Chronobiological aspects of the host-parasite relationships between Biomphalaria glabrata and Schistosoma mansoni: cercarial production and infectivity, and growth kinetics of the host

In the Biomphalaria glabrata-Schistosoma mansoni association, variations in cercarial production, in cercarial infectivity, and in the growth of infected snails are rhythmic. These chronobiological aspects are correlated with the dynamics of the intramolluscan larval stages of the parasite during the course of the infection. Rhythmic variations in the growth kinetics of infected snails are interpreted in terms of host-parasite metabolic exchanges.     

Sequential histological changes in Biomphalaria glabrata during the course of Schistosoma mansoni infection

Biomphalaria glabrata, highly susceptible to Schistosoma mansoni, were seen to shed less and less cercariae along the time of infection. Histological examination kept a close correlation with this changing pattern of cercarial shedding, turning an initial picture of no-reaction (tolerance) gradually into one of hemocyte proliferation with formation of focal encapsulating lesions around disintegrating sporocysts and cercariae, a change that became disseminated toward the 142nd day post miracidial exposure. Findings were suggestive of a gradual installation of acquired immunity in snails infected with S. mansoni.     

Interaction between the Biomphalaria glabrata-Schistosoma mansoni host-parasite system and the non-target molluscs: influence on cercarial production

The cercarial production of Schistosoma mansoni by Biomphalaria glabrata (target mollusc: TM) was studied in relation to the presence of non-target molluscs (NTM). Nine species of NTM were tested. Whatever the type of TM-NTM association (lasting association, during prepatent period and production period, association only during the exposure of the molluscs to the miracidia), the presence of NTM involved a significant increase of S. mansoni cercarial production. This increased cercarial production was more or less pronounced according to the NTM species present.     

Schistosoma mansoni infections in neonatal Biomphalaria glabrata snails.

In areas endemic for schistosomiasis, the population dynamics of the snail intermediate hosts have a direct effect on parasite transmission. The present study focused on the potential for neonatal Biomphalaria glabrata snails to become infected with Schistosoma mansoni and to produce cercariae under various conditions. It was found that snails as small as 0.74 mm in shell diameter could survive miracidial penetration and could release cercariae when as small as 1.6 mm in diameter. Cercariae produced by small snails were equally infectious for mice when compared with those shed by larger snails. Likewise, histological examination of neonatally exposed snails revealed normally developing parasites at all stages of infection. It was found that in 2 snail populations expressing either high or low susceptibility to the parasite, peak susceptibility occurred at 25 days of age in both groups. Daily cercarial production for neonatally exposed snails was initially low but increased dramatically as the snails grew, eventually reaching values as high as 2,100 cercariae/snail/day. A moderate to high percentage of snails infected as neonates was eventually capable of simultaneously producing both eggs and cercariae. These studies emphasize the potential importance of neonatal and preadult snails in helping to maintain foci of S. mansoni infection in endemic areas.     

Schistosoma mansoni: influence of Biomphalaria glabrata size on susceptibility to infection and resultant cercarial production

Biomphalaria glabrata snails of the same age, but different sizes, were used to determine size-related susceptibility to Schistosoma mansoni miracidial infection and the influence of snail size on total cercarial production. Snails with shell diameters from less than 5 to greater than 17 mm were individually exposed to one or several miracidia, depending on the experiment. In snails exposed to multiple numbers of miracidia, the percentage of snails which developed patent infections was lower in snails with larger shell sizes. This was also reflected by fewer primary sporocysts per infected snail found in tissues of the larger snails. Upon determining cercarial production in these groups over a 1-month period there were no statistical differences between any groups in the numbers of cercariae produced per snail. However, upon determining the number of successful primary sporocysts found in cohort snails of each size group, cercarial production increased as a function of the number of successful primary sporocysts. This was verified by examining cercarial production in various size snails with known monomiracidial infections. Our data therefore confirm and extend earlier work using snails infected with unknown numbers of miracidia and clearly show that total S. mansoni cercarial development and decreased susceptibility of snails is a direct reflection of snail size and not necessarily age of the snail.     

Schistosoma rodhaini: dynamics and cercarial production for mono- and pluri-miracidial infections of Biomphalaria glabrata

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.     

