Uninfected Snails Research Articles

Field Evidence of Trematode-Induced Gigantism in Hydrobia Spp. (Gastropoda: Prosobranchia)

The study of growth rates in the prosobranch snails Hydrobia ulvae and H. ventrosa under field conditions showed that growth rate of snails infected by different trematode species is species-specific. Trematodes from the families Microphallidae and Heterophyidae cause gigantic growth whereas species from families Notocotylidae and Bunocotylidae have no effect on growth rate. This discrepancy is attributed to the different pathogenicity of the parasites. However, under experimental conditions with different host population densities the effect of infection by Bunocotyle progenetica effect varies from a tendency to stunt the growth (under high density) to a significant increase in growth rate (under low density). The effect of Himasthla sp. (Echinostomatidae) was shown to be population-specific. These findings agree with a previously reported hypothesis that the growth response of trematode-infected snails depends (among other things) on supplement of host-parasite system with food. Trematode infection caused parasitic castration in all studied host-parasite combinations. It is argued that the phenomenon of gigantism in infected snails is a consequence of the reduced sexual ability of the host. The correlation between snail growth rate and penis size of infected males was negative. The correlation between growth rate and penis size was not found in uninfected snails from the same age group

RELACIONAMENTO DE Tanaisia bragai (DIGENEA, EUCOTYLIDAE) E SEU HOSPEDEIRO INTERMEDIARIO, Subulina octona (GASTROPODA, SUBULINIDAE) SOB CONDICOES EXPERIMENTAIS

The action of the parasitism by the digeneic trematode Tanaisia bragai over the mortality, reproductive function, growth rate, calcium concentration in shells and oxygen consumption of its intermediate host, the snail Subulina octona were experimentally studied in infected and in infected snails maintained under laboratory conditions. A significant increase in mortality was observed in the infected snails. The alteration in the reproductive function evaluated through the egg production, suggested parasitic castration of infected snails. The Student's «t» test showed that there was a significant difference in relation to the shell weight and calcium concentration in the shells of infected and uninfected snails and there was no significant difference in relation to shell lenght, growth rate and oxygen consumption in the infected and uninfected snails.

The Effect of Diet and Reproductive Maturity on the Growth and Reproduction of Helisoma anceps (Pulmonata) Infected by Halipegus occidualis (Trematoda)

Inhibition of snail reproductive activity by larval trematodes often results in increased growth of the snail, termed gigantism, during which the snail is hypothesized to allocate excess energy normally used for reproduction to somatic growth. Using the planorbid snail Helisoma anceps and the hemiurid trematode Halipegus occidualis, this hypothesis was examined by raising snails on low and high quality diets under 3 infection conditions: uninfected (UNI) and infected prior to (IPR) and after (IAR) reproductive maturity. Snails exhibited gigantism as an increase in wet mass only in the IPR condition on a high quality diet; snails in the IAR condition were not significantly different in size from UNI controls. Throughout the experiment, snails raised on a high quality diet were larger than those on the low quality diet, regardless of infection status, primarily because of increased growth rates prior to the onset of reproduction. Egg production by UNI snails on the high quality diet was 20 times higher than snails on the low diet, even though diet quality only differed by a factor of 4, and infected snails (both IPR and IAR) showed significantly reduced reproductive activity compared to UNI controls. The number of cercariae shed by each snail on a daily basis was not significantly different between IPR and IAR conditions when each diet quality was examined separately; more cercariae were produced by snails raised on the high quality diet. The prevalence of infection at the end of the experiment was lowest among snails raised on the low quality diet, suggesting the diet, i.e., energy intake, influences the loss of infections. Snail mortality did not differ among the various treatment conditions. These data suggest that gigantism is dependent upon snail reproductive maturity at the time of infection and food quality, and comparison to growth rates and fecundity data from previous field studies indicates that H. anceps in the field are probably protein deprived. Gigantism does not appear to be adaptive for the snail because survival was not increased, or for the parasite because cercariae production was the same for snails exhibiting gigantism or not. These results also stress the need for caution when examining the fecundity and growth rates of both uninfected and infected snails in the laboratory, especially when comparing these data to other laboratory or field studies.

