Host–parasite coevolution may result in life-history changes in hosts that can limit the detrimental effects of parasitism. Fecundity compensation is one such life-history response, occurring when hosts increase their current reproductive output to make up for expected losses in future reproduction due to parasitic infection. However, the potential trade-offs between this increase in quantity and the quality of offspring have been relatively unexplored. This study uses the trematode, Schistosoma mansoni, and its snail intermediate host, Biomphalaria glabrata, to better understand how this host life-history response, fecundity compensation, impacts host reproduction. Measures of host reproductive output as well as offspring hatching success and survival were collected to assess the reproductive consequences of infection. Infected snails exhibited fecundity compensation by increasing the number of eggs laid and the overall probability of laying eggs compared to uninfected snails. Parental infection status did not play a significant role in hatching or offspring survival to maturity. Offspring from a later reproductive bout demonstrated a higher hatching success rate. Overall, the lack of an apparent trade-off between quantity and quality of offspring suggests that infected parental snails invest more resources towards reproduction not only to increase reproductive output, but also to maintain the fitness of their offspring, possibly at the expense of their own longevity.

Aelurostrongylus abstrusus is a worldwide occurring lungworm affecting felids. This metastrongyloid nematode has an indirect lifecycle relying on slugs and snails as intermediate hosts. In the present study the development of first-stage (L1) to third-stage larvae (L3) in the tropical freshwater pulmonate snail Biomphalaria glabrata was assessed. A total of 306 snails were individually exposed to 300 A. abstrusus L1, which were obtained from a naturally infected stray cat. The species was confirmed by biomolecular analysis. Second stage larvae (L2) and L3 were first isolated by artificial digestion of snails in the second and fourth week post exposure (wpe), respectively. From 8 wpe onwards, all larvae had developed into L3. Snails remained infected for up to 26 wpe. Only 0.4% of the L1 had pursued their development into L3, indicating low suitability of this artificial intermediate host for production of infective A. abstrusus L3.

Activity of the following glycosidases was detected in the plasma of the freshwater snail Biomphalaria glabrata: β-D- fucosidase, β-D-glucosidase, β-D-galactosidase, β-D-mannosidase, β-D-glucuronidase, N-acetyl-β-D-galactosaminidase, N-acetyl-β-D-glucosaminidase, and lysozyme. At the physiological pH (7·2–7·4) of snail haemolymph, enzymatic activity was about 10–50% of the maximum activity at each enzyme's respective acid pH-optimum. Schistosome-susceptible B. glabrata showed lower plasma protein concentration and significantly lower enzymatic activities (U/mg protein) than schistosome-resistant snails. Changes in glycosidase activity levels correlate with the progress of infection. After successful schistosome invasion, activities of plasma glycosidases but not the concentration of total plasma proteins increased significantly during the first 2 days in both snail strains. Thus, most tegumental glycoproteins of schistosome larvae can be altered by humoral host glycosidases. The detection of only very low activities of hexosaminidases leads to the hypothesis that GalNAc/GlcNAc may be involved in the process of non-self recognition. At 4 days post-infection, glycosidase activities were identical or slightly below the levels found in naive snails. At this time of infection the parasite is encapsulated and destroyed by haemocytes of resistant snails. In susceptible snails, however, the schistosomes have transformed into sporocysts and will complete their life-cycle without eliciting effective defence reactions. After >30 days post-infection, when cercariae are fully developed in susceptible snails, plasma protein concentration decreased significantly, whereas glycosidase activities were elevated.

During co-infection of Biomphalaria glabrata with Schistosoma mansoni and Echinostoma liei, S. mansoni cercariae released before the complete resorption of S. mansoni sporocysts show a very strong decrease of their infectivity in mice. Under conditions of high interspecific competition (i.e. when the snails are infected by E. liei 8 days after infection by S. mansoni), the mean overall worm return is five times lower than that of the control experiment. A marked decrease of the infectivity of cercariae is also noted when snails, infected exclusively with either sporocysts of S. mansoni or rediae of E. liei, are associated in the same tank.

