Cercarial Survival Research Articles

Each coin has 2 sides: a positive role of alien Potamopyrgus antipodarum (Grey, 1843) snails in reducing the infection of native lymnaeids with trematodes.

The change in the distribution of organisms in freshwater ecosystems due to natural or manmade processes raises the question of the impact of alien species on local communities. Although most studies indicate a negative effect, the positive one is more difficult to discern, especially in multispecies systems, including hosts and parasites. The purpose of the study was to check whether the presence of an alien host, Potamopyrgus antipodarum, reduces the intensity of Echinoparyphium aconiatum metacercariae in a native host, Radix spp. We additionally tested the impact of water temperature and the biomass of the alien host on the dilution effect. We experimentally studied (1) the lifespan of echinostome cercariae in different temperatures, (2) the infectivity of cercariae toward the alien host and native host, and (3) the impact of different biomass of the alien host on the intensity of metacercariae in the native host. We found that cercarial survival and infectivity were temperature dependent. However, cercarial survival decreased with increasing temperature, contrary to cercarial infectivity. Echinostome cercariae entered the renal cavity of both the native host and alien host, and successfully transformed into metacercariae. The number of metacercariae in the native host decreased with the increasing biomass of the alien host. Our results indicate that lymnaeids may benefit from the co-occurrence with P. antipodarum, as the presence of additional hosts of different origins may reduce the prevalence of parasites in native communities. However, the scale of the dilution effect depends not only on the increased spectrum of susceptible hosts but also on the other variables of the environment, including water temperature and host density.

Opportunity or catastrophe? effect of sea salt on host-parasite survival and reproduction.

Seawater intrusion associated with decreasing groundwater levels and rising seawater levels may affect freshwater species and their parasites. While brackish water certainly impacts freshwater systems globally, its impact on disease transmission is largely unknown. This study examined the effect of artificial seawater on host-parasite interactions using a freshwater snail host, Biomphalaria alexandrina, and the human trematode parasite Schistosoma mansoni. To evaluate the impact of increasing salinity on disease transmission four variables were analyzed: snail survival, snail reproduction, infection prevalence, and the survival of the parasite infective stage (cercariae). We found a decrease in snail survival, snail egg mass production, and snail infection prevalence as salinity increases. However, cercarial survival peaked at an intermediate salinity value. Our results suggest that seawater intrusion into freshwaters has the potential to decrease schistosome transmission to humans.

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.

Freshening rather than warming drives trematode transmission from periwinkles to mussels

In the Western Baltic Sea, climate change is happening at much faster rate than in most other seas and organisms are additionally exposed to a steep and variable salinity gradient. Climate change has previously been shown to affect parasite transmission in other marine ecosystems, yet little is known about potential effects of warming and desalination on parasite–host interactions. In laboratory experiments, we determined the combined effects of projected seawater warming and freshening on the emergence, activity, survival, and infectivity of cercariae (free-swimming infectious stage) of the trematode Himasthla elongata (Mehlis 1831), shed from its first intermediate host, the periwinkle Littorina littorea (Linnaeus 1758), in the Baltic Sea. We also assessed the susceptibility of the second intermediate host, the mussel Mytilus edulis Linnaeus, 1758, to cercarial infections. Generally, salinity was the main driver, particularly of cercarial activity, infectivity, and mussel susceptibility to infection. At the lowest salinity (13), cercariae were 50% less active compared to the highest salinity (19). Infection success and host susceptibility followed a similar pattern, with 47% and 43% less metacercariae (encysted stage) present at salinity 13 than at salinity 19, respectively. In contrast, effects of simulated warming were found only for cercarial survival, with cercarial longevity being higher at 19 than at 23 °C. No significant interactions between temperature and salinity were found. In contrast to the literature, the results suggest that a climate change-driven freshening (partly also warming) may lead to a general decline of marine trematodes, with possible beneficial effects for the involved hosts.

