Number Of Cercariae Research Articles

Risk of Human Exposure to the Intestinal Schistosome, Schistosoma mansoni, across Seasons along the Senegal River

Background: Schistosomiasis is an emerging disease associated with changes to the environment that have increased human contact rates with disease-causing parasites, flatworms that are released from freshwater snails. For example, schistosomiasis remains a major public health problem in Northern Senegal, where prevalence in schoolchildren often reaches 90%. Aim: This study focuses on the impact of seasonality on the risk of human exposure (RHE) to Schistosoma mansoni, defined as the total number of cercariae (the free-living life stage that infects humans) shed from all Biomphalaria pfeifferi snails collected at a site using standardized methods. We focus on RHE because it is rarely quantified and a recent study demonstrated that snails stop shedding cercariae when snail densities increase and thus per capita snail resources become limited [2], suggesting that densities of snails might not be directly proportional to RHE to schistosomes. Method: We sampled four water access points in three villages every other week during the early (Dry1) and later dry seasons (Dry2) and the rainy season, quantifying the abundance of infected and non-infected snail intermediate hosts, cercariae released per infected snail, and water chemistry. We used simple and multiple linear regressions to assess how seasonality and environmental parameters affect non-infected and infected snail abundance and RHE. Results: Although RHE was found across all seasons, the abundance of infected and non-infected snail intermediate hosts and cercariae, as well as prevalence (23.71%), were all highest in the rainy season. In the rainy season, RHE was positively associated with the density of snail hosts and their periphyton food resource. Conclusion: Although previous studies have examined the influence of seasonality on snail densities, few studies have explored the effects of seasonality on cercarial densities, which is the primary source of infection to humans. Our study demonstrates that cercarial densities are greater in the rainy season than in the early or late dry seasons. Given that cercarial densities directly pose risk of infection to humans, unlike non-infected or infected snails, these finding should help to inform decision making and schistosomiasis control efforts in West Africa.

The genetic variation of different developmental stages of Schistosoma japonicum: do the distribution in snails and pairing preference benefit the transmission?

BackgroundSchistosoma japonicum is a waterborne parasite that causes schistosomiasis in humans and in more than 40 animal species. Schistosoma japonicum shows distinct genetic differentiation among geographical populations and multiple hosts, but the genetic diversity of different developmental stages of S. japonicum from is less studied. Such studies could elucidate ecological mechanisms in disease transmission by analysing feedbacks in individual physiology and population state.MethodsAfter infection using cercariae from a pool of snails shedding together (Method I) and infection using mixed equal numbers of cercariae from individually shed snails (Method II), different developmental stages of S. japonicum were genotyped with microsatellite loci, including 346 cercariae, 701 adult worms and 393 miracidia. Genetic diversity and molecular variation were calculated at different population levels. Kinships (I′) among cercariae at intra-snail and inter-snail levels were evaluated. Genetic distance (Dsw) was compared between paired and unpaired worms, and partner changing was investigated through paternity identification for miracidia.ResultsThe cercaria clones in individual snails varied from 1 to 8 and the kinship of cercariae within individual snails was significant higher (P < 0.001) than that among different snails after deleting near-identical multi-locus genotypes (niMLGs). The allelic diversity of worms in Method I was lower (P < 0.001) than that in Method II, and allele frequency among mice in Method I was also less consistent. The parents of some miracidia were worms that were not paired when collected. The Dsw between each female of paired and unpaired males was much larger (P < 0.001) than that between the female and male in each pair.ConclusionsMost of the infected snails contained multiple miracidia clones. The aggregation of genetically similar S. japonicum miracidia in individual snails and the unbalanced distribution of miracidia among snails suggests a non-uniform genetic distribution of cercariae among snails in the field. This further influenced the genetic structure of adult worms from infections with different cercariae sampling methods. Schistosoma japonicum in mice can change paired partner, preferring to mate with genetically similar worms. These characteristics provide implications for understanding the balance in genetic diversity of S. japonicum related to the transmission of schistosomiasis.