The Probability of a Marine Gastropod Being Infected by a Trematode

Sentinel snails were employed to determine the likelihood of an estuarine snail becoming infected with a trematode under natural conditions. In summers of 1991 and 1993, Ilyanassa obsoleta (Mollusca: Gastropoda) (n = 1,400) were collected from a saltmarsh in Lewes, Delaware, where parasite prevalence was low, and screened for infections. Putatively uninfected snails were individually marked and released onto the nearby Cape Henlopen sandflat where prevalence of trematodes among native snails is very high (approximately 80%). Most sentinels were free in the field for 1-5 mo and 186 were ultimately recovered. The overall estimate of probability of infection is 1.6%. The snail lives for many years and this low probability of becoming infected indicates that high prevalence of trematodes in this host is brought about not by rapid colonization, but by slow accumulation over time.

Biogenic monoamines in the freshwater snail, Biomphalaria glabrata: Influence of infection by the human blood fluke, Schistosoma mansoni

The biogenic monoamines, serotonin (5-HT), dopamine (DA) and L-dopa were measured using high performance liquid chromatography with electrochemical detection (HPLC-ED) in the extracts of the central nervous system (CNS) and plasma of uninfected freshwater snails, Biomphalaria glabrata, and in snails at 7, 14, 21 and 28 days postexposure (PE) to the miracidia of the human blood fluke, Schistosoma mansoni. Relative to age-matched uninfected snails, a general depression of biogenic amine levels was observed in the plasma (cell-free haemolymph) and the CNS of infected snails, especially during the latter phase of the prepatency period. Significant decreases were first observed in the CNS of infected snails beginning at Day 14 PE for DA and 5-HT and Day 21 PE for L-dopa. Parasite-exposed snails also exhibited an early and persistent suppression of plasma 5-HT concentrations, starting at 7 days PE and continuing throughout the infection test period. In order to determine the effect of 5-HT on reproduction and, thereby, establish a possible relationship between the observed parasite-induced reduction in 5-HT levels and parasitic castration, the effect of exogenous 5-HT on individual infected and uninfected B. glabrata was investigated. Repeated treatment with 10 microM 5-HT promoted both ovulation and oviposition in B. glabrata. Snails treated with 5-HT consistently layed more eggs than did sham-treated controls. Infected snails that were treated with 5-HT exhibited similar egg-laying rates as those of both serotonin-treated and untreated, uninfected snail groups, thus reversing the castrating effects of larval infection. These findings suggest that 5-HT acts as a stimulant for egg production in B. glabrata, and that parasitic castration may be due, at least in part, to larval-induced suppression of 5-HT in the snail's CNS and plasma during the course of infection with S. mansoni.

Male Reproductive Success of Schistosoma mansoni-Infected Biomphalaria glabrata Snails

Infection of Biomphalaria glabrata by Schistosoma mansoni results in a dramatic reduction in the snail's ability to produce eggs. We studied the ability of such parasitically castrated snails to fertilize the eggs of uninfected snails. Pigmented B. glabrata snails (13141 stock) were infected with S. mansoni miracidia and reared individually until they ceased laying eggs. These infected snails were then given the opportunity to mate with uninfected albino (NMRI) snails. Each of the infected snails was paired with a different albino partner each subsequent week. Sperm transfer by the infected snails was evident from the production of pigmented progeny by the uninfected albino snails. Infected snails successfully acted as males for up to 6 wk after parasitic castration had occurred. The duration of allosperm use by uninfected recipients was lengthy, regardless of the infection status of the pigmented sperm donor.

The effects of size, reproductive condition, and parasitism on foraging behaviour in a freshwater snail, Potamopyrgus antipodarum

