Life-history traits indicate local adaptation of the schistosome parasite, Schistosoma mansoni, to its snail host, Biomphalaria pfeifferi

Human-water interactions associated to cercarial emergence pattern and their influences on urinary schistosomiasis transmission in two endemic areas in Mali

BACKGROUND

Mali is known to be a schistosomiasis-endemic country with a limited supply of clean water. This has forced many communities to rely on open freshwater bodies for many human-water contact (HWC) activities. However, the relationship between contact with these water systems and the level of schistosome infection is currently receiving limited attention. This study assessed human-water interactions including cercarial emergence pattern and their influences on urinary schistosomiasis transmission in two communities in the Kayes district of Mali.

METHODS

We carried out a parasitological study first in children in September 2021, then a cross-sectional study of quantitative observations of human-water contact activities in the population, and finally a study of snail infectivity at contact points in September 2022. The study took place in two communities, Fangouné Bamanan and Diakalèl in the Kayes region of western Mali. The chronobiological study focused on cercarial release from naturally infected snails. Released cercariae were molecularly genotyped by targeting the cox1 region, and the ITS and 18S ribosmal DNA gene (18S rDNA) regions of the DNA. Links between sociodemographic parameters, human water-contact points and hematuria were established using multivariate statistical analysis or the logistic regression model.

RESULTS

The main factor predisposing the 97 participants to water contact was domestic activity (62.9%). Of the 378 snails collected at 14 sampling sites, 27 (7.1%) excreted schistosome cercariae, with 15.0% (19/126) at Fangouné Bamanan and 3.3% (8/252) at Diakalel. The release of Schistosoma cercariae shows three different patterns in Fangouné Bamanan: (i) an early release peak (6:00-8:00 AM), (ii) a mid-day release peak (10:00 AM-12:00 PM) and (iii) a double peak: (6:00-8:00 AM) and (6:00-8:00 PM) cercariae release; and two release patterns in Diakalel: early release (6:00-8:00 AM) and (ii) mid-day release (12:00-2:00 PM). All cercariae released during early diurnal (6:00-8:00 AM) or nocturnal emission patterns (6:00-8:00 PM) were hybrids parasite having an cox1 S. bovis or S. curassoni associated with an ITS and 18S rDNA of S. haematobium while the cercariae released during diurnal, or mid-day patterns (8:00 AM-6:00 PM) were pure S. haematobium.

CONCLUSIONS

Our study showed that domestic activity is the main source of exposure in the Kayes region. Two and three cercariae emission patterns were observed at Diakalel and Fangouné Bamanan respectively. These results suggest that the parasite adapts to the human-water contact period in order to increase its infectivity.

Impact of Schistosoma sp., Infection on Biological, Feeding, Physiological, Histological, and Genotoxicological Aspects of Biomphalaria alexandrina and Bulinus truncatus Snails

BACKGROUND

Trematode infections of the genus Schistosoma can induce physiological and behavioral changes in intermediate snail hosts. This is because the parasite consumes essential resources necessary for the host's survival, prompting hosts to adapt their behavior to maintain some level of fitness before parasite-induced mortality occurs.

METHODS

In this study, the reproductive and biochemical parameters of Biomphalaria alexandrina and Bulinus truncatus were examined during the cercareal shedding stage of infection with Schistosoma mansoni and Schistosoma haematobium, respectively, compared with controls.

RESULTS

The study revealed an infection rate of 34.7% for S. mansoni and 30.4% for S. haematobium. In B. alexandrina infected with S. mansoni, a survival rate of 65.2% was recorded, along with a mean prepatent period of 30.3 ± 1.41 days, a mean shedding duration of 14.2 ± 0.16 days, and a mean lifespan of 44.1 ± 0.24 days. Meanwhile, in B. truncatus infected with S. haematobium, a survival rate of 56.4% was observed, with a mean prepatent period of 44.3 ± 1.41 days, a mean shedding duration of 22.6 ± 2.7 days, and a mean lifespan of 66.9 ± 1.6 days. Feeding increased in both infected species of snails, while the net reproductive rate (Ro) of the infected snails decreased. Total antioxidant (TAO) and lipid peroxidation activity increased in the two infected snail species during shedding, while Glutathione-S-transferase levels decreased. Lipid peroxidase activity and nitrogen oxide levels significantly decreased in infected B. alexandrina and increased in infected Bulinus. Steroid hormone levels were elevated in infected Biomphalaria, whereas they were reduced in infected Bulinus. Comet assay parameters showed an increase in the two infected genera after infection compared to control snails, indicating genotoxic damage and histopathological damage was observed.