The Effects of the Commercially Formulated Neonicotinoids Imidacloprid and Thiamethoxam on the Survival of Infectious Stages of Two Trematode Parasites

Neonicotinoids are a popular class of systemic insecticides registered for use in over 120 countries. Recent global water surveys have found neonicotinoid insecticides in waterbodies adjacent to agricultural fields. The presence of neonicotinoids in surface waters raises the concern of the potential risk of exposure to non-target aquatic species, with relevance for both free-living and parasitic taxa. The susceptibility of aquatic free-living parasitic stages to the lethal effects of neonicotinoids may influence the prevalence and intensity of host infection, and their availability as prey for a variety of organisms (e.g., fish, invertebrates). We examined the effects of two commonly used neonicotinoids on the survival of free-living infectious stages (cercariae) from two trematode (flatworm) species (Diplostomum sp. and Haematoloechus sp.). We found that exposure to commercially formulated imidacloprid and thiamethoxam, individually and in a binary mixture, had no significant effect on cercarial survival for either species at environmentally relevant concentrations. Further study is required to understand if the ability of the surviving cercariae to infect the next host in their life cycle is compromised in the presence of neonicotinoids, or if host defenses are compromised, as changes in parasite infectivity or host susceptibility could influence infection dynamics and have community- and ecosystem-level implications.

The effects of temperature and salinity on the longevity of Opisthorchis viverrini cercariae: a climate change concern.

Research on the effects of environmental factors influenced by climate change on parasite transmissibility is an area garnering recent attention worldwide. However, there is still a lack of studies on the life cycle of Opisthorchis viverrini, a carcinogenic trematode found in countries of the Lower Mekong subregion of Lao PDR, Cambodia, Myanmar, Vietnam and Thailand. To evaluate the influences of environmental factors water temperature and salinity on the transmissibility of the liver fluke O. viverrini through cercarial stage, longevity of O. viverrini cercaria was examined at different experimental temperatures (22°C, 30°C and 38°C) and salinities (2.5 parts per thousand (PPT), 3.75 PPT and 5 PPT). The results reveal that different temperatures have statistically significant effects on cercarial longevity. The cercariae exhibited a thermostability zone ranging between 22°C and 30°C. Cercarial longevity was significantly shortened when water temperatures reached 38°C. Salinity also plays a key role in cercarial longevity, with cercarial survival significantly shorter at a salinity of 3.75 PPT than at 2.5 PPT and 5 PPT. A combined analysis of salinity and temperature revealed unique trends in cercarial longevity. At all experimental salinities, cercarial longevity was lowest when incubated in 38°C, but statistically significant from cercarial longevity at temperatures of 22°C and 30°C, and salinities of 2.5 PPT and 5 PPT. The results suggest that higher temperatures negatively impact parasite longevity. This reflects that O. viverrini transmission patterns may be impacted by changes in water temperature and salinity resulting from climate change.

Susceptibility of Nanophyetus salmincola Cercariae to Formalin, Hydrogen Peroxide, and Seawater.

The ability of formalin, PEROX-AID (hydrogen peroxide), and seawater to kill waterborne Nanophyetus salmincola cercariae was evaluated invitro. Newly emerged cercariae survived for extended periods in freshwater, with 53-73% survival occurring in negative control groups after 24h. Exposure to dilutions of formalin reduced this survival time, with 0% of cercariae surviving after30min in 450μL/L, 40min in 225μL/L, and 300min in 113μL/L. Exposure to PEROX-AID (hydrogen peroxide) for 1h resulted in reduced cercarial survival (16.4%) only at the highest concentration (100μL/L), compared with 100% survival in the untreated controls and all lesser concentrations. Exposure to dilutions of seawater resulted in reduced cercarial survival only at high salinities (15.2-30.3‰), where 10-min exposures resulted in 0-20% survival. These results provide insights into options for prophylactic water treatment at salmonid enhancement facilities that experience high mortalities due to infections with Nanophyetus salmincola. Further, the intolerance of live cercariae to high salinities indicates that exposure to fish occurs primarily in the freshwater portions of watersheds.

Cercarial survival in an intertidal trematode: a multifactorial experiment with temperature, salinity and ultraviolet radiation

Parasite transmission takes place in the context of a multitude of simultaneously fluctuating environmental factors. As a particularly vulnerable step in the transmission, trematode cercariae are directly exposed to ambient conditions during their search for a host. Here, we investigated the survival of cercariae of the intertidal trematode Maritrema novaezealandensis in a multifactorial experiment (2 × 2 × 2 design) with temperature (20 and 30°C), salinity (35 and 40practical salinity units (psu)) and ultraviolet radiation (UVR; exposed and not exposed) as main factors. All factors had significant effects, with cercariae dying faster at the higher temperature, increased salinity and when exposed to UVR. Full activity ceased within ~6h in all treatments at 30°C, except at 35psu under no exposure to UV; in all other treatments full activity was maintained for >6h. Several factor interactions were identified, of which the interactive negative effect of temperature and UVR was the most important. These results imply that conditions during the main transmission window of M. novaezealandensis are highly challenging for cercariae. Our findings highlight the importance of considering multiple environmental factors in the study of parasite transmission to gain a more ecologically relevant understanding of transmission dynamics.