Innate antipredator behavior can promote infection in fish even in the absence of predators

AbstractNatural enemies—predators and parasites—largely shape the dynamics of ecosystems. It is known that antipredator and antiparasite defense can be mutually conflicting, however consequences of this trade-off for the regulation of infection burden in animals are still poorly understood. We hypothesize that even in the absence of cues from predators, innate antipredator behavior (“ghost of predation past”) interferes with defense against parasites and can enhance the infection risk. As a case study, we explore interactions between a commercial species, the rainbow trout Oncorhynchus mykiss, and its parasite, the trematode eye-fluke Diplostomum pseudospathaceum. Fish–parasite interactions were tested in compartmentalized tanks where shelters and parasites were presented in different combinations providing various conditions for microhabitat choice and territorial behavior. Shelters were attractive and contestable despite the absence of predators and presence of parasites. The individuals fighting for shelters acquired more than twice the number of cercariae as compared to those in infected shelter-free compartments. Most infected were subordinate fish with a higher ventilation rate. Fish possessing shelters were less vulnerable to parasites than fighting fish. Grouping reduced the infection load, although less efficiently than sheltering. Our data demonstrate that the innate antipredator behavior can undermine antiparasite tactics of the fish and result in higher infection rates. Using our empirical results, we construct a mathematical model which predicts that enriching the environment in fish farming will be beneficial only when a large number of shelters is provided. Using insufficient number of shelters will increase the parasite burden in the fish.

Striking differences in virulence, transmission and sporocyst growth dynamics between two schistosome populations

BackgroundParasite traits associated with transmission success, such as the number of infective stages released from the host, are expected to be optimized by natural selection. However, in the trematode parasite Schistosoma mansoni, a key transmission trait, i.e. the number of cercariae larvae shed from infected Biomphalaria spp. snails, varies significantly within and between different parasite populations and selection experiments demonstrate that this variation has a strong genetic basis. In this study, we compared the transmission strategies of two laboratory schistosome population and their consequences for their snail host.MethodsWe infected inbred Biomphalaria glabrata snails using two S. mansoni parasite populations (SmBRE and SmLE), both isolated from Brazil and maintained in the laboratory for decades. We compared life history traits of these two parasite populations by quantifying sporocyst growth within infected snails (assayed using qPCR), output of cercaria larvae and impact on snail host physiological response (i.e. hemoglobin rate, laccase-like activity) and survival.ResultsWe identified striking differences in virulence and transmission between the two studied parasite populations. SmBRE (low shedder (LS) parasite population) sheds very low numbers of cercariae and causes minimal impact on the snail physiological response (i.e. laccase-like activity, hemoglobin rate and snail survival). In contrast, SmLE (high shedder (HS) parasite population) sheds 8-fold more cercariae (mean ± SE cercariae per shedding: 284 ± 19 vs 2352 ± 113), causes high snail mortality and has strong impact on snail physiology. We found that HS sporocysts grow more rapidly inside the snail host, comprising up to 60% of cells within infected snails, compared to LS sporocysts, which comprised up to 31%. Cercarial production is strongly correlated to the number of S. mansoni sporocyst cells present within the snail host tissue, although the proportion of sporocyst cells alone does not explain the low cercarial shedding of SmBRE.ConclusionsWe demonstrated the existence of alternative transmission strategies in the S. mansoni parasite consistent with trade-offs between parasite transmission and host survival: a “boom-bust” strategy characterized by high virulence, high transmission and short duration infections and a “slow and steady” strategy with low virulence, low transmission but long duration of snail host infections.

First intermediate hosts of Paragonimus spp. in Vietnam and identification of intramolluscan stages of different Paragonimus species