Predation risk is expected to affect the foraging behaviour of individual prey according to their size, sex and reproductive condition. Parasites may also affect the foraging behaviour of their hosts, due to increased energetic demands imposed by the parasite, or due to a parasite strategy to increase the likelihood of its transmission to the next host. Field studies were conducted to determine the effects of size, reproductive condition and trematode infection on foraging behaviour in a freshwater snail, Potamopyrgus antipodarum. Among uninfected snails, small individuals (<3.8 mm in length) foraged more than large individuals during daylight hours when the risk of predation is greatest; non-brooding adult females foraged more during the day than females that were brooding embryos. These results suggest that juveniles and non-brooding females trade off the risk of predation with the energy required for growth and reproduction, respectively. Infection by a digenetic trematode ( Microphallussp.) also caused significant changes in snail behaviour. This trematode has a two-host life cycle. Snails are the intermediate host, and ducks are the final host. Encysted parasite larvae are transmitted to ducks when they ingest snails infected with mature larval cysts. Results showed that snails infected by encysted, transmissible larvae foraged in the early morning hours, and retreated under rocks during the late morning. In contrast, snails infected by unencysted non-transmissible larvae retreated to positions under rocks in the early morning, and mirrored the risk-averse strategy of brooding females. These results suggest that, when the parasite larvae encyst and become transmissible to the final host, they induce the snails to forage later into the morning. Because foraging by waterfowl is greatest in the early morning, encysted larvae may manipulate snail behaviour to increase the likelihood of their transmission to the final host.

Eurytrema coelomaticum (Giard &amp; Billet) (Digenea, Dicrocoeliidae) em Bradybaena similaris (Férussac) (Gastropoda, Xanthonychidae): alterações nos depósitos de cálcio e na função reprodutiva do primeiro hospedeiro intermediário

The influence of the parasitism by the digenetic trematode Eurytrema coelomaticum (Giard & Billet, 1892) over the calcium mobilization and the reproductive function of its first intermediate host, the snail Bradybaena similaris (Ferussac, 1821) was experimentally studied in infected and uninfected snails maintained in laboratory under controlled conditions of nutrition, humidity and temperature. The calcium of the shells of infected and uninfected snails was quantified by using a technique of volumetric complexation. The amount of calcium present in the tissue was evaluated by using the histochemistry techniques of Dahl and von Kossa. The alteration in the reproductive function was evaluated through the egg production, and by histological follow up of the snails gametogenesis, in the infected and uninfected snails. The Student's test t showed that there was no significant difference in relation to size, weight and calcium concentration in the shells of infected and uninfected snails. The analysis of nonlinear regression, polynomial of 4th order, showed an highly positive relation to weight and size of shell to infected and uninfected snails, and an weakly significant relation to calcium concentration in the shells and shells size of infected snails. The histochemistry techniques demonstrated a large calcium deposition in the cells of the digestive glands of the infected snails when compared to the uninfected ones. The histological study of the ovotestis of the infected snails demonstrated that the gametogenesis of these snails was abnormal. The ovulogenesis was the most alterated and at 45 days post-infection the histological sections showed no ovocytes present in the ovotestis.

Growth, Trematode Parasitism, and Longevity of a Long-Lived Marine Gastropod (Ilyanassa Obsoleta)

Ilyanassa obsoleta (Say) (Mollusca: Neogastropoda) is a conspicuous biotic element of marine / estuarine habitats along the eastern North American coast. Growth among larger individuals (-18–30 mm shell height) has not been assessed. Trematode infections are common in this group and their effect on growth must be considered. Snails were individually marked, measured, and released on the Cape Henlopen sand-flat at the mouth of Delaware Bay. Shell height change (growth) and trematode parasitism were assessed in recaptured snails. Most or all growth occurred in June and July. Mean growth of uninfected snails was 1–2 mm per season. Snails that started out larger grew more slowly, but there was much individual variability. Mean growth of parasitized snails was 0–2 mm per season and independent of initial size. The maximum age of this snail is currently documented at 7 y, which is too low. Size reflects age only approximately. However, the minimum and maximum age of a snail of given size, based on maximal and minimal growth per season, can be estimated. A mid-range estimate for lifespan of unparasitized and parasitized individuals is 30–40 y.

