Molecular ecology of Schistosoma mansoni transmission inferred from the genetic composition of larval and adult infrapopulations within intermediate and definitive hosts

Trematode genetic patterns at host individual and population scales provide insights about infection mechanisms

Multiple parasites can infect a single host, creating a dynamic environment where each parasite must compete over host resources. Such interactions can cause greater harm to the host than single infections and can also have negative consequences for the parasites themselves. In their first intermediate hosts, trematodes multiply asexually and can eventually reach up to 20% of the host's biomass. In most species, it is unclear whether this biomass results from a single infection or co-infection by 2 or more infective stages (miracidia), the latter being more likely a priori in areas where prevalence of infection is high. Using as model system the trematode Bucephalus minimus and its first intermediate host cockles, we examined the genetic diversity of the cytochrome c oxidase subunit I region in B. minimus from 3 distinct geographical areas and performed a phylogeographic study of B. minimus populations along the Northeast Atlantic coast. Within localities, the high genetic variability found across trematodes infecting different individual cockles, compared to the absence of variability within the same host, suggests that infections could be generally originating from a single miracidium. On a large spatial scale, we uncovered significant population structure of B. minimus, specifically between the north and south of Bay of Biscay. Although other explanations are possible, we suggest this pattern may be driven by the population structure of the final host.

Different trematode parasites in the same snail host: Species-specific or shared microbiota?

The concept that microbes associated with macroorganisms evolve as a unit has swept evolutionary ecology. However, this idea is controversial due to factors such as imperfect vertical transmission of microbial lineages and high microbiome variability among conspecific individuals of the same population. Here, we tested several predictions regarding the microbiota of four trematodes (Galactosomum otepotiense, Philophthalmus attenuatus, Acanthoparyphium sp. and Maritrema novaezealandense) that parasitize the same snail host population. We predicted that each parasite species would harbour a distinct microbiota, with microbial composition similarity decreasing with increasing phylogenetic distance among parasite species. We also predicted that trematode species co-infecting the same individual host would influence each other's microbiota. We detected significant differences in alpha and beta diversity, as well as differential abundance, in the microbiota of the four trematode species. We found no evidence that phylogenetically closely related trematodes had more similar microbiota. We also uncovered indicator bacterial taxa that were significantly associated with each trematode species. Trematode species sharing the same snail host showed evidence of mostly one-sided bacterial exchanges, with the microbial community of one species approaching that of the other. We hypothesize that natural selection acting on specific microbial lineages may be important to maintain differences in horizontally acquired microbes, with vertical transmission also playing a role. In particular, one trematode species had a more consistent and diverse bacteriota than the others, potentially a result of stronger stabilizing pressures. We conclude that species-specific processes shape microbial community assembly in different trematodes exploiting the same host population.

Atypical life cycle does not lead to inbreeding or selfing in parasites despite clonemate accumulation in intermediate hosts

Many parasites utilize asexual and sexual reproduction and multiple hosts to complete their life cycles. How these taxa avoid inbreeding is an essential question for understanding parasite evolution and ecology. Aquatic trematodes that require multiple host species may benefit from diverse genetic parasite assemblages accumulating within second intermediate hosts prior to sexual reproduction in definitive hosts. However, Cotylurus species are able to utilize the same snail species as first and second intermediate hosts, potentially resulting in the accumulation of genetically identical clones (clonemates) prior to sexual reproduction. In this study, we developed and analysed novel microsatellite loci to determine if clones are accumulating within snail hosts prior to ingestion by bird hosts and the effects this could have on parasite inbreeding. Contrary to previous studies of aquatic trematodes, significantly large numbers of clonemates were present within snails, but full-sibs were not. Genetic structure was present over a relatively small geographical scale despite the use of vagile definitive hosts. Phylogenetic analysis identified the Cotylurus sp. clones as belonging to a single species. Despite the presence of clones within snails, mating between clones/selfing was not common and heterozygosity is maintained within individuals. Potential issues with clones mating may be mitigated by the presence of snails with numerous clones, the consumption of many snails by bird hosts and parasite clone recognition/avoidance. Use of the same host species for multiple life stages may have advantages when parasites are able to avoid inbreeding and the required hosts are common.

Schistosoma mansoni Epidemiology Among Snails, Rodents and Children: A One Health Approach

Background

Schistosoma is one of the prevalent parasitic infection in humans and animals. Schistosomiasis in children is particularly serious and results in liver and spleen enlargement, anemia, stunting, reduced ability to learn and death. The aim of this study was to measure the prevalence and distribution of schistosomiasis in children, rodents and snail populations in Aleffa and Takusa districts, north-west Ethiopia.