Thermodynamics of cercarial survival and metabolism in a changing climate

Cercariae are non-feeding free-living stages in the life cycles of trematodes, highly influenced by temperature. Their life span is brief, limited by the depletion of a non-renewable glycogen store. Warmer temperatures under the influence of climate change may promote the transmission of parasites and therefore understanding their thermobiology forms an important step in discerning the future dynamics of parasite populations. An empirical relationship exists between cercarial mean expected life span and the half-life of the population (t0·5) and therefore t0·5 is a good indicator of glycogen utilization. In this study experimental data on the effects of temperature on cercarial survival is compiled from the scientific literature and evaluated in terms of metabolism using Q10 and Arrhenius activation energy (E* or μ), common measures of temperature-mediated reaction rates. Cercariae have a variable response to temperature, which does not appear to be influenced by their life-history attributes or size. There were little differences in Q10 and E* values between most temperature ranges. In almost half the studies examined (7 of 16) cercariae demonstrated a discrete zone of thermostability over a range equivalent to typical individual mean summer temperatures. Distinct intraspecific differences in temperature responses between 3 laboratory strains of Schistosoma mansoni and 2 natural strains of Echinoparyphium recurvatum sensu stricto were apparent. The importance of these results for cercarial biology under global climate change is discussed.

Parasites and global warming: net effects of temperature on an intertidal host–parasite system

Climate changes, particularly global warming, are likely to impact host-parasite inter- actions. However, our understanding of the effects of environmental factors on marine host-parasite systems is limited. We conducted a series of laboratory experiments on the effects of temperature on all transmission steps of the intertidal trematode Maritrema novaezealandensis from its first interme- diate snail host Zeacumantus subcarinatus to the second intermediate amphipod host Paracalliope novizealandiae. By measuring output of cercarial transmission stages from snails, cercarial survival and infectivity, susceptibility of amphipods to infections, amphipod survival and parasite develop- ment within amphipods, we evaluated overall net temperature effects. At low temperatures (<20°C), transmission was low and amphipod survival unaffected. At intermediate temperatures (20 to 25°C), output and infectivity of cercarial transmission stages was at an optimum, which may increase the risk of infection intensity-dependent mortality of amphipods. Also, temperature directly increased amphipod mortality, but accelerated parasite development within amphipods. At high temperatures (≥30°C), transmission of the parasite was reduced (few cercariae, low infectivity), but temperature- induced mortality of amphipods was most pronounced. Our approach revealed that temperature strongly, but differentially, affects the various steps of the transmission process, pointing to the amphipod as the most vulnerable component. An increased impact of parasites on amphipod popula- tions with global warming is predicted and the possible disruption of the host-parasite system seems realistic under unusual future circumstances such as prolonged heatwaves. We suggest that more holistic studies of host-parasite interactions are essential for a better understanding of potential responses of host-parasite systems to global changes.

UNDERSTANDING THE NET EFFECTS OF PESTICIDES ON AMPHIBIAN TREMATODE INFECTIONS

Anthropogenic factors can have simultaneous positive and negative effects on parasite transmission, and thus it is important to quantify their net effects on disease risk. Net effects will be a product of changes in the survival and traits (e.g., susceptibility, infectivity) of both hosts and parasites. In separate laboratory experiments, we exposed cercariae of the trematode Echinostoma trivolvis, and its first and second intermediate hosts, snails (Planorbella trivolvis) and green frog tadpoles (Rana clamitans), respectively, to one of four common pesticides (atrazine, glyphosate, carbaryl, and malathion) at standardized, ecologically relevant concentrations (201.0, 3700.0, 33.5, and 9.6 microg/L, respectively). We measured effects of pesticide exposure on six mechanisms important to this host-parasite interaction: (1) survival of E. trivolvis cercariae over 26 hours, (2) tadpole survival over two weeks, (3) snail survival over four weeks, (4) snail growth and fecundity, (5) cercarial infectivity, and (6) tadpole susceptibility to a fixed number of cercariae. Pesticides, in general, caused significantly greater mortality of E. trivolvis cercariae than did control treatments, but atrazine was the lone chemical to significantly reduce cercarial survival (LC50 value = 267 mg/L) and then only at concentrations greater than commonly found in aquatic ecosystems (> or =200 microg/L). None of the pesticides significantly enhanced E. trivolvis virulence, decreased tadpole survival, or reduced snail survival, growth, or fecundity. Sublethal exposure of the cercariae to the pesticides (4 h) did not significantly affect trematode encystment in R. clamitans. In contrast, sublethal exposure of R. clamitans to each of the four pesticides increased their susceptibility as measured by the percentage of cercariae that encysted. The reduction in exposure to trematodes due to pesticide-induced cercarial mortality (a density-mediated effect) was smaller than the pesticide-induced increase in amphibian susceptibility (a trait-mediated effect), suggesting that the net effect of exposure to environmentally realistic levels of pesticides will be to elevate amphibian trematode infections. These findings highlight the importance of elucidating the lethal and sublethal effects of anthropogenic factors on both hosts and parasites to understand the mechanisms underlying changes in parasite transmission and virulence, an approach that is especially needed for amphibians, a taxon experiencing global disease-related declines.