CONCLUSIONS

These findings demonstrate that infection with larva species diverse biochemical, hormonal, genotoxic, and histopathological changes in the tissues responsible for fecundity and reproduction in B. alexandrina and B. truncates comparing with controls.

Comparative Vectorial Competence of Biomphalaria sudanica and Biomphalaria choanomphala, Snail Hosts of Schistosoma mansoni, From Transmission Hotspots In Lake Victoria, Western Kenya

Schistosoma mansoni, which causes human intestinal schistosomiasis, continues to be a major public health concern in the Lake Victoria basin in western Kenya, with Biomphalaria sudanica (a shoreline inhabiting snail) and Biomphalaria choanomphala (a deep-water snail) playing roles in transmission. A recent study showed that B. sudanica was abundantly present near all study villages on the lakeshore, but B. choanomphala was significantly more abundant near villages known to be persistent transmission hotspots. The present study investigated the relative compatibility of B. sudanica and B. choanomphala with S. mansoni. A reciprocal cross-infection experiment used young adult F1 generation B. sudanica and B. choanomphala that were exposed to either 1, 5, or 10 sympatric or allopatric human-derived S. mansoni miracidia. Three weeks post-exposure (PE) and weekly thereafter, the snails were counted and screened for schistosome cercariae, and at 7 wk PE, total cercariae shed during a 2 hr period by each infected snail was determined. Pre-patent periods for S. mansoni in both B. sudanica and B. choanomphala were similar, and most snails in all exposure combinations started shedding cercariae 5 wk PE. Prevalences were significantly higher in B. choanomphala (12.2-80.9%) than in B. sudanica (5.2-18.6%) at each dose, regardless of whether miracidia were of an allopatric or a sympatric source (P < 0.0001). Overall, the odds of a snail becoming infected with 5 or 10 miracidia were significantly higher than the odds of being infected with 1 miracidium, (P < 0.0001), and fewer cercariae were produced by snails exposed to single as compared to 5 or 10 miracidia. On average, B. choanomphala produced more cercariae ( = 458, SD = 414) than B. sudanica ( = 238, SD = 208) (P < 0.0001). These results suggest that B. choanomphala is more compatible with S. mansoni than B. sudanica. Though B. choanomphala can be found in shallow shoreline waters, it is, for the most part, a deeper-water taxon. Because dredging is a relatively inefficient means of sampling, B. choanomphala is likely underestimated with respect to its population size, the number of S. mansoni-positive snails, and its role in maintaining transmission.

Patterns of Sphaeridiotrema pseudoglobulus infection in sympatric and allopatric hosts (Bithynia tentaculata) originating from widely separated sites across the USA

In circumstances where populations of invasive species occur across variable landscapes, interactions among invaders, their parasites, and the surrounding environment may establish local coevolutionary trajectories for the participants. This can generate variable infection patterns when parasites interact with sympatric versus allopatric hosts. Identifying the potential for such patterns within an invasive-species framework is important for better predicting local infection outcomes and their subsequent impacts on the surrounding native community. To begin addressing this question, we exposed an invasive snail (Bithynia tentaculata) from two widely separated sites across the USA (Wisconsin and Montana) to the digenean parasite, Sphaeridiotrema pseudoglobulus, collected from Wisconsin. Parasite exposures generated high infection prevalences in both sympatric and allopatric snails. Furthermore, host survival, host growth, the proportion of patent snails, and the timing of patency did not differ between sympatric and allopatric combinations. Moreover, passaging parasites through snails of different origins had no effect on transmission success to subsequent hosts in the life cycle. However, the number of parasites emerging from snails and the pattern of their release varied based on snail origin. These latter observations suggest the potential for local adaptation in this system, but subsequent research is required to further substantiate this as a key factor underlying infection patterns in the association between S. pseudoglobulus and B. tentaculata.