BackgroundMembers of the genus Paragonimus require at least three hosts in their life-cycles. The obligatory first intermediate hosts are freshwater snails. In Vietnam, although seven Paragonimus species have been recorded, the natural first intermediate hosts of almost all species have not been confirmed. The aim of this study was, therefore, to investigate snail hosts of Paragonimus species in Vietnam, and to identify Paragonimus species at intramolluscan stages.MethodsFreshwater snails were collected from streams in Yen Bai and Quang Tri Provinces, where high prevalences of Paragonimus metacercariae in crab hosts have been reported. Snails were morphologically identified and then examined individually for Paragonimus cercariae using shedding and crushing methods. Chaetomicrocercous cercariae, the morphological class to which Paragonimus cercariae belong, were collected for morphological description and molecular species identification by analyses of ITS2 sequences. The infected snail species were identified based on analyses of nucleotide sequences of the cox1 gene.ResultsThree snail species were found to be infected with Paragonimus cercariae at low infection rates, ranging between 0.07–1.0%. The molecular analyses identified them as Sulcospira quangtriensis and 2 species of subfamily Triculinae. In a phylogenetic tree, these two triculine snails were related to the genera Gammatricula and Tricula with low posterior probabilities. Thus we named them as Triculinae sp. 1 and Triculinae sp. 2. Cercariae from the three snail species, Sulcospira quangtriensis, Triculinae sp. 1 and Triculinae sp. 2, were molecularly identified as Paragonimus westermani, P. heterotremus and P. proliferus, respectively. The cercariae of the three species are morphologically similar to each other, but their daughter rediae can be distinguished by the length of the intestine and the number of cercariae per redia. The rediae of P. westermani have a long intestine and each contain 6–8 cercariae. In contrast, those of P. heterotremus and P. proliferus have a short intestine and each redia contain 10–12 and 5–6 cercariae, respectively.ConclusionsThree snail species, Sulcospira quangtriensis, Triculinae sp. 1 and Triculinae sp. 2, serve as the first intermediate hosts of P. westermani, P. heterotremus and P. proliferus, respectively, in Vietnam. The length of the intestine of rediae and the number of cercariae per redia are valuable characteristics for distinguishing between larvae of these Paragonimus species.

Assessment of the experimental infection by Echinostoma paraensei (Lie &amp; Basch, 1967) (Trematoda: Echinostomatidae) in two Biomphalaria tenagophila (D’Orbigny, 1835) (Gastropoda: Planorbidae) isolates resistant and susceptible to Schistosoma mansoni (Sambon, 1907) (Trematoda: Schistosomatidae)

http://dx.doi.org/10.5007/2175-7925.2017v30n2p7Diferentes linhagens de Biomphalaria tenagophila mostram amplo espectro de compatibilidade para Schistosoma mansoni, que vai desde refratária a altamente suscetível. O objetivo deste estudo foi verificar o padrão de compatibilidade de duas linhagens geográficas de B. tenagophila, resistentes e suscetíveis ao S. mansoni, para a infecção com Echinostoma paraensei. Foram utilizadas diferentes cargas miracidianas onde se avaliou a mortalidade, características histopatológicas e emissão de cercárias. Observou-se correlação entre o número de miracídios e a taxa de infecção em B. tenagophila (TAIM). Não houve correlação entre a carga de miracídios utilizados e a emissão cercariana em ambas as linhagens de B. tenagophila. Biomphalaria tenagophila (SJC) mostrou pouca susceptibilidade à infecção por E. paraensei. Os resultados demonstraram diferentes graus de compatibilidade das duas linhagens de B. tenagophila ao E. paraensei, podendo contribuir para os estudos de relação parasito-hospedeiro.

Atrazine reduces the transmission of an amphibian trematode by altering snail and ostracod host-parasite interactions.

Trematodes are ubiquitous members of aquatic environments, have many functional roles in ecosystems, and can cause diseases in humans, livestock, and wild animals. Despite their importance and reports of parasite population declines, few studies have concurrently assessed the effects of aquatic contaminants on multiple hosts, multiple parasite life cycle stages, and on transmission-related host-parasite interactions. Here, we test the effects of environmentally relevant concentrations of the herbicide atrazine (0, 3, 30 μg/L) on the establishment and development of an amphibian trematode (Halipegus eccentricus) in a first-intermediate snail host (Physa acuta) and in a second-intermediate ostracod host (Cypridopsis sp.). Additionally, we test the interactive effects of atrazine and parasitism on snail and ostracod survival. Our results indicate that atrazine negatively affects trematode transmission by altering snail and ostracod host-parasite interactions. Although atrazine did not affect the survival of uninfected snails alone, atrazine acted synergistically with parasitism to reduce the longevity of infected snails. As a result, the number of cercariae (i.e., larval trematodes) produced by snails was 50.7 % (3 μg/L) and 14.9 % (30 μg/L) relative to controls. Atrazine exhibited direct negative effects on ostracod survival at 30 μg/L. However, when ostracods were also exposed to trematodes, the negative effects of atrazine on survival were diminished. Although infected ostracod survival remained high, trematode development was significantly reduced, resulting in reduced infectivity of metacercariae (i.e., nongravid adult cysts infective to definite host) to 32.2 % (3 μg/L) and 28.6 % (30 μg/L) relative to the controls. The combination of reduced cercaria production and reduced metacercarial infectivity in the 3 and 30 μg/L atrazine treatment groups reduced the net number of infective worms produced to 16.4 and 4.3 % (respectively) relative to the control. These results demonstrate the complex nature of pesticide effects on trematode infections and indicate that trematodes can affect their first- and second-intermediate hosts differently under different pesticide concentrations. Our work has broad implications for parasite transmission and conservation and provides a testable mechanism for understanding trematode population declines in contaminated wetlands.