Passive transfer of immunity with serum in mice infected with Nematospiroides dubius: in vitro effect of immune serum on larval infectivity

Dobson C. and Cayzer C. J. R. 1982. Passive transfer of immunity with serum in mice infected with Nematospiroides dubius: in vitro effect of immune serum on larval infectivity. International Journal for Parasitology12: 413–421. Incubation of Nematospiroides dubius larvae in serum in vitro induced 15% exsheathment after 3h. Larvae incubated in immune mouse serum at 37°C for 3h lost 20% of their infectivity for mice. Immune serum from donors given 1–7 concurrent or anthelmintic abbreviated infections all depressed larval infectivity to the same extent. Larvae incubated in immune sera were protected from the effects of passively transferred immune serum in mice following infection. The effects of incubation of larvae in immune serum were prolonged into the adult stages of the parasite and were seen as stunting of worms and a reduction in the male-female sex ratio of the parasites.     

Effects of environmental calcium on fecundity and cercarial production of Biomphalaria glabrata (Say) infected with Schistosoma mansoni Sambon

Biomphalaria glabrata were reared in stock culture and subjected to either 7-day or 60-day acclimation periods in complex CaCO3 media with calcium values ranging from 1.5 mg/L to 75 mg/L. Following 60-day acclimation, snails from series I were each exposed to 8 miracidia of Schistosoma mansoni. Snails of series II were each exposed to a single miracidium. Snails of both experimental regimens were observed for mortality, growth, rate of infection, and number of cercariae shed. Series I snails were also monitored for fecundity during acclimation and following miracidial exposure. Calcium levels of 1.5 and 75 mg/L resulted in significant snail mortality. Shell growth and rates of infection were positively correlated with calcium maintenance level. Snails with high fecundity prior to miracidial exposure subsequently shed more cercariae. In contrast, post-exposure (PE) fecundity of snails reared in media with up to 30 mg/L Ca++ were negatively correlated with calcium level, infection rate, and number of cercariae shed. Maximal cercarial emergence occurred at 30 mg/L Ca++. These results suggest that environmental calcium affects both the distribution patterns of snail hosts of human schistosomes and the productivity of intramolluscan schistosome infection.     

Influence of saccharose on the development of cercariae from Schistosoma mansoni strains BH and SJ.

The development of cercariae from Schistosoma mansoni strains BH and SJ in Biomphalaria glabrata and Biomphalaria tenagophila treated with saccharose was studied. The molluscs were maintained in dechlorinated tap water containing 0.01% saccharose. After one week of treatment with saccharose, B. glabrata and B. tenagophila were exposed to ten S. mansoni miracidia, from BH and SJ strains respectively. Control snails of both species were maintained in dechlorinated tap water without saccharose and exposed to the same number of miracidia. There was no significant difference between the infection rates of snails treated or not with saccharose. However, the two groups of B. glabrata had significantly greater infection rates than the corresponding B. tenagophila groups. Molluscs treated with saccharose had a lower survival rate, with the greatest mortality occurring immediately before and at the beginning of cercariae release. Treatment with saccharose did not result in the release of more cercariae, but larvae from molluscs so treated showed a greater capacity to penetrate mouse skin, which was attributed to the greater energy supply during larval development in the mollusc.     

Disentangling phylogenetic constraints from selective forces in the evolution of trematode transmission stages

The transmission stages of parasites are key determinants of parasite fitness, but they also incur huge mortality. Yet the selective forces shaping the sizes of transmission stages remain poorly understood. We ran a comparative analysis of interspecific variation in the size of transmission stages among 404 species of parasitic trematodes. There are two transmission steps requiring infective stages in the life cycle of trematodes: transmission from the definitive to the first intermediate (snail) host is achieved by eggs and/or the miracidia hatched from those eggs, and transmission from the first to the second intermediate host is achieved by free-swimming cercariae. The sizes of these stages are under strong phylogenetic constraints. Our results show that taxonomy explains >50% of the unaccounted variance in linear mixed models, with most of the variance occurring at the superfamily level. The models also demonstrated that mollusc size is positively associated with egg volume, miracidial volume and cercarial body volume, but not with the relative size of the cercarial tail. In species where they encyst on substrates, cercariae have significantly larger bodies than in species penetrating chordates, although the relative size of the cercarial tail of species using chordates as second intermediate hosts was larger than in other trematode species. Habitat also matters, with larger cercarial tails seen in freshwater trematodes than in marine ones, and larger miracidial volumes in freshwater species than in marine ones. Finally, the latitude (proxy for local temperature) at which the trematode species were collected had no effect on the sizes of transmission stages. We propose that resource availability within the snail host, the probability of contacting a host, and the density and viscosity of the water medium combine to select for different transmission stage sizes.     

The effects of temperature change on the infection rate of Biomphalaria glabrata with Schistosoma mansoni

The aim of this study was to investigate the influence of temperature on the development of Schistosoma mansoni infections in Biomphalaria glabrata. The snails were infected at 15, 20, and 30 degrees C, and the cercarial release was analyzed after 30 and 60 days post-infection. Our results showed that a decrease in the temperature has a substantial influence on the development of S. mansoni infection in B. glabrata, with significant differences (p < 0.05) between 15 and 30 degrees C. These data could provide a better understanding of the epidemiological aspects of schistosomiasis.     

