clams, can consume substantial numbers of cercariae. However, we found no strong evidence that the presence of these predators causes cercariae to encyst faster as a way to escape from predation. We also found that grazing snails can reduce numbers of encysted metacercariae, either by eating or crushing them. Our findings add to the growing evidence that trematode transmission success can be strongly affected by the local diversity of non-host organisms. They also reinforce the notion that parasites are potentially important food items for many organisms, thus playing roles other than consumers in many food webs.