The effect of increasing water temperatures on Schistosoma mansoni transmission and Biomphalaria pfeifferi population dynamics: an agent-based modelling study.

IntroductionThere is increasing interest in the control and elimination of schistosomiasis. Little is known, however, about the likely effects of increasing water-body temperatures on transmission.MethodsWe have developed an agent-based model of the temperature-sensitive stages of the Schistosoma and intermediate host snail life-cycles, parameterised using data from S. mansoni and Biomphalaria pfeifferi laboratory and field-based observations. Infection risk is calculated as the number of cercariae in the model, adjusted for their probability of causing infection.ResultsThe number of snails in the model is approximately constant between 15–31°C. Outside this range, snail numbers drop sharply, and the snail population cannot survive outside the range 14–32°C. Mean snail generation time decreases with increasing temperature from 176 days at 14°C to 46 days at 26°C. Human infection risk is highest between 16–18°C and 1 pm and 6–10 pm in calm water, and 20–25°C and 12–4 pm in flowing water. Infection risk increases sharply when temperatures increase above the minimum necessary for sustained transmission.ConclusionsThe model suggests that, in areas where S. mansoni is already endemic, warming of the water at transmission sites will have differential effects on both snails and parasites depending on abiotic properties of the water-body. Snail generation times will decrease in most areas, meaning that snail populations will recover faster from natural population reductions and from snail-control efforts. We suggest a link between the ecological properties of transmission sites and infection risk which could significantly affect the outcomes of interventions designed to alter water contact behaviour – proposing that such interventions are more likely to reduce infection levels at river locations than lakes, where infection risk remains high for longer. In cooler areas where snails are currently found, increasing temperatures may significantly increase infection risk, potentially leading to new, high-intensity foci of infection.

Fasciola hepatica: effect of the natural light level on cercarial emergence from temperature-challengedGalba truncatula

As abrupt changes in water temperature (thermal shock) triggered a significantly greater cercarial emergence of Fasciola hepatica from experimentally infected Galba truncatula, laboratory investigations were carried out to study the influence of light on cercarial emergence in snails subjected to a thermal shock every week (a mean of 12 °C for 3 h) during the patent period. Thermal shock for these temperature-challenged (TC) snails was carried out outdoors under artificial or natural light, or indoors under constant artificial light. Compared with the infected control snails always reared indoors at 20 °C, the number of cercariae in TC snails subjected to a thermal shock and natural light outdoors was significantly greater. The repetition of this experiment by subjecting TC snails to the same thermal shock indoors under an artificial light level ranging from 600 to 3000 lux did not show any significant difference among the numbers of cercariae in the different subgroups. A detailed analysis of the results noted in the TC snails subjected to natural light during the thermal shock demonstrated that the number of cercariae-releasing snails was significantly higher between 601 and 1200 lux and for the highest nebulosity values (7–8 octas, which corresponds to a sufficiently or completely overcast sky). Contrary to the intensity of artificial light, which did not influence cercarial emergence, the natural light level had a significant effect on this process when F. hepatica-infected snails were subjected to a regular thermal shock during the patent period.

Cercarial emergence pattern ofSchistosoma haematobiumfrom Libreville, Gabon

Although schistosomiasis has been a public health issue in Gabon for nearly a century, little is known about its current transmission dynamics. We analyzed the chronobiology of 137 cercarial emission profiles of Schistosoma haematobium from Libreville, the capital of Gabon, located in an open area for schistosomiasis. We found that 88% of the cercariae were shed between 11 a.m. and 3 p.m. and that the average pattern was of circadian type, with the average peak at 1 p.m., and representing 27% of the total number of cercariae of the day. The rhythms of emergence may be associated with environmental pressures on the parasite, especially those related to their definitive hosts.