Methods

Disease status and exposure were simultaneously measured from December 2020 to December 2021. School children's stool specimens were collected for schistosomiasis examination by Kato Katz and formal-ether techniques. Live rodents and snails were trapped and collected in search of adult schistosoma, eggs, and cercariae, respectively. Multiple logistic regression analytic technique by using SPSS version 20 was conducted.

Results

Of 460 stool specimens examined, 116 (25.22) were found positive for S. mansoni infection. In the present finding, the possible determinants for the occurrence of intestinal schistosomiasis in school children were female sex (AOR = 1.09, 95% CI: 1.37–2.96); working with bare foot (AOR = 1.21, 95% CI: 1.08–1.52); skin cut/abrasion history (AOR = 3.6, 95% CI: 1.08–7.43) and swimming habit (AOR = 1.58, 95% CI: 1.33–1.99). The overall (n = 108) rodent prevalence of S. mansoniwas 23 (21.3%). Only 6 snails were shedding the infective stage of schistosoma cercariae.

Conclusion

The study revealed that there is a moderate prevalence of schistosomiasis in different hosts. Thus, multi-host intervention is crucial to achieving the goal of interrupting transmission of schistosomiasis in the study area. Further research to better understand and exploit the broader environmental, ecological context and encompassing dynamic interactions between all hosts over time will be crucial for building predictive models beyond the known fact of having or not having reservoirs/hybridization of schistosoma in our study area.

A review of the genetic determinants of praziquantel resistance in Schistosoma mansoni: Is praziquantel and intestinal schistosomiasis a perfect match?

Schistosomiasis is a neglected tropical disease (NTD) caused by parasitic trematodes belonging to the Schistosoma genus. The mainstay of schistosomiasis control is the delivery of a single dose of praziquantel (PZQ) through mass drug administration (MDA) programs. These programs have been successful in reducing the prevalence and intensity of infections. Due to the success of MDA programs, the disease has recently been targeted for elimination as a public health problem in some endemic settings. The new World Health Organization (WHO) treatment guidelines aim to provide equitable access to PZQ for individuals above two years old in targeted areas. The scale up of MDA programs may heighten the drug selection pressures on Schistosoma parasites, which could lead to the emergence of PZQ resistant schistosomes. The reliance on a single drug to treat a disease of this magnitude is worrying should drug resistance develop. Therefore, there is a need to detect and track resistant schistosomes to counteract the threat of drug resistance to the WHO 2030 NTD roadmap targets. Until recently, drug resistance studies have been hindered by the lack of molecular markers associated with PZQ resistance. This review discusses recent significant advances in understanding the molecular basis of PZQ action in S. mansoni and proposes additional genetic determinants associated with PZQ resistance. PZQ resistance will also be analyzed in the context of alternative factors that may decrease efficacy within endemic field settings, and the most recent treatment guidelines recommended by the WHO.

Phylogenomics and Diversification of the Schistosomatidae Based on Targeted Sequence Capture of Ultra-Conserved Elements

Schistosomatidae Stiles and Hassall 1898 is a medically significant family of digenetic trematodes (Trematoda: Digenea), members of which infect mammals or birds as definitive hosts and aquatic or amphibious gastropods as intermediate hosts. Currently, there are 17 named genera, for many of which evolutionary interrelationships remain unresolved. The lack of a resolved phylogeny has encumbered our understanding of schistosomatid evolution, specifically patterns of host-use and the role of host-switching in diversification. Here, we used targeted sequence capture of ultra-conserved elements (UCEs) from representatives of 13 of the 17 named genera and 11 undescribed lineages that are presumed to represent either novel genera or species to generate a phylogenomic dataset for the estimation of schistosomatid interrelationships. This study represents the largest phylogenetic effort within the Schistosomatidae in both the number of loci and breadth of taxon sampling. We present a near-comprehensive family-level phylogeny providing resolution to several clades of long-standing uncertainty within Schistosomatidae, including resolution for the placement of the North American mammalian schistosomes, implying a second separate capture of mammalian hosts. Additionally, we present evidence for the placement of Macrobilharzia at the base of the Schistosoma + Bivitellobilharzia radiation. Patterns of definitive and intermediate host use and a strong role for intermediate host-switching are discussed relative to schistosomatid diversification.

