Effects of temperature on survival, infectivity and in vitro encystment of the cercariae of Echinostoma caproni

The impact of climate change and pollution on trematode-bivalve dynamics

Coastal ecosystems and their marine populations are increasingly threatened by global environmental changes. Bivalves have emerged as crucial bioindicators within these ecosystems, offering valuable insights into biodiversity and overall ecosystem health. In particular, bivalves serve as hosts to trematode parasites, making them a focal point of study. Trematodes, with their life cycles intricately linked to external factors, provide excellent indicators of environmental changes and exhibit a unique ability to accumulate pollutants beyond ambient levels. Thus, they act as living sentinels, reflecting the ecological condition of their habitats. This paper presents a comprehensive review of recent research on the use of bivalve species as hosts for trematodes, examining the interactions between these organisms. The study also investigates the combined impact of trematode infections and other pollutants on bivalve molluscs. Trematode infections have multifaceted consequences for bivalve species, influencing various aspects of their physiology and behavior, including population-wide mortality. Furthermore, the coexistence of trematode infections and other sources of pollution compromises host resistance, disrupts parasite transmission, and reduces the abundance of intermediate hosts for complex-living parasites. The accumulation process of these parasites is influenced not only by external factors but also by host physiology. Consequently, the implications of climate change and environmental factors, such as temperature, salinity, and ocean acidification, are critical considerations. In summary, the intricate relationship between bivalves, trematode parasites, and their surrounding environment provides valuable insights into the health and sustainability of coastal ecosystems. A comprehensive understanding of these interactions, along with the influence of climate change and environmental parameters, is essential for effective management and conservation strategies aimed at preserving these delicate ecosystems and the diverse array of species that rely on them.

Heat sensitivity of first host and cercariae may restrict parasite transmission in a warming sea

To predict global warming impacts on parasitism, we should describe the thermal tolerance of all players in host-parasite systems. Complex life-cycle parasites such as trematodes are of particular interest since they can drive complex ecological changes. This study evaluates the net response to temperature of the infective larval stage of Himasthla elongata, a parasite inhabiting the southwestern Baltic Sea. The thermal sensitivity of (i) the infected and uninfected first intermediate host (Littorina littorea) and (ii) the cercarial emergence, survival, self-propelling, encystment, and infection capacity to the second intermediate host (Mytilus edulis sensu lato) were examined. We found that infection by the trematode rendered the gastropod more susceptible to elevated temperatures representing warm summer events in the region. At 22 °C, cercarial emergence and infectivity were at their optimum while cercarial survival was shortened, narrowing the time window for successful mussel infection. Faster out-of-host encystment occurred at increasing temperatures. After correcting the cercarial emergence and infectivity for the temperature-specific gastropod survival, we found that warming induces net adverse effects on the trematode transmission to the bivalve host. The findings suggest that gastropod and cercariae mortality, as a tradeoff for the emergence and infectivity, will hamper the possibility for trematodes to flourish in a warming ocean.

Circadian rhythms of trematode parasites: applying mixed models to test underlying patterns

Circadian rhythms of parasites and their hosts can influence processes such as transmission, pathology and life cycle evolution. For trematode parasites that depend on free-living infectious stages (i.e. cercariae) to move among host species, the timing of parasite release is hypothesized to increase the likelihood of contacting a host. Yet, a persistent challenge in studying such biorhythms involves selection of appropriate analytical techniques. Here, we extend a generalized linear mixed modelling (GLMM) framework to cosinor analyses, thereby allowing flexibility in the statistical distribution of the response variable, incorporation of multiple covariates and inclusion of hierarchical grouping effects. By applying this approach to 93 snails infected with trematode parasites from freshwater pond ecosystems, we detected non-random rhythms in six of eight species, with variation in both the timing of peak cercariae release (between 5:10 and 21:46 h) and its magnitude (between 13 and 386). The use of GLMM yielded more accurate and precise estimates of the cosinor parameters compared with classical least-squares (LS) based on a simulation-based sensitivity analysis. The sensitivity analysis revealed that the amplitude and rhythm-adjusted mean values from the LS models diverged from the true values at some limits. We highlight the importance of novel analytical approaches for evaluating parasite circadian rhythms and investigating their underlying mechanisms.

Variation among genotypes in responses to increasing temperature in a marine parasite: evolutionary potential in the face of global warming?

Climates are changing worldwide, and populations are under selection to adapt to these changes. Changing temperature, in particular, can directly impact ectotherms and their parasites, with potential consequences for whole ecosystems. The potential of parasite populations to adapt to climate change largely depends on the amount of genetic variation they possess in their responses to environmental fluctuations. This study is, to our knowledge, the first to look at differences among parasite genotypes in response to temperature, with the goal of quantifying the extent of variation among conspecifics in their responses to increasing temperature. Snails infected with single genotypes of the trematode Maritrema novaezealandensis were sequentially acclimatised to two different temperatures, 'current' (15°C) and 'elevated' (20°C), over long periods. These temperatures are based on current average field conditions in the natural habitat and those predicted to occur during the next few decades. The output and activity of cercariae (free-swimming infective stages emerging from snails) were assessed for each genotype at each temperature. The results indicate that, on average, both cercarial output and activity are higher at the elevated acclimation temperature. More importantly, the output and activity of cercariae are strongly influenced by a genotype-by-temperature interaction, such that different genotypes show different responses to increasing temperature. Both the magnitude and direction (increase or decrease) of responses to temperature varied widely among genotypes. Therefore, there is much potential for natural selection to act on this variation, and predicting how the trematode M. novaezealandensis will respond to the climate changes predicted for the next century will prove challenging.