Concomitant Immunity to Schistosoma mansoni in Mice

The aim of the present study was to confirm observations on the concomitant immunity to Schistosoma mansoni in mice and assess its effects on the resistance of mice to a challenge infection. Cercariae from infected Biomphalaria glabrata were used to infect mice. Twenty mice were infected with a single dose of S. mansoni cercariae. The animals were randomly divided into two groups: experimental group (Group A) and control group (Group B). Group A mice were challenged with the same number of cercariae six weeks after the primary infection. Perfusion of all mice was done 9 weeks after infection in order to obtain worm burdens in relation to their initial cercarial dose. The livers of all mice were obtained for parasitological and pathological assessments. Our results showed that all the exposed animals became infected with S. mansoni. After a challenge infection, Group A mice had a 54.66% worm reduction rate, 41.45% liver egg reduction rate, and 51.76% granuloma size reduction rate compared to their respective controls. This study shows that mice with persistent adult S. mansoni infection are able to mount a very strong regulatory response to a challenge infection. It is concluded that concomitant immunity does occur in mice. These results describe novel imaging methods that permit visualization of live schistosomes within their living hosts and may help to elucidate mechanisms of infection and also be of value not only for epidemiological investigations, but also in designing government control programs for schistosomiasis.

The investigation of protective concomitant immunity in murine schistosomiasis mansoni

The present study was conducted mainly to establish a new concomitant immunity model to Schistosoma mansoni in mice and assess its effects on the resistance of mice to a challenge infection with a possibility of using as a diagnostic marker. Three groups (A, B and C) of BALB/c mice were infected with a single dose of S. mansoni cercariae obtained from infected Biomphalaria glabrata snail. The group A mice were used as infected control group. The group B mice were intraperitoneally injected with allylthiourea (ATU) 22 days post-primary infection then they were challenged 3 weeks post-ATU treatment. The group C mice were challenged with the same number of cercariae 6 weeks post-primary infection. Perfusion of all mice was done 9 weeks after infection in order to obtain worm burdens. The livers of all mice were obtained for parasitological and pathological assessments. Our results showed that the group B mice had a 29.11% worm reduction rate, 25.37% liver egg reduction rate, and 37.48% granuloma size reduction rate compared to their respective controls. While the group C mice showed superior results and had a 54.66% worm reduction rate, 41.45% liver egg reduction rate, and 51.76% granuloma size reduction rate. It was concluded that these results described novel imaging methods that permit visualization of live schistosomes within their living hosts and may have important implications not only for epidemiological and diagnostic investigations, but also in designing control programs for schistosomiasis including anti-schistosome vaccine.

Haplometra cylindracea(Zeder, 1800) (Trematoda: Plagiorchiidae): variation in the dates of cercarial shedding for overwinteringGalba truncatula

Natural infections of Galba truncatula with Haplometra cylindracea were followed from 2001 to 2009 to determine if their characteristics were similar when snails came from water collections frequented by Bufo bufo or by frogs and newts for their egg-laying. Snail samples were collected from both types of sites to count shed cercariae for three days and also free cercariae when snails were dissected. In sites only frequented by B. bufo, cercarial shedding occurred earlier than in those colonized by frogs and newts (March instead of April-May). In contrast, the number of cercariae shed during three successive days was significantly higher in May. This variation in the dates of cercarial shedding might be due, either to a synchronism between cercaria-releasing snails and the presence of the definitive host (tadpoles) in water collections, or to an earlier infection of overwintering snails in autumn by H. cylindracea miracidia in the case of toad-frequented sites.

Biological and biochemical responses of Biomphalaria alexandrina to some extracts of the plants Solanum siniacum and Artemisia judaica L.