No significant difference in cercarial production was noted when Biomphalaria glabrata from Brazil were exposed to one or several miracidia of Schistosoma rodhaini from Burundi, even though the dynamics seemed different. The minor differences can be explained by the intramolluscal larval development of this parasite characterized by the production of a high number of daughter sporocysts over a long period.

Sterols in the digestive gland-gonad (DGG) complex of Biomphalaria glabrata (Gastropoda) infected with patent larval stages of Echinostoma caproni (Trematoda) versus uninfected control snails were studied by gas-liquid chromatography. The major sterol present in both populations was cholesterol, 59% in the infected snails and 51% in the controls. Infected snails contained the same phytosterols as the controls—desmosterol, campesterol, stigmasterol, and beta-sitosterol–but the percent composition was reduced. The reduction was significant for both campesterol and stigmasterol.

The number of circulating haemocytes, the size of the haematopoietic organ, and the size of haemocyte capsules around the parasite were studied in M-line Biomphalaria glabrata snails exposed to 100 or 400 first-stage larvae of Angiostrongylus cantonensis. The number of haemocytes in exposed snails increased significantly at 1 day post-exposure, decreased to control value, and then increased again. The decrease in number of circulating haemocytes is probably due to the removal of cells from the circulation to participate in encapsulation of larvae. The majority of circulating haemocytes in M-line B. glabrata are fully-spread granulocytes, which increase significantly in number in snails following exposure to A. cantonensis larvae. However, populations of partially-spread granulocytes, round cells, hyalinocytes and miscellaneous haemocytes were relatively constant. The size of capsules around the parasite increased during the 42-day interval of the experiment. The haematopoietic organ increased in size in response to infection.

Production of elevated haemolymph agglutination titres by Biomphalaria glabrata following exposure to Echinostoma paraensei miracidia was investigated, to characterize this parasite-induced response and to understand its functional relevance. Both the dose of infection (1, 10 or 100 miracidia per snail) or the number of separate exposures to infection (between one and three, over a 4 or 8 day interval) were varied, and assuming a threshold dosage (10 miracidia per snail or higher) was exceeded, titres of juvenile snails peaked at 8–16 times the values for unexposed control snails, regardless of the exposure regimen. Adult snails, which are relatively refractory to infection, have slightly higher resting titres than juveniles, but exhibit only a 2- to 4-fold increase in titre following exposure. Juveniles exposed to infection but lacking demonstrable infection had lower titres than snails with confirmed infections. Exposure to infection increased heterogeneity of plasma agglutinins and provoked production of unique specificities not found in unexposed snails. However, the overall pattern of agglutination responses for snails with successfully developed parasites did not differ from those in which parasite development was unsuccessful. Agglutinating activity was inhibitable by several different monosaccharides, although plasma from infected snails was relatively unaffected by N-acetyl-glucosamine or N-acetyl-galactosamine. Wounding of snails provoked no change in plasma agglutination activity. As the highest agglutination titres were produced in snails with successfully developing parasites and agglutinin composition did not differ between snails with successful or unsuccessful parasites, the functional relevance of the response remains enigmatic. The production of unique agglutinins following exposure deserves additional study.

Trematode cercariae typically become unable to successfully infect a host many hours before they die. We examined the hypothesis that both time to 50% mortality and time to loss of infective capacity are controlled to the same degree by rates of energy expenditure, by determining the relative effects of temperature on both parameters. Infective capacity was assessed by exposing Echinostoma trivolvis cercariae of different ages to a suitable second intermediate host (the gastropod Biomphalaria glabrata) and counting 1–2 days later the number of metacercarial cysts formed. Temperature had a remarkably similar effect on time to 50% mortality and loss of infective capacity, supporting the hypothesis that both absolute and functional cercarial life-spans are limited by the rates at which energy stores are utilized.