Schistosoma japonicum proteins that interact with the gynecophoral canal protein identified using a yeast two-hybrid system

The large amount of schistosome eggs produced by mature female worms not only induce major pathological damage to the host but also lead to the transmission of schistosomiasis. Mature female schistosome worms need constant pairing contact with a male partner as male signaling is indispensable to female growth, development, and reproduction. The gynecophoral canal protein (GCP), a cell-surface glycoprotein, plays a potential role in the interaction between males and females and in stimulating female development and maturation. In this study, a yeast two-hybrid cDNA library of Schistosoma japonicum (Sj) parasites 18 days post-infection (dpi) was constructed; the Sjgcp gene was inserted into a pGBKT7-BD bait plasmid and used as a bait protein to screen for its molecular interactions using a yeast mating procedure. Twenty-four prey proteins that interacted with the SjGCP were selected after excluding false positives; the interactions between S.japonicum lethal giant larvae (SjLGL) and SjGCP, S.japonicum type V collagen (SjColV) and SjGCP, were verified by co-immunoprecipitation. The RNA interference against SjGCP, SjColV and SjGCP + SjColV led to severe underdevelopment of tegument in male worms and vitelline globules in female worms as well as reduced reproductive capacity of the females. Collectively, SjGCP and its interacting proteins may play pivotal roles in growth and development. The findings also suggested that SjGCP and its interacting protein partners might represent new candidate targets for drug development against schistosomiasis.

Somatic Dimorphism in Cercariae of a Bird Schistosome

Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. “peregra”, in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.

Whole-genome sequencing of Schistosoma mansoni reveals extensive diversity with limited selection despite mass drug administration

Control and elimination of the parasitic disease schistosomiasis relies on mass administration of praziquantel. Whilst these programmes reduce infection prevalence and intensity, their impact on parasite transmission and evolution is poorly understood. Here we examine the genomic impact of repeated mass drug administration on Schistosoma mansoni populations with documented reduced praziquantel efficacy. We sequenced whole-genomes of 198 S. mansoni larvae from 34 Ugandan children from regions with contrasting praziquantel exposure. Parasites infecting children from Lake Victoria, a transmission hotspot, form a diverse panmictic population. A single round of treatment did not reduce this diversity with no apparent population contraction caused by long-term praziquantel use. We find evidence of positive selection acting on members of gene families previously implicated in praziquantel action, but detect no high frequency functionally impactful variants. As efforts to eliminate schistosomiasis intensify, our study provides a foundation for genomic surveillance of this major human parasite.

Multihost Transmission of Schistosoma mansoni in Senegal, 2015-2018

In West Africa, Schistosoma spp. are capable of infecting multiple definitive hosts, a lifecycle feature that may complicate schistosomiasis control. We characterized the evolutionary relationships among multiple Schistosoma mansoni isolates collected from snails (intermediate hosts), humans (definitive hosts), and rodents (definitive hosts) in Senegal. On a local scale, diagnosis of S. mansoni infection ranged 3.8%-44.8% in school-aged children, 1.7%-52.6% in Mastomys huberti mice, and 1.8%-7.1% in Biomphalaria pfeifferi snails. Our phylogenetic framework confirmed the presence of multiple S. mansoni lineages that could infect both humans and rodents; divergence times of these lineages varied (0.13-0.02 million years ago). We propose that extensive movement of persons across West Africa might have contributed to the establishment of these various multihost S. mansoni clades. High S. mansoni prevalence in rodents at transmission sites frequented by humans further highlights the implications that alternative hosts could have on future public health interventions.

Population genetics of African Schistosoma species

Blood flukes within the genus Schistosoma (schistosomes) are responsible for the major disease, schistosomiasis, in tropical and sub-tropical areas. This disease is predominantly present on the African continent with more than 85% of the human cases. Schistosomes are also parasites of veterinary importance infecting livestock and wildlife. Schistosoma population genetic structure and diversity are important characteristics that may reflect variations in selection pressures such as those induced by host (mammalian and snail) environments, habitat change, migration and also treatment/control interventions, all of which also shape speciation and evolution of the whole Schistosoma genus. Investigations into schistosome population genetic structure, diversity and evolution has been an area of important debate and research. Supported by advances in molecular techniques with capabilities for multi-locus genetic analyses for single larvae schistosome genetic investigations have greatly progressed in the last decade. This paper aims to review the genetic studies of both animal and human infecting schistosome. Population genetic structures are reviewed at different spatial scales: local, regional or continental (i.e. phylogeography). Within species genetic diversities are discussed compared and the compounding factors discussed, including the effect of mass drug administration. Finally, the ability for intra-species hybridisation questions species integrities and poses many questions in relation to the natural epidemiology of co-endemic species. Here we review molecularly confirmed hybridisation events (in relation to human disease) and discuss the possible impact for ongoing and future control and elimination.