The molluscicidal activity of cold water, boiled water, methanol, ethanol, acetone and chloroform extracts of Solanum siniacum and Artemisia judaica L. plants against Biomphalaria alexandrina snails was carried out. The tests revealed plant’s ethanol extract was more toxic to the snails than the other tested extracts. Therefore, it was tested against snails’ fecundity ( M x ), reproduction rate ( R 0) and their infection with Schistosoma mansoni miracidia. In addition, biochemical parameters and the activities of some enzymes in tissues of snails treated with the two tested plants were determined. As well, glucose concentration in snails’ hemolymph was evaluated. Exposure of B. alexandrina snails to plant’s ethanol extract led to a significant reduction in their survival and snails’ fecundity, reproduction rate. In addition, it caused a considerable reduction in the infectivity of S. mansoni miracidia to the snails. Also, it caused a reduction in number of cercariae per snail during the patent period and in the period of cercarial shedding. The results revealed that the glucose concentration in hemolymph and Lactate level in soft tissues of treated snails were increased ( P < 0.001) while glycogen, total protein, the lipid content and the pyruvate level in snail’s tissues decreased ( P < 0.001). The activities of lactic dehydrogenase (LDH), pyruvatekinase (PK) and cytochrome oxidase (CY) enzymes in homogenate of snail’s tissues were reduced ( P < 0.001) in response to treatment with the two tested plants while protease (PR) activity increased ( P < 0.001). It is concluded that the application of LC 25 of ethanol extract of S. siniacum and A. judaica L. may be helpful in snail control as it interferes with the snails’ biology and physiology.

Impact profenophos (pesticide) on infectivity of &amp;lt;i&amp;gt;Biomphalaria alexandrina&amp;lt;/i&amp;gt; snails with &amp;lt;i&amp;gt;schistosoma mansoni&amp;lt;/i&amp;gt; miracidiaand on their physiological parameters

Profenophos is an organophosphorus pesticide which are used effectively against cotton insects and mites.The present work was carried out to evaluate the molluscicidal activity of pesticides (Profenophos) against Biomphalaria alexandrina snails. Also, the effect of sublethal concentrations of pesticide on the infection of B. alexandrina with Schistosoma mansoni and some enzymes of energy metabolism were studied. The results showed that the infection of B. alexandrina with schistosoma mansoni was greatly reduced after exposure to LC0, LC10, LC25 of pesticide and also, reduction in number of cercariae per snail during the patent period and in the period of cercarial shedding. The present study indicated that the increase in levels of aminotransaminases, acid phosphatase and alkaline phosphatases enzymes in haemolymph and soft tissue of snails and the activity level of lactate dehydrogenase, hexokinase and pyruvatekinase was also significantly reduced in response to treatment.

The Interaction of Light and Gravity on the Transmission of Echinostoma caproni (Digenea: Echinostomatidae) Cercariae to the Second Intermediate Host, Biomphalaria glabrata (Gastropoda: Pulmonata)

The current experiments were designed to assess the interaction of light and gravity on the transmission of Echinostoma caproni cercariae to the second intermediate host, Biomphalaria glabrata. Transmission chambers were constructed of clear polyvinyl chloride pipe covered with a black sleeve to exclude light. Snails were constrained within the chamber to prevent movement, while permitting the cercariae to swim freely. A trial consisted of 2 infected B. glabrata shedding E. caproni cercariae placed at the center of the chamber with 5 uninfected B. glabrata placed 10 cm above and below the shedding snails as sentinels. Three experiments, consisting of 12 trials each, were conducted under the following lighting conditions, i.e., above and below the transmission chamber, and in complete darkness. In all 3 experiments, the proportion of metacercariae was significantly higher in snails at the top of the chamber. The results suggest that a negative geotaxis is the primary factor in the initial dispersal of E. caproni cercariae. Coupling negative geotaxis and positive phototaxis (light from above) resulted in a significantly higher proportion of metacercariae in sentinel snails at the top of the transmission chamber when corrected for cercarial density. There was no significant difference in the proportion of metacercariae in snails at the top or bottom of the transmission chamber with light at the bottom of the chamber or in complete darkness. Cercariae of E. caproni only respond to light in context, i.e., from above, and ignore the light stimulus when it comes from an unexpected location (bottom of the water column). Significantly greater numbers of cercariae were released from shedding snails when light was present, suggesting that emergence of cercariae from B. glabrata is dependent on light regardless of the position of the light source.