Infection by Schistosoma mansoni resulted in morphological and biochemical changes to the foot of its intermediate host, Biomphalaria glabrata. Migration through, and emergence of cercariae from, the foot was observed and evidenced by lesions on the ciliated foot surface. This was accompanied by a significant decrease in the velocity of movement by infected individuals. In vivo31P NMR spectral analyses demonstrated that the foot of infected snails had a lower phosphoarginine (PA)/adenosine 5' triphosphate (ATP) ratio than that of uninfected controls. Moreover, kinetic experiments, employing saturation transfer, demonstrated the pseudo-first-order rate constant for the arginine kinase-catalysed exchange reaction in the forward direction, that is, PA→ATP was decreased by infection. The reverse reaction was not observed by the NMR methods used. PA was depleted upon exposure to hypoxic conditions suggesting its traditional role in preserving ATP level. Partly oxidized metabolic end-products were not observed in snails maintained under aerobic conditions, but succinate, propionate, acetate and lactate rapidly accumulated under hypoxic conditions.

Praziquantel, administered over a 72 h period in the food of mature Biomphalaria glabrata harbouring 7-week-old Schistosoma mansoni infections, dramatically reduced the numbers of cercariae shed. Doses of 20–30 μg/g body weight (including shell weight) reduced shedding by 85–95% over 5 weeks before recovery was evident. Suppression of cercarial shedding in these infections was accompanied by the temporary recovery of the snail reproductive regression due to S. mansoni infection. Snail fecundity was subsequently re-suppressed 2 weeks prior to recovery of the parasite as evidenced by a resumption of cercarial shedding. Praziquantel destroyed mature and developing cercariae within the daughter sporocysts but had no apparent effect on daughter sporocysts; this may account for the eventual resumption of cercarial production. Reproductive failure of the snail is apparently related to the latter stages of cercarial development specifically. Reproductive recovery did not occur when snails were infected as juveniles or when mature-infected snails harboured 12–5 week i.e. older infections: drug treatment temporarily inhibited cercarial production but no snails produced eggs.

The effect of temperature (19–36 °C) and snail host density (0.014–10 snails/1) on the snail-finding capacity of Echinostoma caproni cercariae is described. The initial swimming speed increased whereas the length of the infective period decreased with increasing temperature. The combined effect resulted in the E. caproni cercarial snail-finding capacity being temperature independent in the range 19 to 36 °C at a snail density of 0·014 snails/1. A moderate temperature dependency was, however, seen at a lower snail density. The cercarial snail-finding capacity was snail-host density dependent in the density range 0.014 to 1 snail/1. The findings from this study show that a relatively low and biologically realistic snail host density must be used in experimental studies if realistic estimates of the dynamics of cercarial transmission are to be obtained.

An in vitro bioassay was used to examine [14C]glucose incorporation into polysaccharides in albumen glands (AGs) of susceptible M-line Biomphalaria glabrata infected with the NMRI strain of Schistosoma mansoni. Polysaccharide and galactogen synthesis were unaffected by larval trematode infection in AGs of snails at days 14, 21, and 28 post-infection (p.i.) when compared to uninfected controls. Further experiments were conducted to determine if daughter sporocysts, hypothesized to be primary mediators of parasitic castration in this system, were able to exert direct effects on synthetic activity of uninfected AGs via haemolymph-borne molecules or in vitro culture-generated larval excretory–secretory (ES) products. When AGs were incubated in the presence of infected snail haemolymph, significant differences in quantities of polysaccharides and galactogen were detected only in test organs incubated in day 28 p.i. haemolymph. Daughter sporocyst ES products generated during the first 48 h of culture caused a significant reduction in polysaccharide and galactogen synthesis in test organs. When ES products from days 3 to 6 of in vitro culture were tested similarly, no significant differences in either polysaccharide or galactogen synthesis were observed between control and test organs. These data demonstrate that daughter sporocysts are able to modulate a specific aspect of the reproductive activity of the snail host through haemolymph-borne molecules of host or parasite origin, or directly through in vitro culture-generated ES products.