Effect of temperature on emergence, survival and infectivity of cercariae of the marine trematode Renicola roscovita (Digenea: Renicolidae)

Marine bivalves harbour a diversity of trematode parasites affecting population and community dynamics of their hosts. Although ecologically and economically important, factors influencing transmission between first (snail) and second (bivalve) intermediate hosts have rarely been studied in marine systems. In laboratory experiments, the effect of temperature (10, 15, 20, 25 degrees C) was investigated on (1) emergence from snails, (2) survival outside hosts and (3) infectivity in second intermediate hosts of cercariae of the trematode Renicola roscovita (Digenea: Renicolidae), a major parasite in North Sea bivalves. Emergence of cercariae peaked at 20 degrees C (2609 +/- 478 cercariae snail(-1) 120 h(-1)) and was considerably lower at 10 degrees C (80 +/- 79), 15 degrees C (747 +/- 384) and 25 degrees C (1141 +/- 334). Survival time decreased with increasing temperature, resulting in 50% mortality of the cercariae after 32.8 +/- 0.6 h (10 degrees C), 26.8 +/- 0.8 h (15 degrees C), 20.2 +/- 0.5 h (20 degrees C) and 16.6 +/- 0.3 h (25 degrees C ). Infectivity of R. roscovita cercariae in cockles Cerastoderma edule increased with increasing temperature and was highest at 25 degrees C (42.6 +/- 3.9%). However, mesocosm experiments with infected snails and cockle hosts in small aquaria, integrating cercarial emergence, survival and infectivity, showed highest infection of cockles at 20 degrees C (415 +/- 115 metacercariae host(-1)), indicating 20 degrees C to be the optimum temperature for transmission of this species. A field experiment showed metacercariae of R. roscovita to appear in C. edule with rising water temperature in April; highest infection rates were in August, when the water temperature reached 20 degrees C. Since another trematode species (Himasthla elongata; Digenea: Echinostomatidae) occurring at the experimental site showed a similar temporal pattern, trematode transmission to second intermediate bivalve hosts may peak during especially warm (> or = 20 degrees C) summers in the variable climate regime of the North Sea.

Toxicity of cadmium and zinc mixtures to the decaudized cercarial life span of Diplostomum spathaceum

The effects of cadmium and zinc mixtures at concentrations ranging from 0.1 to 10,000 microg l(-1) on the life-span of decaudized cercarial bodies (cercariae that have shed their tails) of Diplostomum spathaceum (Trematoda: Diplostomatidae) was investigated. Cercariae were exposed to metal mixtures of equal and unequal concentrations, and a low-dose pre-treatment followed by a high-dose exposure mixtures. Metal mixtures demonstrated variable effects on decaudized cercariae either by increasing or reducing their life-span compared to single metal exposures dependent on concentration and the type of mixed metal treatment. Prolonged exposure to equal metal mixtures at low concentrations (0.1-100 microg l(-1)) resulted in a reduction in the life-span of decaudized cercariae at 0.1 and 100 microg l(-1) in those individuals decaudized during the initial 24 h exposure period compared with those decaudized during the final 24 h period of cercarial survival, whilst in controls there was no significant life-span change between the two time periods. Decaudized cercariae which were exposed to low concentrations (0.1-100 microg l(-1)) of equal metal mixtures were also evaluated for their role as an indicator of larval 'fitness' for migrating through the tissues of their target fish host for those individuals decaudized during the initial 24 h exposure period, and demonstrated only a limited change in their life-span compared to control and single metal exposures. The importance of metal mixtures in parasite establishment in the fish host is discussed.