The redial and cercarial production of a Digenean in the snail host is lower when no cercarial shedding occurs

Single- and double-miracidium exposures of Galba truncatula with Fasciola hepatica (two groups) or with Paramphistomum daubneyi (two groups) were carried out under laboratory conditions to compare parasite production in cercaria-shedding snails (CS snails) with that found in snails without emission (NCS snails). Free rediae and cercariae were thus counted in snails from both categories after their dissection at regular intervals (at 24 degrees C). In the four groups, the numbers of free rediae and free cercariae found at day 75 post-exposure (F. hepatica) or at day 85 (P. daubneyi) were significantly greater in CS snails than in NCS ones. The number of cercariae in NCS subgroups did not show any significant variation from day 45 p.e. to day 75 (F. hepatica, the two groups) or from day 55 to day 85 (P. daubneyi, single-miracidium infections), while it significantly decreased with increasing time of infection in the double-miracidium infections with P. daubneyi. In NCS snails, the presence of too numerous free cercariae within the snail's body (the volume of the body allows development only of a given number of rediae) might rapidly block out redial development and intraredial differentiation of other cercariae. The numerical diminution of P. daubneyi cercariae in the NCS snails (double-miracidium group) might probably be due to the lysis of new cercariae just formed, probably in reason of a lack of nutrients available for these larvae within the snail.

Infection of Mytilus edulis by the trematode Echinostephilla patellae (Digenea: Philophthalmidae)

The blue mussel Mytilus edulis is described as second intermediate host for Echinostephilla patellae from the common limpet Patella vulgata. Mussels were infected with metacercariae of E. patellae under laboratory conditions. Average infection rates increased with increasing temperature, whereas numbers of cercariae, to which individual mussels were exposed, had no effect on relative infection success. The round to slightly oval metacercariae with an average cyst diameter of 208 microm (range 186-243 microm) encysted exclusively in the foot tissue of M. edulis. Morphologically similar metacercariae were found in naturally infected mussels at sites where parasitized P. vulgata and M. edulis are sympatric. This is the first report of E. patellae in blue mussels. The detection of M. edulis being a second intermediate host is of particular interest with regard to the abundance of the parasite and host organisms in intertidal rocky shore ecosystems. The potential role of the common limpet P. vulgata as an alternative secondary host is discussed.

Use of some plant extracts to control Biomphalaria alexandrina snails with emphasis on some biological effects

The present work was carried out to evaluate, the molluscicidal activity of cold water, boiled water, methanol, ethanol, acetone and chloroform extracts of some plant species against Biomphalaria alexandrina snails. Preliminary screening tests on 10 plant species showed that the highest molluscicidal potency was recorded for Euphorbia splendens, Atriplex stylosa and Guayacum officinalis. Exposure of B. alexandrina snails to plant’s methanol extracts led to a significant reduction in their survival and growth rates. In addition, newly hatched snails were susceptible to plant’s methanol extracts than elder ones. LC 25 of methanol extract from these plants caused a considerable reduction in the infectivity of Schistosoma mansoni miracidia to the snails. It caused a reduction in number of cercariae per snail during the patent period and in the period of cercarial shedding. The results, also, revealed that the glucose concentration in treated snails was increased in haemolymph, while soft tissue glycogen decreased. The activities of glycogen phosphorylase, succinate dehydrogenase (SDH), glucose-6-phosphatase and acetylcholinesterase (AchE) in homogenate of snail’s tissues were reduced ( P < 0.001) in response to treatment with plants methanol extract, while glucose-6-phosphate dehydrogenase (G-6-PD) activity increased ( P < 0.001). It was concluded that the application of LC 25 of methanol extracts of E. splendens, A. stylosa and G. officinalis may be helpful in snail control as it interferes with the snails’ biology and physiology.

A Studies on Chemical Control of S. mansoni Intermediate Host

This study was conducted to screen different snail species to determine their susceptibility for different parasitic infections and also to determine LC50 and LC90 values. The screening of snail species infected with Schistosome parasite and mean number of cercariae shed by individual infected snail was 184+12. The experiment on relationship of snail size and dose of Niclosamide showed that juvenile snails were more susceptible to chemical when compared with adult snails. A series of seven experiments indicated that concentration of 0.067 and 0.109 mg L(-1) were required to kill 50 and 90% snails, respectively in an exposure period of 24 h and no death occurred in recovery period.