Effect of miracidial dose on adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host, Biomphalaria glabrata

Adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host Biomphalaria glabrata was measured as a function of miracidial challenge dose. Schistosome-susceptible snails implanted with the amebocyte-producing organ (APO) from resistant donors showed 29 and 39% prevalences of infection after challenge with 5 and 10 miracidia, respectively, but 68-83% prevalences when exposed to 25-200 miracidia. Prevalences in control (untampered) susceptible snails ranged from 97 to 100% at the different miracidial doses. Higher infection prevalences at elevated doses suggest that a range of transferred resistance occurs and possibly that low levels of APO-derived plasma factors or hemocytes in some recipients can be overwhelmed by larger numbers of parasites.     

Schistosoma mansoni: ultrastructural demonstration of a miracidial glycocalyx that cross-reacts with antibodies raised against the cercarial glycocalyx

Cercariae are covered by a glycocalyx that is highly antigenic. Here, we have examined the surface of miracidia for a similar structure. The miracidia are covered by epithelial plates and syncytial ridges. By transmission electron microscopy, the plates and ridges were covered by a 0.5-micron-thick glycocalyx composed of a mesh of 9- to 10-nm fibrils that were stained by ruthenium red delivered in the aldehydes or ferrocyanide-reduced osmium tetroxide. Rabbit antibodies prepared against phenol extracted and chromatographed cercarial glycocalyx were detected by immunoelectron microscopy with secondary antibodies conjugated to horseradish peroxidase. Reaction product bound to both the miracidial and cercarial glycocalyx. In addition, the outer leaflets of the cercarial tegumental membrane and membranes of the miracidial surface structures, including plates, ridges, terebratorium, and sensory papillae, had reaction product. Controls incubated with nonspecific rabbit serum had no reaction product. By indirect immunofluorescence, antibodies against the cercarial glycocalyx stained both plates and ridges. As the miracidia transformed to sporocysts, the glycocalyx remained associated with the plates as they were sloughed. These studies demonstrate that miracidia possess a glycocalyx similar in structure and antigenicity to the cercarial glycocalyx.     

Schistosoma mansoni: male-biased sex ratios in snails and mice

Adult sex ratios of Schistosoma mansoni, in mice, were shown to be biased toward males (3:1) despite the finding that sex ratios of miracidia were 50:50. The adult male bias was caused by greater male infectivity of miracidia for snails and cercariae for mice. A significantly higher percentage of male miracidia developed to cercarial production in unimiracidial infections (57 male, 34 female), and a significantly higher percentage of male cercariae developed to adulthood in mice (143 male, 79 female worms resulted from 900 male and 900 female cercariae). No significant differences were found between male and female parasites for longevity of miracidia (both sexes, 10 hr) and cercariae (males 21.3 +/- 5.75 hr, females 25.0 +/- 7.02 hr), prepatent periods in snail hosts (male 34 +/- 2.92 days, females 33 +/- 2.36 days), longevity of snail infections (males 96.6 +/- 25.15 days, females 115.2 +/- 82.43 days), and the numbers of cercariae produced per snail lifetime (males 30,751.44 +/- 18,064.33, females 34,083.00 +/- 33,732.82). Present results provide a better understanding of the life cycle of S. mansoni, are of theoretical significance for theories of biased sex ratios (which at present cannot account for the male-biased ratio of S. mansoni), and also suggest that schistosomiasis transmission models assuming a 50:50 sex ratio at all stages of the life cycle should be reassessed.     

Resistance to desiccation of Biomphalaria glabrata adults infested by various miracidia of Schistosoma mansoni

1200 adult Biomphalaria glabrata were submitted during 6 weeks to anhydrobiosis condition. Some snails were healthy, some were previously infected 3 days or 12 days ago with 8 +/- 2 miracidia of Schistosoma mansoni, others were shedding cercariae. The snails were put on soil or buried into hermetically closed, or ventilated, plastic boxes. There was no survival of snails kept in sealed boxes, or among positive snails, but 44% of control healthy snails and 40.6% of infected (for 3 or 12 days) snails in ventilated boxes were living at the term of the desiccation stage. Survival was better for "on soil" snails than for "buried" snails, but no difference was shown between 3-days and 12-days infection. The surviving desiccated B. glabrata had a lesser death rate and a lesser cercarial production than infected snails kept in water. An inferior production of male cercariae comparatively to female and to "mixed" cercariae was demonstrated by statistical analysis of the cercarial sheddings. In all positive snails, periodic variations of cercarial production was shown, whatever the sex of those cercariae. In addition many pauses of the sheddings were established by the authors.     

