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BackgroundThe full scope of the genes expressed by schistosomes during intramolluscan development has yet to be characterized. Understanding the gene products deployed by larval schistosomes in their snail hosts will provide insights into their establishment, maintenance, asexual reproduction, ability to castrate their hosts, and their prolific production of human-infective cercariae. Using the Illumina platform, the intramolluscan transcriptome ofSchistosoma mansoniwas investigated in field-derived specimens of the prominent vector speciesBiomphalaria pfeifferiat 1 and 3 days post infection (d) and from snails shedding cercariae. TheseS. mansonisamples were derived from the same snails used in our complementaryB. pfeifferitranscriptomic study. We supplemented this view with microarray analyses ofS. mansonifromB. glabrataat 2d, 4d, 8d, 16d, and 32d.Principal FindingsTranscripts representing at least 7,740 (66%) of knownS. mansonigenes were expressed during intramolluscan development, with the greatest number expressed in snails shedding cercariae. Many transcripts were constitutively expressed throughout development featuring membrane transporters, and metabolic enzymes involved in protein and nucleic acid synthesis and cell division. Several proteases and protease inhibitors were expressed at all stages, including some proteases usually associated with cercariae. Transcripts associated with G-protein coupled receptors, germ cell perpetuation, and stress responses and defense were well represented. We noted transcripts homologous to planarian anti-bacterial factors, several neural development or neuropeptide transcripts including neuropeptide Y, and receptors that may be associated with schistosome germinal cell maintenance and that could also impact host reproduction. In at least one snail the presence of larvae of another digenean species (an amphistome) was associated with repressedS. mansonitranscriptional activity.Conclusions/SignificanceThisin vivostudy, particularly featuring field-derived snails and schistosomes, provides a distinct view from previous studies of development of cultured intramolluscan stages from lab-maintained organisms. We found many highly represented transcripts with suspected or unknown functions, with connection to intramolluscan development yet to be elucidated.AUTHOR SUMMARYSchistosoma mansoniis one of the most important schistosome species causing the neglected tropical disease human intestinal schistosomiasis. By focusing onS. mansoni in vivowith its broadly distributed sub-Saharan African snail intermediate host,Biomphalaria pfeifferi, we uncover new insights and basic knowledge of this host-parasite relationship that are critical for understanding schistosomiasis transmission. We show thatin vivostudies, particularly using field-derived specimens, provides a distinct view from the uniformed transcriptional responses traditionally seen fromin vitrostudies onS. mansoniandBiomphalariasnails. With the growing consensus that we need to supplement chemotherapy with other control methods, understanding howS. mansoniinteracts with its obligatory snail host becomes integral for future planning of control programs. The data provided within provides specific analysis on how the schistosomes successfully protect themselves from host defenses and the necessary transcriptional responses required for its amplifying asexual proliferation that result in human-infective cercariae.

BackgroundSchistosomiasis is one of the world’s most common NTDs. Successful control operations often target snail vectors with the molluscicide niclosamide. Little is known about how niclosamide affects snails, including forBiomphalaria pfeifferi, the most important vector forSchistosoma mansoniin Africa. We used Illumina technology to explore how field-derivedB. pfeifferi, either uninfected or harboring cercariae–producingS. mansonisporocysts, respond to a sublethal exposure of niclosamide. This study afforded the opportunity to determine if snails respond differently to biotic or abiotic stressors, and if they reserve unique responses for when presented with both stressors in combination. We also examined how sporocysts respond when their snail host is exposed to niclosamide.Principal FindingsCercariae-producing sporocysts within snails exposed to niclosamide express ~68% of the genes in theS. mansonigenome, as compared to 66% expressed by intramolluscan stages ofS. mansoniin snails not exposed to niclosamide. Niclosamide does not disable sporocysts nor does it seem to provoke from them distinctive responses associated with detoxifying a xenobiotic. ForB. pfeifferi, niclosamide treatment alone increases expression of several features not up-regulated in infected snails including particular cytochrome p450s and heat shock proteins, glutathione-S-transferases, antimicrobial factors like LBP/BPI and protease inhibitors, and also provokes strong down regulation of proteases. Exposure of infected snails to niclosamide resulted in numerous up-regulated responses associated with apoptosis along with down-regulated ribosomal and defense functions, indicative of a distinctive, compromised state not achieved with either stimulus alone.Conclusions/SignificanceThis study helps define the transcriptomic responses of an important and under-studied schistosome vector toS. mansonisporocysts, to niclosamide, and to both in combination. It suggests the response ofS. mansonisporocysts to niclosamide is minimal and not reflective of a distinct repertoire of genes to handle xenobiotics while in the snail host. It also offers new insights for how niclosamide affects snails.Author’S SummarySchistosomaisis control programs often employ the use of chemical molluscicides, such as niclosamide, to control the obligatory intermediate snail hosts. Despite its widespread use, we know little about how niclosamide affects snails likeBiomphalaria pfeifferi, the most important vectorSchistosoma mansoniin Africa. By sequencing the transcriptomes of uninfected andS. mansoni-infectedB. pfeifferiexposed to niclosamide, we analyze the snail’s response to both biotic and abiotic stressors. We can also examine the response ofS. mansonito niclosamide exposure during intramolluscan development.Biomphalaria pfeifferisnails exposed only to niclosamide showed unique up-regulation of stress and defense-related transcripts not seen in snails infected with a biotic, likeS. mansoniinfection, alone.Schistosoma mansoni-infectedB. pfeifferiexposed to niclosamide were clearly unable to regulate normal metabolic and detoxification processes. Cercariae-producing sporocysts within snails exposed to niclosamide are largely unaffected and continue to produce transcripts required for cercariae production.

Following a four-year annual praziquantel treatment campaign the resulting prevalence ofS. mansoniwas seen to differ among individual villages along the Kenyan shore of Lake Victoria. We have investigated possible inherent differences in snail-related aspects of transmission among such 10 villages, including six persistent hotspot (PHS) villages (≤30% reduction in prevalence following repeated treatments) located along the west-facing shore of the lake, and four PZQ-responding (RESP) villages (>30% prevalence reduction following repeated treatment) along Winam Gulf. When taking into account all sampling sites and times and water hyacinth presence/absence, shoreline-associatedB. sudanicafrom PHS and RESP villages did not differ in relative abundance or prevalence ofS. mansoniinfection. Water hyacinth intrusions were associated with increasedB. sudanicaabundance. The deeper water snailBiomphalaria choanomphalawas significantly more abundant in the PHS villages and prevalence ofS. mansoniamong villages both before and after control was positively correlated withB. choanomphalaabundance. Worm recoveries from sentinel mice did not differ between PHS and RESP villages, and abundance of non-schistosome trematode species was not associated withS. mansoniabundance.Biomphalaria choanomphalaprovides an alternative, deepwater mode of transmission that may favor greater persistence ofS. mansoniin PHS villages. As we found evidence for ongoingS. mansonitransmission in all 10 villages, we conclude conditions conducive for transmission and reinfection occur ubiquitously. This argues for an integrated, basin-wide plan for schistosomiasis control to counteract rapid reinfections facilitated by large snail populations and movements of infected people around the lake.