Infection success of Echinoparyphium aconiatum (Trematoda) in its snail host under high temperature: role of host resistance

Background

Extreme weather events such as summer heat waves become more frequent owing to global climate change and are predicted to alter disease dynamics. This is because high temperatures can reduce host immune function. Predicting the impact of climate change on host-parasite interactions is, however, difficult as temperature may also affect parasite infective stages and other host characteristics determining the outcome of interaction.

Methods

Two experiments were conducted to investigate these phenomena in a Lymnaea stagnalis–Echinoparyphium aconiatum (Trematoda) interaction. In the first experiment, the effects of exposure of snails to experimental heat waves [maintenance at 25°C vs. 15°C (control)] with different durations (3 days, 7 days) on the infection success of parasite cercariae was examined. In the second experiment, the infection success was examined under similar conditions, while controlling for the possible temperature effects on cercariae and at least partly also for host physiological changes that take place rapidly compared to alterations in immune function (exposure to cercariae at intermediate 20°C).

Results

In the first experiment, increased infection success at 25°C was found independently of the duration of the heat wave. In the second experiment, increased infection success was found only in snails maintained at 25°C for 7 days, a treatment in which snail immune defence is known to be impaired.

Conclusions

These results suggest that the effects of host resistance in determining overall parasite infection success can be overridden by effects of temperature on parasite transmission stages and/or alterations in other host traits than immune defence.

On the efficacy of anti-parasite behaviour: a case study of tadpole susceptibility to cercariae of Echinostoma trivolvis

Many animals respond behaviourally to the infective stages of parasites, but the efficacy of such responses in reducing risk of parasitism often is not established. It was found that tadpoles of Rana clamitans Latr., 1801 (green frogs) and R. sylvatica LeConte, 1825 (wood frogs) increased their activity when exposed to live infective stages (cercariae) of the trematode Echinostoma trivolvis Rudolphi, 1809. The susceptibility to parasitism for green frog tadpoles subjected to three different treatments was compared. Tadpoles were housed at 20 °C and allowed to respond to cercariae, held at 6–8 °C and showing reduced behavioural responses, or anesthetized and showing no responses. Low levels of parasitism were found for tadpoles that responded behaviourally to cercariae; such responses are expected to occur under normal field conditions in the absence of factors suppressing activity of tadpoles. We also demonstrate that infectivity of E. trivolvis cercariae to non-responding (anesthetized) wood frog tadpoles was higher at warm than at cool temperatures. Thus, lowered parasitism at warm temperatures in the first experiment likely resulted from host behavioural responses and not from low infectivity of cercariae. These results have implications for observing effects of environmental factors on susceptibility to parasitism where susceptibility is thought or known to be mediated by host behaviour.

Morphological observations of Echinochasmus japonicus cercariae and the in vitro maintenance of its life cycle from cercariae to adults

The cercaria morphology of Echinochasmus japonicus was investigated using light and scanning electron microscopy. Cercariae, liberated from naturally infected snails (Parafossarulus manchouricus), had ovoid bodies and diminutive tails. The cercaria tegument was covered with minute spines. Four type II sensory papillae were observed on the dorsal side of the oral sucker, and type I papillae were distributed on the dorsal tegument surfaces. When cercariae were kept in the same bath as the freshwater fish, Pseudorasbora parva, which were free from trematode infections, parasites encysted only in the gills of fishes at day 4 postinfection (PI). The outermost metacercaria wall was fully formed in host tissues at day 7 PI. Adult worms were recovered from the intestines of rats, chicks, and ducks 28 days after experimental exposure to metacercariae. The head crown of the adult was armed with 24 collar spines, which were interrupted dorsal to the oral sucker, and the species was identified as E. japonicus.

Global warming and temperature-mediated increases in cercarial emergence in trematode parasites