Field Studies on the Transmission and Survival of Cyclocoelum mutabile (Digenea) Infections in Natural Snail Populations in Southern Manitoba, Canada

The transmission of Cyclocoelum mutabile to snails was examined under natural conditions by sampling the snail communities of 4 natural ponds that had been exposed experimentally to infection by laboratory-infected coots (Fulica americana). Five of 6 snail species in the ponds, Physa jennessi, Promenetus exacuous, Armiger crista, Gyraulus parvus, and Stagnicola elodes, became infected. No natural infections were found in the few Helisoma trivolvis examined. The second most abundant species Promenetus exacuous was infected most often, whereas Physa jennessi, the most abundant species present, was rarely infected. The temporal pattern of infections in the snail community suggests the transmission window of this parasite in southern Manitoba is limited by both the 14 C hatching threshold of the fluke eggs and the seasonality of ovigerous infections in the coot host. No naturally overwintering infections were found in snails from these ponds, which were examined the following spring. None of the 1,120 laboratory-infected snails placed in cages and held overwinter in the ponds survived, whereas 14 of the 1,120 uninfected control snails kept in the same cages survived. These results are consistent with the hypothesis that C. mutabile must be reestablished in northern waterfowl breeding areas each spring.

Effects of diet on the lipid composition of the digestive gland-gonad complex of Biomphalaria Glabrata (Gastropoda) infected with larval Echinostoma Caproni (Trematoda)

This study examined the effects of a larval Echinostoma caproni infection on the neutral lipid composition of the digestive gland-gonad complex (DGG) of Biomphalaria glabrata snails fed hen's egg yolk supplemented with lettuce (Y-L) or lettuce supplemented with Tetramin (L-T). Snails were experimentally infected with the miracidial stage of this echinostome, and their DGGs containing daughter rediae were analyzed for neutral lipids five weeks post-infection by qualitative and quantitative thin-layer chromatography. Light microscopy using Oil Red O (ORO) staining and transmission electron microscopy (TEM) were used to localize neutral lipids in the rediae. The DGGs of infected snails maintained on the Y-L diet showed a significant increase in free sterols and a significant decrease in triacylglycerols compared to uninfected snails maintained on the Y-L diet. The DGGs of infected snails maintained on the L-T diet showed no significant difference in free sterols or triacylglycerols compared to uninfected snails maintained on the L-T diet. ORO staining and TEM showed the presence of lipid droplets in rediae from snails on the Y-L diet. The significant decrease in triacylglycerols in the DGGs of infected snails maintained on the Y-L diet suggests that triacylglycerols were utilized by the rediae.

Thin-Layer Chromatographic and Histochemical Analyses of Neutral Lipids in the Intramolluscan Stages of Leucochloridium variae (Digenea, Leucochloridiidae) and the Snail Host, Succinea ovalis

Thin-layer chromatographic and histochemical analyses were used to analyze neutral lipids in the intramolluscan larval stages of Leucochloridium variae and in tissues of the snail host Succinea ovalis. Thin-layer chromatography showed that the major neutral lipids in uninfected snails were triacylglycerols and free sterols. The major neutral lipids in brood sacs minus their encysted metacercariae and in sporocysts were triacylglycerols and free sterols. Encysted metacercariae showed a significant free fatty acid fraction in addition to triacylglycerols and free sterols. Residual snail tissue (mainly head, foot, and visceral mass) from infected snails from which parasites were removed showed only a free sterol fraction. Histochemical staining of tissues with oil red O (ORO) showed the presence of neutral lipid droplets in the brood sac and sporocyst walls and mainly in the suckers, parenchyma, and excretory system of the encysted metacercariae. The residual snail tissue was ORO negative.

The effect of larval trematodes on the growth and burrowing behaviour of Hydrobia ulvae (gastropoda:prosobranchiata) in the Ythan estuary, north-east Scotland

Hydrobia ulvae were collected from five areas on the Ythan estuary, and the prevalences of digenean parasites in the samples compared. Differences in mean shell height were best explained by differences in parasite prevalence. All snails larger than 6.1 mm were infected, with a strong correlation between size and probability of infection. Comparisons of depth distributions of different size classes of snail in the substratum showed that larger snails were disproportionately represented in the top 5 mm. Snails exposed on the surface of the mud at low tide were found to have a higher chance of infection with metacercarial species compared with those that were buried. Snails infected with metacercarial species were less likely to burrow than those infected with other species, or with uninfected snails, in laboratory experiments. Large snails crawled more slowly and did not react as rapidly to simulated rainfall as small snails. These observations are interpreted as evidence for parasite-induced changes in snail behaviour.