Consistency of trematode infection prevalence in host populations across large spatial and temporal scales

Parasites can impart heavy fitness costs on their hosts. Thus, understanding the spatial and temporal consistency in parasite pressure can elucidate the likeliness of parasites' role as agents of directional selection, as well as revealing variable environmental factors associated with infection risk. We examined spatiotemporal variation in digenetic trematode infection in 18 populations of an intertidal host snail (Littorina littorea) over a 300 km range at an 11-yr interval, more than double the generation time of the snail. Despite a complete turnover in the snail host population, the average change in infection prevalence among populations was <1% over the 11-yr span, and all but three populations remained within 5 percentage points. This consistency of prevalence in each population over time suggests remarkable spatiotemporal constancy in parasite delivery vectors in this system, notably gulls that serve as definitive hosts for the parasites. Thus, despite gulls' high mobility, their habitat usage patterns are ostensibly relatively fixed in space. Importantly, this spatiotemporal consistency also implies that sites where parasites are recruitment limited remain so over time, and likewise, that parasite hotspots stay hot.

Bioenergetic theory predicts infection dynamics of human schistosomes in intermediate host snails across ecological gradients

Epidemiological dynamics depend on the traits of hosts and parasites, but hosts and parasites are heterogeneous entities that exist in dynamic environments. Resource availability is a particularly dynamic and potent environmental driver of within-host infection dynamics (temporal patterns of growth, reproduction, parasite production and survival). We developed, parameterised and validated a model for resource-explicit infection dynamics by incorporating a parasitism module into dynamic energy budget theory. The model mechanistically explained the dynamic multivariate responses of the human parasite Schistosoma mansoni and its intermediate host snail to variation in resources and host density. At the population level, feedbacks mediated by resource competition could create a unimodal relationship between snail density and human risk of exposure to schistosomes. Consequently, weak snail control could backfire if reductions in snail density release remaining hosts from resource competition. If resource competition is strong and relevant to schistosome production in nature, it could inform control strategies.

A Comparison of Kenyan Biomphalaria pfeifferi and B. Sudanica as Vectors for Schistosoma mansoni, Including a Discussion of the Need to Better Understand the Effects of Snail Breeding Systems on Transmission

In Kenya, schistosomes infect an estimated 6 million people with >30 million people at risk of infection. We compared compatibility with, and ability to support and perpetuate, Schistosoma mansoni of Biomphalaria pfeifferi and Biomphalaria sudanica, 2 prominent freshwater snail species involved in schistosomiasis transmission in Kenya. Field-derived B. pfeifferi (from a stream in Mwea, central Kenya) and B. sudanica (from Nawa, Lake Victoria, in western Kenya) were exposed to S. mansoni miracidia isolated from fecal samples of naturally infected humans from Mwea or Nawa. Juvenile (<6 mm shell diameter), young adult (6-9 mm), and adult snails (>9 mm) were each exposed to a single miracidium. Schistosoma mansoni developed faster and consistently had higher infection rates (39.6-80.7%) in B. pfeifferi than in B. sudanica (2.4-21.5%), regardless of the source of S. mansoni or the size of the snails used. Schistosoma mansoni from Nawa produced higher infection rates in both B. pfeifferi and B. sudanica than did S. mansoni from Mwea. Mean daily cercariae production was greater for B. pfeifferi exposed to sympatric than allopatric S. mansoni (583-1,686 vs. 392-1,232), and mean daily cercariae production among B. sudanica were consistently low (50-590) with no significant differences between sympatric or allopatric combinations. Both non-miracidia-exposed and miracidia-exposed B. pfeifferi had higher mortality rates than for B. sudanica, but mean survival time of shedding snails (9.3-13.7 wk) did not differ significantly between the 2 species. A small proportion (1.5%) of the cercariae shedding B. pfeifferi survived up to 40 wk post-exposure. Biomphalaria pfeifferi was more likely to become infected and to shed more cercariae than B. sudanica, suggesting that the risk per individual snail of perpetuating transmission in Kenyan streams or lacustrine habitats may differ considerably. High infection rates exhibited by the preferential self-fertilizing B. pfeifferi relative to the out-crossing B. sudanica point to the need to investigate further the role of host breeding systems in influencing transmission of schistosomiasis by snail hosts.