Carbohydrates associated with the tegumental surface of Schistosoma mansoni primary sporocyst may serve as potential receptors for mediating recognition by the internal defence system of the molluscan host, Biomphalaria glabrata. Therefore, a combination of SDS-PAGE and lectin probe analyses were carried out on biotin-labelled tegumental glycopeptides as a first step to defining the carbohydrates expressed at the sporocyst surface. The majority of surface polypeptides, ranging in relative molecular masses from 27 to 113 kDa, reacted with horseradish peroxidase-labelled Canavalia ensiformis (Con A), Erythrina corallodendron (ECA), Glycine max (SBA) and Triticum vulgaris (WGA) lectins indicating that most, if not all, tegumental proteins are glycosylated. However, differences in the binding of some lectins to individual glycopeptides suggest a degree of heterogeneity in the structure/composition of sugar moieties comprising these surface glycoconjugates. This notion is supported by the finding that the fucose-specific Tetragonolobus purpureas (TPA) lectin only reacted with approximately 50% of glycopeptides identified at the tegumental surface. Experiments employing biotin-labelled plasma (cell-free haemolymph) from S. mansoni-susceptible and -resistant B. glabrata snails as probes, further demonstrated that many of the identified surface glycoproteins also serve as plasma-binding sites for both snail strains. Binding interactions between plasma and sporocyst surface glycoproteins appeared to be, at least in part, mediated by carbohydrates since periodate treatment of sporocyst proteins or pre-incubation of plasma with the glycoproteins, fetuin or mucin, resulted in a decrease in plasma reactivity to blotted larval proteins.

The thermal preferences of two strains of the snail Biomphalaria glabrata, one resistant to, and one susceptible to, the parasite Schistosoma mansoni were determined in an aquatic thermal gradient. Snails were tested without exposure to the parasite, and 2 h and 5 weeks after exposure to trematode miracidia. The mean temperature selected by susceptible strain snails 2 h post-exposure tended to be lower than that of unexposed controls, although this was not statistically significant. In this strain, at 5 weeks post-exposure, the preferred temperature dropped by 1.9±0.5°C. The resistant strain displayed a significant drop of 1.8±0.6°C 2 h post-exposure. These results are consistent with the hypothesis that a drop in mean temperatures selected by snails is due to altered levels of endogenous cytokines such as IL-1 or TNF in association with parasite activation of the snail internal defense system.

Infection of the snail, Biomphalaria glabrata, by the free-swimming miracidial stage of the human blood fluke, Schistosoma mansoni, and its subsequent development to the parasitic sporocyst stage is critical to establishment of viable infections and continued human transmission. We performed a genome-wide expression analysis of the S. mansoni miracidia and developing sporocyst using Long Serial Analysis of Gene Expression (LongSAGE). Five cDNA libraries were constructed from miracidia and in vitro cultured 6- and 20-day-old sporocysts maintained in sporocyst medium (SM) or in SM conditioned by previous cultivation with cells of the B. glabrata embryonic (Bge) cell line. We generated 21 440 SAGE tags and mapped 13 381 to the S. mansoni gene predictions (v4.0e) either by estimating theoretical 3′ UTR lengths or using existing 3′ EST sequence data. Overall, 432 transcripts were found to be differentially expressed amongst all 5 libraries. In total, 172 tags were differentially expressed between miracidia and 6-day conditioned sporocysts and 152 were differentially expressed between miracidia and 6-day unconditioned sporocysts. In addition, 53 and 45 tags, respectively, were differentially expressed in 6-day and 20-day cultured sporocysts, due to the effects of exposure to Bge cell-conditioned medium.

The genetic control of compatibility between laboratory strains of schistosomes and their snail hosts has been studied intensively since the 1970s. These studies show (1) a bewildering array of genotype-by-genotype interactions – compatibility between one pair of strains rarely predicts compatibility with other strains, and (2) evidence for a variety of (sometimes conflicting) genetic mechanisms. Why do we observe such variable and conflicting results? One possibility is that it is partly an artifact of the use of laboratory strains that have been in culture for many years and are often inbred. Here we show that results of compatibility trials between snails and schistosomes – all derived from the same natural population – depend very much on whether one uses laboratory-cultured or field-collected individuals. Explanations include environmental effects of the lab on either host or parasite, and genetic changes in either host or parasite during laboratory culture. One intriguing possibility is that genetic bottlenecks during laboratory culture cause the random fixation of alleles at highly polymorphic loci that control the matched/mismatched status of hosts and parasites. We show that a simple model of phenotype matching could produce dose response curves that look very similar to empirical observations. Such a model would explain much of the genotype-by-genotype interaction in compatibility observed among strains.