Modulation of the bacterial clearance activity of haemocytes from the freshwater mollusc, Lymnaea stagnalis, by the avian schistosome, Trichobilharzia ocellata.

The ability of haemocytes, from the haemolymph of the gastropod mollusc Lymnaea stagnalis, to recognize and eliminate the bacterium Aeromonas salmonicida was shown using an in vitro bacterial clearance assay. The assay employs a dye which is reduced by A. salmonicida in direct proportion to the number of viable bacteria resulting in a colour change which can be determined spectrophotometrically. Addition of cytochalasin B resulted in a marked decrease in bacterial clearance, implicating both intracellular and extracellular cytotoxicity of haemocytes. A comparison of haemocytes from uninfected snails and snails infected with the avian schistosome parasite Trichobilharzia ocellata showed that both juveniles and adults of L. stagnalis were susceptible to infection with T. ocellata. After exposure to the trematode for 1.5 h the haemocytes from these infected snails had an enhanced clearance capacity, whilst cells obtained from snails with 24-96 h infections showed decreased clearance of the bacteria, indicating suppression by the parasite. Haemocytes, as well as plasma, which was tested on haemocytes from uninfected snails, were used and hence a distinction was made between cell and humoral-associated effects. The results show that both cellular and humoral components of immunity were activated, then suppressed, following exposure to the parasite. Infection with T. ocellata seems to have a modulating effect on the bactericidal activity of the internal defence system of the snail host, L., stagnalis.

Eurytrema coelomaticum (Digenea, Dicrocoeliidae): the effect of infection on carbohydrate contents of its intermediate snail host, Bradybaena similaris (Gastropoda, Xanthonychidae)

The interface Eurytrema coelomaticum/Bradybaena similaris was studied by quantifying the amount of glucose on the hemolymph and the content of glycogen in the cells of the digestive gland and the cephalopedal mass of infected and uninfected snails. Samples were analyzed on days 0, 30, 90 and 150 post-infection. The infected snails had less glucose in the hemolymph, with a reduction of 67.05 por cento at 30 days, and 62.09 por cento at 90 days post-infection. The reduction in glycogen content was 86.41 por cento in the digestive gland and 79.1 por cento in the cephalopedal mass at 30 days, and 92.71 por cento and 90.89 por cento in these organs respectively at 90 days post-infection. It is proposed that the sporocysts absorb glucose directly from the hemolymph.

The enigma of gigantism: effect of larval trematodes on growth, fecundity, egestion and locomotion in hydrobia ulvae (pennant) (gastropoda:prosobranchia)

The effect of larval trematodes on growth, fecundity, egestion and locomotor activity in naturally infected Hydrobia ulvae (Pennant) was studied in the laboratory. Infected snails showed increased growth rates (shell height, body wet weight) compared with uninfected controls. C/N analysis of the snails suggested that the additional growth of infected specimens included shell material as well as dry soft tissue. Infection caused a significant reduction in penis size and an almost complete cessation of oviposition. As judged by their egestion rates, food consumption rates of infected and uninfected snails were roughly similar, but infection caused a significant reduction in locomotor activity. It is suggested that the energetic basis for parasite metabolism and excess host growth is in part reallocated reproductive energy following castration, and in part energy saved by reduced locomotor activity. Some previously reported hypotheses explaining the equivocal evidence on parasite-induced growth enhancement are discussed. It is argued that the phenomenon is not necessarily species-specific, but should also be addressed at the level of subpopulations both in and outside the snails' reproductive season.

Plagiorchis elegans: emergence, longevity and infectivity of cercariae, and host behavioural modifications during cercarial emergence

We investigated changes in the behaviour of Stagnicola elodes associated with the emergence of Plagiorchis elegans cercariae. Within 15 min of the reduction in light intensity, which triggered the onset of cercarial emergence, infected snails moved to the top of the water column and remained there for 2-3 h. Seventy-nine percent of all cercariae that emerged from the snail did so during this period. Uninfected snails showed no such behavioural changes following the change in light intensity. Cercariae were released in a dense cloud around the snail at the water surface and dispersed passively. Within 3-4 h more than 80% of all cercariae had settled in the bottom 5 cm of the water column. The infectivity of cercariae increased from less than 20% upon emergence from the snail to greater than 75% 4-6 h post-emergence, and then declined steadily to below 5% by 24 h post-emergence. Cercarial longevity was greater than 30 h and exceeded the period of infectivity. This may be related to steadily falling endogenous glycogen levels. A delay in attaining maximum infectivity may represent an adaptive mechanism allowing time for cercarial dissemination, thus reducing superinfection, and subsequent parasite-associated mortality, of second intermediate hosts.