Transcriptomic responses of Biomphalaria pfeifferi to Schistosoma mansoni: Investigation of a neglected African snail that supports more S. mansoni transmission than any other snail species

BACKGROUND

Biomphalaria pfeifferi is highly compatible with the widespread human-infecting blood fluke Schistosoma mansoni and transmits more cases of this parasite to people than any other snail species. For these reasons, B. pfeifferi is the world's most important vector snail for S. mansoni, yet we know relatively little at the molecular level regarding the interactions between B. pfeifferi and S. mansoni from early-stage sporocyst transformation to the development of cercariae.

METHODOLOGY/PRINCIPAL FINDINGS

We sought to capture a portrait of the response of B. pfeifferi to S. mansoni as it occurs in nature by undertaking Illumina dual RNA-Seq on uninfected control B. pfeifferi and three intramolluscan developmental stages (1- and 3-days post infection and patent, cercariae-producing infections) using field-derived west Kenyan specimens. A high-quality, well-annotated de novo B. pfeifferi transcriptome was assembled from over a half billion non-S. mansoni paired-end reads. Reads associated with potential symbionts were noted. Some infected snails yielded fewer normalized S. mansoni reads and showed different patterns of transcriptional response than others, an indication that the ability of field-derived snails to support and respond to infection is variable. Alterations in transcripts associated with reproduction were noted, including for the oviposition-related hormone ovipostatin and enzymes involved in metabolism of bioactive amines like dopamine or serotonin. Shedding snails exhibited responses consistent with the need for tissue repair. Both generalized stress and immune factors immune factors (VIgLs, PGRPs, BGBPs, complement C1q-like, chitinases) exhibited complex transcriptional responses in this compatible host-parasite system.

SIGNIFICANCE

This study provides for the first time a large sequence data set to help in interpreting the important vector role of the neglected snail B. pfeifferi in transmission of S. mansoni, including with an emphasis on more natural, field-derived specimens. We have identified B. pfeifferi targets particularly responsive during infection that enable further dissection of the functional role of these candidate molecules.

Relative compatibility of Schistosoma mansoni with Biomphalaria sudanica and B. pfeifferi from Kenya as assessed by PCR amplification of the S. mansoni ND5 gene in conjunction with traditional methods

Background

Schistosoma mansoni is hosted by several species of Biomphalaria spp. snails in Africa. We were interested in determining if there were differences in compatibility of S. mansoni with Biomphalaria sudanica from Lake Victoria, or with B. pfeifferi from streams and smaller water bodies in Kenya. Does this parasite develop with equal efficiency in both snail species, and does this have implications for transmission in different habitat types?

Methods

Primers for PCR amplification of the S. mansoni ND5 gene were designed and tested for sensitivity and specificity. We exposed laboratory-reared B. sudanica and field-derived B. pfeifferi to single miracidium infections and at 1, 2, 4, 8, 16 and 24 days post-exposure (dpe), snails were extracted for the PCR assay. Snails were also shed for cercariae and/or dissected prior to extraction. Additionally, B. sudanica and B. pfeifferi were collected from field locations and tested with the PCR assay.

Results

The ND5 PCR assay was sensitive (>0.1 fg S. mansoni genomic DNA) and allowed S. mansoni to be differentiated from other relevant schistosome species or snails. The number of PCR positive snails at 1–4 dpe was higher for B. pfeifferi than for B. sudanica, but not significantly so (P = 0.052). From 8–24 dpe, more B. pfeifferi harbored successfully developing parasites (positive by both dissection and PCR) than did B. sudanica (P = 0.008). At 40 dpe, more B. pfeifferi than B. sudanica shed cercariae or harbored dissection positive/PCR positive infections (P < 0.001). Both immature and failed (dissection negative but PCR positive) S. mansoni infections could also be detected in naturally infected snails of both species.