The presence of antigenic carbohydrate epitopes shared by Biomphalaria glabrata as well as by the sporocysts and miracidia representing snail-pathogenic larval stages of Schistosoma mansoni was assayed by immunohistochemical staining of paraformaldehyde-fixed tissues. To this end, both polyclonal rabbit antiserum raised against soluble egg antigens (SEA) of S. mansoni and monoclonal antibodies recognizing the carbohydrate epitopes LDN [GalNAc(β1-4)GlcNAc(β1-)], F-LDN [Fuc(α1-3)GalNAc(β1-4)GlcNAc(β1-)], LDN-F [GalNAc(β1-4)[Fuc(α1-3)]GlcNAc(β1-)], LDN-DF [GalNAc(β1-4)[Fuc(α1-2)Fuc(α1-3)]GlcNAc(β1-)] and Lewis X [Gal(β1-4)[Fuc(α1-3)]GlcNAc(β1-)] were used. Intriguingly, anti-SEA serum as well as anti-F-LDN antibodies displayed significant binding in the foot region, anterior tissue and the hepatopancreas of uninfected snails, whereas the Lewis X epitope was only weakly detectable in the latter tissue. In contrast, increased binding of antibodies recognizing LDN, LDN-F and LDN-DF was observed in infected snail tissue, in particular in regions involved in sporocystogenesis, in addition to an enhanced binding of anti-SEA serum and antibodies reacting with F-LDN. A pronounced expression of most of these carbohydrate antigens was also observed at the surface of miracidia. Hence, the detection of shared carbohydrate determinants in uninfected snail tissue, sporocysts and miracidia may support the hypothesis of carbohydrate-based molecular mimicry as a survival strategy of S. mansoni.

Competition between parasite species has been predicted to be an important force shaping parasite and host ecology and evolution, although empirical data are often lacking. Using the Mus musculus-Schistosoma mansoni and Schistosoma rodhaini host-parasite systems we characterized mate choice and inter-specific competition between these two schistosome species. Simultaneous infections revealed species-specific mate preferences for both species as well as suggesting mating competition, with male S. rodhaini appearing dominant over male S. mansoni. S. rodhaini homologous pairs were also shown to have increased reproduction per paired female in the presence of a competitor in simultaneous infections. Overall total reproductive success was, however, similar between the two species under conditions of direct competition due to the greater initial infectivity of S. mansoni in comparison to S. rodhaini. Inter-specific competition was also implicated as increased parasite virulence to the host. The potential effects of such interactions on parasite and host ecology and evolution in nature are discussed.

Molecular interplay during snail-schistosome interactions is poorly understood and there is much to discover concerning the effect of snail host molecules on molecular processes in schistosomes. Using the Biomphalaria glabrata – Schistosoma mansoni host-parasite system, the effects of exposure to haemolymph, derived from schistosome-resistant and susceptible snail strains, on protein tyrosine phosphorylation in miracidia have been investigated. Western blotting revealed several tyrosine phosphorylated proteins in this larval stage. Exposure of miracidia to haemolymph from susceptible snails for 60 min resulted in a striking, 5-fold, increase in the tyrosine phosphorylation of a 56 kDa (p56) S. mansoni protein. In contrast, haemolymph from resistant snails had little effect on protein tyrosine phosphorylation levels in miracidia. Confocal microscopy revealed that tyrosine phosphorylation was predominantly associated with proteins present in the tegument. Finally, treatment of miracidia with the tyrosine kinase inhibitor genistein significantly impaired their development into primary sporocysts. The results open avenues for research that focus on the potential importance of phospho-p56 to the outcome of schistosome infection in snails, and the significance of protein tyrosine kinase-mediated signalling events to the transformation of S. mansoni larvae.