Population Structure and Dynamics of Helminthic Infection: Schistosomiasis

While disease and outbreaks are mainly clonal for bacteria and other asexually reproducing organisms, sexual reproduction in schistosomes and other helminths usually results in unique individuals. For sexually reproducing organisms, the traits conserved in clones will instead be conserved in the group of organisms that tends to breed together, the population. While the same tools are applied to characterize DNA, how results are interpreted can be quite different at times (see another article in this collection, http://www.asmscience.org/content/journal/microbiolspec/10.1128/microbiolspec.AME-0002-2018). It is difficult to know what the real effect any control program has on the parasite population without assessing the health of this population, how they respond to the control measure, and how they recover, if they do. This review, part of the Microbiology Spectrum Curated Collection: Advances in Molecular Epidemiology of Infectious Diseases, concentrates on one approach using pooled samples to study schistosome populations and shows how this and other approaches have contributed to our understanding of this parasite family's biology and epidemiology. *This article is part of a curated collection.

Epidemiological surveillance of schistosomiasis outbreak in Corsica (France): Are animal reservoir hosts implicated in local transmission?

Environmental and anthropogenic changes are expected to promote emergence and spread of pathogens worldwide. Since 2013, human urogenital schistosomiasis is established in Corsica island (France). Schistosomiasis is a parasitic disease affecting both humans and animals. The parasite involved in the Corsican outbreak is a hybrid form between Schistosoma haematobium, a human parasite, and Schistosoma bovis, a livestock parasite. S. bovis has been detected in Corsican livestock few decades ago raising the questions whether hybridization occurred in Corsica and if animals could behave as a reservoir for the recently established parasite lineage. The latter hypothesis has huge epidemiological outcomes since the emergence of a zoonotic lineage of schistosomes would be considerably harder to control and eradicate the disease locally and definitively needs to be verified. In this study we combined a sero-epidemiological survey on ruminants and a rodent trapping campaign to check whether schistosomes could shift on vertebrate hosts other than humans. A total of 3,519 domesticated animals (1,147 cattle; 671 goats and 1,701 sheep) from 160 farms established in 14 municipalities were sampled. From these 3,519 screened animals, 17 were found to be serologically positive but were ultimately considered as false positive after complementary analyses. Additionally, our 7-day extensive rodent trapping (i.e. 1,949 traps placed) resulted in the capture of a total of 34 rats (Rattus rattus) and 4 mice (Mus musculus). Despite the low number of rodents captured, molecular diagnostic tests showed that two of them have been found to be infected by schistosomes. Given the low abundance of rodents and the low parasitic prevalence and intensity among rodents, it is unlikely that neither rats nor ruminants play a significant role in the maintenance of schistosomiasis outbreak in Corsica. Finally, the most likely hypothesis is that local people initially infected in 2013 re-contaminated the river during subsequent summers, however we cannot definitively rule out the possibility of an animal species acting as reservoir host.

There is an increasing interest on the effect of global changes on the transmission of infectious diseases. Both environmental and anthropogenic changes are expected to promote outbreaks and spread of pathogens. In particular, tropical infectious diseases are expected to move towards more temperate latitudes. Until 2013, urogenital schistosomiasis was restricted to tropical and sub-tropical areas. In summer 2013, a schistosomiasis outbreak has emerged in Corsica (France) with more than 100 cases. Corsica is a French Mediterranean island, which is very popular for tourists from throughout Europe due to the natural beauty of the environment. Surprisingly, in summer 2015 and 2016, the contamination has resumed, and schistosomiasis has been classified in the list of French notifiable infectious disease. In this context it has been hypothesised that reservoir vertebrate hosts, either human and/or animal are at the origin of the maintenance of the local transmission. This paper shows that ruminants (cow, sheep and goats) should not play a role of reservoir host but we found that rodents living in the vicinity of the transmission sites have been infected by the parasite. Considering the low abundance of rodents and the low parasitic prevalence/intensity among rodents, it is unlikely that rats play a significant role in the maintenance of schistosomiasis outbreak in Corsica and that other animals or human could maintain the parasite locally.