The stress of Lymnaea truncatula just before miracidial exposure with Fasciola hepatica increased the prevalence of infection.

Single-miracidium infections of Lymnaea truncatula with Fasciola hepatica were carried out under laboratory conditions to determine whether the stress of snails just before miracidial exposure had any influence on the prevalence of Fasciola infection, redial burden, and cercarial shedding. Three methods, i.e., the fasting of L. truncatula for 3 days in water filtered through a Millipore membrane, the effect of 6-8 degrees C water for 15 min, or the immersion of L. truncatula in a detergent solution at low concentration for 15 min, were used to stress snails. Enhanced susceptibility of snails to F. hepatica infection was noted in stressed groups (93-96% vs 48-50% in controls). The number of free rediae did not show any variation in controls as well as in stressed groups, except for fasted snails in which free rediae were significantly fewer. No differences in cercarial production between controls and the cold group were noted. Fasting, cold shock, or detergent exposure prior to exposure to F. hepatica miracidia might have weakened the snails so that they were not as efficient in avoiding miracidial penetration, thus leading to higher infection rates.     

Studies of the relationships between Schistosoma nasale and S. spindale and their snail host Indoplanorbis exustus

Infectivity and cercarial production of Indoplanorbis exustus related to variation of miracidial dose (1, 4, 10 or 20) with Schistosoma nasale and S. spindale from Sri Lanka were studied. The intermediate host-parasite relationships of the two schistosome species showed marked differences under the conditions of observation recorded in this study. Prepatent death rates (PDR) were on average higher for S. spindale (30%) than for S. nasale (10%). The size of the miracidial dose to which snails had been exposed had no effect on PDR. The infection rates (IR) were on average higher for S. nasale (41%) compared with S. spindale (27%). Highest IR occurred after exposure to 4 miracidia in S. nasale infections (79%) and after exposure to 10 miracidia in S. spindale infections (60%). The highest daily average cercarial production per snail was recorded for S. nasale at a level of 4 miracidia (1311), and for S. spindale at a level of 10 miracidia (1615). At low level (1 or 4 miracidia) of exposure, I. exustus showed a better compatibility with S. nasale than with S. spindale. An opposite tendency was observed at higher levels (10 or 20 miracidia) of exposure. Unsuccessful infections of Lymnaea luteola with either S. nasale or S. spindale indicate that this species is not involved in transmission.     

Morpho-anatomic and functional sectorization of Schistosoma mansoni daughter sporocysts

During the larval development of S. mansoni in the snail host, morpho-anatomic changes occur in the daughter sporocyst by a sectorization of this larval stage. Three sectors can be distinguished: an anterior zone with a well-differentiated birth pore; dilated zones containing the developing cercariae; constricted zones without cercarial embryo. The photonic and electronic microscopical study shows variations in the tegumental structure of these sectors. This evolution of the daughter sporocysts is discussed in relation with the dynamics of larval stages and the replication process of sporocysts.     

Re-establishing a life cycle of Schistosoma mansoni from cryopreserved larvae

To study the feasibility of re-establishing a life cycle of Schistosoma mansoni (NMRI strain) from cryopreserved larvae, schistosomules were suspended in the cryoprotectant 1,2-ethanediol and cryopreserved in liquid nitrogen. Mice were injected intramuscularly with samples thawed after 3 days, 3 wk, or 6 mo in liquid nitrogen storage. Two to 5% of the cryopreserved larvae and approximately 18% of corresponding unfrozen control larvae developed into adult worms. Infectivity did not decrease as a function of storage time. The adult worms showed no structural damage or changes in overall size and morphology when examined by light and transmission electron microscopy. Female worms derived from cryopreserved larvae had the same or slightly elevated egg production as controls, but tissue egg distributions were comparable. Subsequent passages through Biomphalaria glabrata snails and mice revealed no difference in snail prepatent death rate, percentage of snails infected, cercarial production per snail, or cercarial infectivity.     

Influence of the nontarget mollusc Marisa cornuarietis on the hourly cercarial production of Schistosoma mansoni from Biomphalaria glabrata

A comparative study of hourly cercarial productivities of Schistosoma mansoni from infected Biomphalaria glabrata was carried out in the presence of either healthy B. glabrata (control) or healthy Marisa cornuarietis (experimental). The results showed that, with M. cornuarietis, almost all the hourly cercarial productivities increased by a factor varying from 1.3 to 2.5 without modification of the shedding period.