Changes in survival characteristics of Diplostomum spathaceum cercariae emerged from cadmium-exposed Lymnaea stagnalis

The effect of exposing Lymnaea stagnalis (Gastropoda: Pulmonata), infected with Diplostomum spathaceum (Trematoda: Diplostomatidae), to 100 microg l(-1) cadmium for 7 days on survival characteristics (survival, tail loss, decaudized cercarial life-span) of emerged cercariae was investigated. Exposure of L. stagnalis to cadmium resulted in significantly increased D. spathaceum cercarial survival and an inhibited tail loss compared to controls. The normal parallel relationship which exists over time between decreasing cercarial survival and increasing tail loss in controls was changed in cercariae from cadmium-exposed hosts with an increased proportion of cercarial deaths occurring without tail loss. The decaudized cercarial life-span over the survival period of the cercarial population did not significantly change. However comparisons between individuals decaudized during the initial 24 h time period with those which were decaudized during the final period of cercarial survival showed a significantly altered life span which did not occur in the control population. As a potential indicator of penetration 'fitness' comparisons were also undertaken between control and exposed cercariae decaudized during the initial 24 h time period, which revealed that the decaudized cercarial life-span from the exposed hosts was significantly different from controls. This may have important implications for the ability of cercariae to migrate through the tissues of their target host. The importance and relevance of these results to parasite transmission are discussed.

Toxicity of cadmium and zinc mixtures to cercarial tail loss in Diplostomum spathaceum (Trematoda:Diplostomidae)

The effects of cadmium and zinc mixtures on tail loss of the free-living cercarial stage of the parasitic fluke Diplostomum spathaceum were investigated at concentrations ranging from 0.1 to 10,000 μg/L. Cercariae were exposed to metal mixtures of equal concentrations, metal mixtures of unequal concentrations, and a low-dose pretreatment followed by high-dose exposure mixtures. Under control conditions as the cercarial population aged tail loss occurred naturally a few hours before death, with an increasing tail loss over time associated with a decrease in cercarial survival. Under all mixture exposures the period of tail loss was prolonged compared to single-metal exposures. In most test solutions this was associated with a similar increase in survival of cercariae. Inhibition of tail loss occurred in low concentrations (0.1–100 μg/L) of equal mixed exposures. In some unequal, and in all pretreatment mixture exposures, a significant stimulation of tail loss occurred compared to the pattern of decreasing survival over time within the cercarial population. The importance of toxic metal mixtures to cercarial tail loss and potential implications for successful parasite transmission are discussed.

Toxicity of cadmium and zinc to the decaudised cercarial life-span of Diplostomum spathaceum (Trematoda: Diplostomidae).

The toxicity of cadmium and zinc at concentrations ranging from 0.1 to 10000 microg/l to the life-span of decaudised cercarial bodies (cercariae that have shed their tails) of Diplostomum spathaceum was investigated. The effects of metal exposure at 3 temperatures (12, 20, and 25 degrees C) and 3 levels of water hardness (distilled water, soft water and hard water) were studied. In general, under most experimental conditions increasing metal concentrations reduced the life-span of decaudised cercariae. Increasing water hardness and decreasing water temperature caused an increase in the life-span of both control and metal exposed decaudised cercariae. However, at certain isolated metal concentrations, associated with a specific level of water hardness and temperature, increased survival above controls occurred. Differences in the relative toxicity of cadmium and zinc were dependent on the environmental conditions of exposure. The decaudised cercarial life-span under metal exposure was found to be generally independent of the overall cercarial survival and tail loss in most experimental conditions. Prolonged exposure to cadmium and zinc caused changes in the decaudised cercarial life-span when compared to individuals decaudised during the initial 24 h exposure period to those which were decaudised during the final 24 h period of cercarial survival. The validity of studying the decaudised cercarial life-span as an indicator of 'fitness' of larvae to migrate through the tissues of the target fish host, in terms of glycogen utilization, was assessed for those cercariae decaudised during the initial 24 h exposure period only. A limited reduction in the decaudised cercarial life-span during this period compared to controls was recorded, which may possibly indicate a reduced penetration 'fitness' of cercariae exposed to cadmium and zinc. The importance and relevance of these findings to parasite migration and establishment in the fish host are discussed.