Global warming can affect the world's biota and the functioning of ecosystems in many indirect ways. Recent evidence indicates that climate change can alter the geographical distribution of parasitic diseases, with potentially drastic consequences for their hosts. It is also possible that warmer conditions could promote the transmission of parasites and raise their local abundance. Here I have compiled experimental data on the effect of temperature on the emergence of infective stages (cercariae) of trematode parasites from their snail intermediate hosts. Temperature-mediated changes in cercarial output varied widely among trematode species, from small reductions to 200-fold increases in response to a 10 °C rise in temperature, with a geometric mean suggesting an almost 8-fold increase. Overall, the observed temperature-mediated increases in cercarial output are much more substantial than those expected from basic physiological processes, for which 2- to 3-fold increases are normally seen. Some of the most extreme increases in cercarial output may be artefacts of the methods used in the original studies; however, exclusion of these extreme values has little impact on the preceding conclusion. Across both species values and phylogenetically independent contrasts, neither the magnitude of the initial cercarial output nor the shell size of the snail host correlated with the relative increase in cercarial production mediated by rising temperature. In contrast, the latitude from which the snail-trematode association originated correlated negatively with temperature-mediated increases in cercarial production: within the 20 ° to 55 ° latitude range, trematodes from lower latitudes showed more pronounced temperature-driven increases in cercarial output than those from higher latitudes. These results suggest that the small increases in air and water temperature forecast by many climate models will not only influence the geographical distribution of some diseases, but may also promote the proliferation of their infective stages in many ecosystems.

EFFECTS OF COPPER SULFATE TOXICITY ON CERCARIAE AND METACERCARIAE OF ECHINOSTOMA CAPRONI AND ECHINOSTOMA TRIVOLVIS AND ON THE SURVIVAL OF BIOMPHALARIA GLABRATA SNAILS

Copper in the form of copper sulfate (CuSO4) decreases the survival of Biomphalaria glabrata snails, but the effects of this molluscicide on Echinostoma caproni and Echinostoma trivolvis, 2 species of digeneans that use B. glabrata as intermediate hosts, are not known. Studies were done on the effects of various concentrations of CuSO4 in artificial spring water (ASW) on the survival and infectivity of E. caproni and E. trivolvis cercariae. Solutions containing 1.0, 0.1, and 0.01% CuSO4 were 100% lethal within 2 hr of exposure for both species. Time to 50% mortality in 0.001% CuSO4 was 8 hr for E. caproni and 16 hr for E. trivolvis; at 24 hr, the controls showed 50 and 65% mortality, respectively. Treatment of cercariae of both species for 0.5 hr in 0.001% CuSO4 had no effect on the ability of cercariae to form normal cysts in juvenile B. glabrata snails. However, treatment with 0.01% CuSO4 for 0.5 hr caused a significant reduction in the ability of cercariae of both species to encyst in snails. Treatment of encysted metacercariae of both species in 0.001% CuSO4 for I hr had no effect on subsequent excystation of these echinostomes in a trypsin-bile salts medium, whereas concentrations of 1.0, 0.1, and 0.01% CuSO4 and 1.0 and 0.1% CuSO4 decreased chemical excystation of E. caproni and E. trivolvis cysts, respectively. Survival studies on the effects of CuSO4 in Locke's solution on chemically excysted metacercariae of both species were also done. Excysted metacercariae of both species were killed by 2 hr in either 0.1 or 0.01% CuSO4 in Locke's solution. However, time to 50% mortality for both species of excysted metacercariae in 0.001% CuSO4 was approximately 5 hr. Time to 50% mortality for the controls was about 12 hr. Survival of juvenile B. glabrata snails was also examined. All B. glabrata snails were dead by 6 hr in 1 and 0.1% CuSO4 in ASW. Biomphalaria glabrata snails showed 50% mortality by about 6 hr in 0.01% CuSO4 and about 80% were still alive at 24 hr in 0.001% CuSO4. All controls were alive at 24 hr, at which time the experiment was terminated. Concentrations greater than 0.001% CuSO4 increased snail mortality, as well as that of the cercariae and excysted metacercariae of E. caproni and E. trivolvis. Our findings suggest that concentrations of copper sufficient to eliminate juvenile B. glabratta snails are also sufficient to kill the cercariae and excysted metacercariae of these digeneans but not the encysted metacercariae, which may be protected by their cyst walls.

Effects of tonicity, digestive enzymes and bile salts, and nutrient media on the survival of excysted metacercariae of Echinostoma caproni

The effects of tonicity, digestive enzymes and bile salts, and various nutrients added to Locke's solution were studied on the chemically excysted metacercariae of Echinostoma caproni. Metacercariae were maintained at 37.5 degrees C in multiwell chambers, ten per 0.5 ml of test solution; each experiment was replicated five times. Most metacercariae maintained in deionized water or Locke's 2x solution were dead within 2 h. About 85% and 55% of the metacercariae were alive at 8 h in Locke's 1x and Locke's 0.5x, respectively. Metacercariae of this species are osmoconformers, as is the case for adult digeneans. All metacercariae were dead in an acid saline or acid pepsin medium by 2 h. About 50% of the metacercariae were alive in an alkaline trypsin-bile salts medium at 4 h. These results suggest that the acidic pepsin environment in the stomach of a definitive host would be detrimental to the survival of excysted metacercariae, but prolonged survival in alkaline trypsin-bile salts would facilitate establishment of this larval stage in the mucosa of the host small intestine. Studies on excysted metacercariae in Locke's 1x supplemented with various nutrients showed that optimal survival occurred in Locke's plus 0.1% glucose and in Locke's 1x plus 1% hen's egg yolk. Significant survival of excysted metacercariae in Locke's 1x supplemented with either 0.1% proline, 0.1% threonine, or 0.1% serine did not occur.