The Effects of Spatial and Temporal Heterogeneity as Structuring Forces for Parasite Communities in Helisoma anceps and Physa gyrina

-A total of 1231 Physa gyrina and 1532 Helisoma anceps were collected over a 12-mo period from Charlie's Pond, in Stokes County, North Carolina. Similarity in the infraand component parasite communities in Helisoma anceps and Physa gyrina provided an opportunity to compare the effects of various life history traits on infection by larval trematodes in a number of different microhabitats. In order to assess the effects of microhabitat partitioning on the infection status of the two snail species, collections were made twice per month. Site location, water depth, snail depth at capture, distance from shore, type of substratum, infection status, and host size were recorded for each snail. Multivariate statistics were used to assess which variables determined a snail's infection status. Data analysis indicated that certain of the microhabitat variables are better predictors of a snail's infection status than others. Comparing uninfected snails to those infected with a specific parasite species, there was much disparity with regard to which microhabitat characteristic varied significantly. Manipulation of infracommunity structure, by excluding certain parasites, allowed examination of the influence of temporal heterogeneity as compared to interspecific trematode interactions in structuring the infraand component communities. Snails were maintained in cages in the field. Enclosures were positioned a few centimeters above the substratum to prevent infection of the snails via egg ingestion and effectively prevented the more prevalent parasite species (Halipegus occidualis in H. anceps and Halipegus eccentricus in P gyrina) from infecting the snails. Statistical analyses indicate that preventing snails from ingesting eggs of some parasites had no affect on the recruitment of other parasites. Parasite prevalences in H. anceps and P gyrina were not different in caged snails compared with those collected from the substrata of the pond.

High Performance Thin-Layer Chromatographic Analysis of Sugars in Biomphalaria glabrata (Gastropoda) Infected with Echinostoma caproni (Trematoda)

High performance thin-layer chromatographic analysis was done on the hemolymph and digestive gland-gonad complex (DGG) of Biomphalaria glabrata snails experimentally infected with the intramolluscan stages of Echinostoma caproni. The major sugars detected in both the DGG and hemolymph of infected and uninfected snails were glucose and trehalose. Quantitative analysis by scanning densitometry showed a significant reduction in glucose in both the hemolymph and DGG of infected snails at 4, 6, and 8 wk postinfection. A similar analysis on trehalose showed that this sugar was significantly reduced at 6 wk postinfection in the hemolymph and DGG of infected snails and could not be detected from these sites in infected snails by 8 wk postinfection. Findings from this study were compared with information on sugars in B. glabrata infected with larval stages of Schistosoma mansoni.

DNA probes for the detection of Fasciola hepatica in snails

Fasciola hepatica, also called the large liver fluke, is a trematode which can infect most mammals. Monitoring the infection rate of snails, which function as intermediate hosts and harbour larval stages of F. hepatica, is an important component of epidemiological studies on fascioliasis. For this purpose, DNA probes were generated which can be used for the detection of F. hepatica larvae in snails. Four highly repetitive DNA fragments were cloned in a plasmid vector and tested by Southern blot hybridization to the DNA of various trematodes for specificity and sensitivity. The probes Fhr-I, Fhr-II and Fhr-III hybridized only to F. hepatica DNA. Fhr-IV contained ribosomal RNA gene sequences and cross-hybridize with the DNA from various other trematode species. Squash blot analysis showed that the different probes were able to detect the parasite larvae in trematode-infected snails even as isolated single larvae. No signals were obtained in squash blots of uninfected snails. Probes Fhr-I, Fhr-II and Fhr-III are thus useful specific tools for studying the epidemiology of fascioliasis. The probe Fhr-IV, because of its broader spectrum, can be used to detect the larvae of a wide range of trematode species of waterbirds, which are the causative agents of swimmer's itch.