Toxicity of cadmium and zinc to cercarial tail loss in Diplostomum spathaceum (Trematoda: Diplostomidae).

The effect of cadmium and zinc at concentrations ranging from 0.1 to 10,000 microg/l on tail loss in cercariae of Diplostomum spathaceum was investigated at 3 temperatures (12, 20 and 25 degrees C) and 3 levels of water hardness (distilled water, soft water and hard water). Increasing tail loss over time was found to be linked with a parallel decrease in cercarial survival in controls. Exposure to the heavy metals induced, especially at high concentrations, a change in the relationship between cercarial tail loss and survival, causing either stimulation or inhibition of tail loss dependent on the individual toxic exposure. Under most environmental conditions the rate of tail loss over time was reduced by increasing metal concentrations. Inhibition of tail loss occurred in a limited number of both control and metal-exposed cercariae, with a number of low metal concentrations inducing greater inhibition than in controls. Stimulation of tail loss causing an increased tail loss rate above controls also occurred at certain high metal concentrations. Increasing water hardness and decreasing water temperature caused a reduced tail loss rate over time in both control and metal-exposed cercariae. However, with decreasing temperature a reduced rate of tail loss over time in metal-exposed cercariae compared to controls occurred at some low metal concentrations. When tail loss was compared against cercarial death of the experimental population toxic exposure induced changes in the parallel relationship of these parameters, dependent on individual metal concentration, water temperature and hardness. Differences in the relative effects of cadmium and zinc on cercariae were dependent on the environmental conditions of exposure. Both metals showed limited effects during the period of maximum cercarial infectivity (0-5 h). The mechanisms and importance of metal toxicity to cercarial tail loss are discussed.

Toxicity of cadmium and zinc mixtures to Diplostomum spathaceum (Trematoda: Diplostomidae) cercarial survival.

The toxicity of cadmium and zinc mixtures at concentrations ranging from 0.1 to 10,000 microg/L was investigated against the survival of the free-living cercarial stage of the parasitic fluke Diplostomum spathaceum. Cercariae were exposed to metal mixtures of equal concentration, metal mixtures of unequal concentration, and low-dose pretreatment followed by high-dose exposure mixtures. Under all exposures cercarial survival was increased compared to that achieved with single metals. At exposures with metal mixture of equal concentration in the range 0.1-100 microg/L survival was increased compared to that achieved by controls. The mechanism of metal toxicity and their effects on cercarial survival are discussed.

Toxicity of cadmium and zinc to Diplostomumspathaceum (Trematoda: Diplostomidae) cercarial survival

The toxicity of cadmium and zinc at concentrations ranging from 0.1 to 10000 μg/l was investigated with cercariae of Diplostomum spathaceum at three temperatures (12, 20, 25°C) and three levels of water hardness (distilled water, soft water, hard water). Under most environmental conditions survival of cercariae was reduced by increasing metal concentration. Increasing water hardness and decreasing water temperature caused an increase in the survival of both control and metal exposed cercariae. However, with decreasing temperature increased survival of metal-exposed cercariae above the control occurred at a number of low metal concentrations. Differences in the relative toxicity of cadmium and zinc to cercariae was dependent on the environmental conditions of exposure. The two heavy metals demonstrated little toxicity to cercariae during the period of maximum cercarial infectivity (0–5 h).The mechanisms of metal toxicity and their effects on cercarial survival are discussed.

Survival and infectivity of Hypoderaeum conoideum and Euparyphium albuferensis cercariae under laboratory conditions

The survival characteristics of the cercariae of Hypoderaeumconoideum and Euparyphiumalbuferensis (Trematoda: Echinostomatidae) at 20°C and 30°C are described, and the age dependency of their infectivity at 20°C is studied to determine their respective transmission efficiencies. Cercarial survival was found to be age-dependent and was higher at 20°C. For both cercariae, the maximum life-span was 26 h at 20°C and 16 h at 30°C, and their respective times to 50% mortality were similar at each temperature. Both cercariae seem to be well adapted to transmission in their natural habitat, though cercarial infectivity of H. conoideum was higher than that of E. albuferensis, this being correlated with their prevalences in nature. The age-dependency of cercarial survival may be related to steadily diminishing endogenous energy levels, though the delay in attaining maximum infectivity suggests that other factors not related to energy considerations are involved in the delimitation of the cercarial infective period. However, this latter observation may constitute an adaptative mechanism allowing cercarial dissemination.