Conclusions

The PCR assay detected S. mansoni infections in snails exposed to one miracidium for one day. Both B. sudanica and B. pfeifferi supported full development of S. mansoni, but B. pfeifferi was more compatible, with significantly more dissection positive/PCR positive or shedding infections, and significantly fewer failed infections (dissection negative/PCR positive). This highlights the relatively lower compatibility of B. sudanica with S. mansoni, and suggests the factors responsible for incompatibility and how they might affect transmission of S. mansoni in habitats like Lake Victoria deserve additional study.

Experimental infection of the digeneans to some congeneric snail species radiated in a single water system: Relative importance of local evolution and phylogenetic constraint

To determine the relative importance of local adaptation caused by host-parasite coevolution and resource tracking by the parasites, the susceptibility of the freshwater snail genus Semisulcospira to the digenean parasite genus Genarchopsis was investigated experimentally. Four snail species endemic to the Lake Biwa system in Japan and two non-endemic species were investigated. All but one species was also tested for local variation in susceptibility. Parasites were obtained from Takashima (mix population of Genarchopsis gigi and Genarchopsis chubuensis) and Nagahama (G. chubuensis). In endemic Semisulcospira, closely related specie pairs (Semisulcospira habei and Semisulcospira niponica, Semisulcospira decipiens and Semisulcospira nakasekoae) showed similar susceptibilities to parasites from both localities. S. habei and S. niponica were highly susceptible to parasites from Takashima, but were resistant to parasites from Nagahama. S. decipiens and S. nakasekoae showed moderate susceptibility to parasites from both localities. None of the endemic snail species showed a clear local variation in susceptibility. These results show that the susceptibility of endemic Semisulcospira to Genarchopsis is conservative and can be regarded as an example of resource-tracking. One of the non-endemic snails, Semisulcospira libertina, showed local variation in susceptibility. This variation was not related to the sympatry of the parasites used for the experimental infection, suggesting that it was not the result of local adaptation by parasites.

Field-derived Schistosoma mansoni and Biomphalaria pfeifferi in Kenya: a compatible association characterized by lack of strong local adaptation, and presence of some snails able to persistently produce cercariae for over a year

Background

Schistosoma mansoni is widely distributed in sub-Saharan Africa with Biomphalaria pfeifferi being its most widespread and important snail intermediate host. Few studies have examined the compatibility of field-derived B. pfeifferi snails with S. mansoni miracidia derived from human hosts. We investigated compatibility (as defined by shedding of cercariae following exposure to miracidia) of two isolates of S. mansoni from school children from Asao (western Kenya) and Mwea (central Kenya) with B. pfeifferi collected directly from Asao stream or the Mwea rice fields.

Methods

We exposed snails from both regions to four different doses of miracidia (1, 5, 10 and 25) from sympatric or allopatric S. mansoni, and maintained them in a shaded, screened out-of-doors rearing facility in Kisian, in western Kenya. Both snail survival and the number of snails that became infected were monitored weekly. This was done for 25 weeks post-exposure (PE). Those infected snails which survived beyond this period were monitored until they all died.

Results

Although overall survival of Mwea snails maintained in western Kenya was generally low, both sympatric and allopatric combinations of parasites and snails exhibited high compatibility (approximately 50% at a dose of one miracidium per snail), with an increase in infection rates as the miracidial dose was increased (P < 0.002). Schistosomes were no more compatible with sympatric than allopatric snails, nor were snails less compatible with sympatric than allopatric schistosomes. Snail mortality increased significantly with dose of miracidia (P < 0.05). Approximately 3% of Asao snails exposed to a low dose of sympatric miracidia (1 or 5) continued to shed cercariae for as long as 58 weeks post exposure.

Conclusions

There were no significant local adaptation effects for either schistosomes or snails. Also, the existence of “super-survivor” snails is noteworthy for its implications for current control initiatives that mostly rely on mass drug administration (MDA). Long-term shedders could provide an ongoing source of cercariae to initiate human infections for many months, suggesting care is required in considering how human MDA treatments are timed. Future control programs should incorporate means to eliminate infected snails to complement chemotherapy interventions in controlling schistosomiasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0533-3) contains supplementary material, which is available to authorized users.