Status of Schistosomiasis Elimination in the Caribbean Region

Schistosomiasis elimination status in the Caribbean is reviewed with information on historical disease background, attempts to control it and current situation for each locality in the region where transmission has been eliminated (Sint Maarten, Saint Kitts, Vieques), eliminated but not yet verified (Puerto Rico, Dominican Republic, Antigua, Montserrat, Guadeloupe, Martinique) and still ongoing (Saint Lucia, Suriname). Integrated control initiatives based on selective and mass treatment and snail control using environmental, chemical and biological methods along with public service improvements (housing, safe water, sanitation) and changes in demography (urbanization) and economy (change from sugarcane and banana production to tourism) have resulted in reduction in the burden of schistosomiasis over the past century. Introduction of Biomphalaria-competitor snails into the region as a cost-effective, low maintenance control method appears to have had the most sustainable impact on transmission reduction. A regional inventory of B. glabrata, other Biomphalaria species and Biomphalaria-competitor snails as well as investigation of possible animal reservoir hosts in persisting endemic areas would be helpful for control. Elimination of schistosomiasis appears achievable in the Caribbean. However, a regional surveillance and monitoring program is needed to verify elimination in the various localities and identify and monitor areas still endemic or at risk.

Population genetic structure of Schistosoma bovis in Cameroon

Background

Schistosomiasis is neglected tropical parasitic disease affecting both humans and animals. Due to the human health impact, population genetic studies have focused on the three main human-infecting schistosome species: Schistosoma mansoni, S. haematobium and S. japonicum. Here we present novel data on the population genetic structure of Schistosoma bovis, a highly widespread and prevalent schistosome infecting ruminants, and therefore of veterinary importance.

Methods

Adult S. bovis were sampled in the two main abattoirs of Cameroon (Yaoundé and Douala). Twenty-two cows originating from four distinct localities were sampled and a total of 218 parasites were recovered. All parasites were genotyped using a panel of 14 microsatellite markers and a sub-sample of 91 parasites were sequenced and characterized with the mitochondrial (cox1) and nuclear (ITS) genetic markers.

Results

No significant difference in allelic richness, heterozygosity, nucleotide diversity and haplotype diversity was observed between the populations. Additionally, no strong genetic structure was observed at the country scale. Our data also show that S. bovis is more polymorphic than its sister species, S. haematobium, and that the haplotype diversity is similar to that of S. mansoni while the nucleotide diversity does not significantly differ from that of S. haematobium. The resulting negative Tajima’s D* and Fu and Li’s D* indices could be a signature of population demographic expansion. No S. haematobium/S. bovis hybrids were observed in our populations, thus all samples were considered as pure S. bovis.

Conclusions

This study provides novel insights into genetic diversity and population genetic structure of S. bovis. No strong genetic structure was observed at the country scale but some genetic indices could be associated as a signature of population demographic expansion.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3307-0) contains supplementary material, which is available to authorized users.

Large and finite sample properties of a maximum-likelihood estimator for multiplicity of infection

Reliable measures of transmission intensities can be incorporated into metrics for monitoring disease-control interventions. Genetic (molecular) measures like multiplicity of infection (MOI) have several advantages compared with traditional measures, e.g., R0. Here, we investigate the properties of a maximum-likelihood approach to estimate MOI and pathogen-lineage frequencies. By verifying regulatory conditions, we prove asymptotical unbiasedness, consistency and efficiency of the estimator. Finite sample properties concerning bias and variance are evaluated over a comprehensive parameter range by a systematic simulation study. Moreover, the estimator's sensitivity to model violations is studied. The estimator performs well for realistic sample sizes and parameter ranges. In particular, the lineage-frequency estimates are almost unbiased independently of sample size. The MOI estimate's bias vanishes with increasing sample size, but might be substantial if sample size is too small. The estimator's variance matrix agrees well with the Cramér-Rao lower bound, even for small sample size. The numerical and analytical results of this study can be used for study design. This is exemplified by a malaria data set from Venezuela. It is shown how the results can be used to determine the necessary sample size to achieve certain performance goals. An implementation of the likelihood method and a simulation algorithm for study design, implemented as an R script, is available as S1 File alongside a documentation (S2 File) and example data (S3 File).

Genetic differences among Haplorchis taichui populations in Indochina revealed by mitochondrial COX1 sequences

Haplorchis taichui is an intestinal heterophyid fluke that is pathogenic to humans. It is widely distributed in Asia, with a particularly high prevalence in Indochina. Previous work revealed that the lack of gene flow between three distinct populations of Vietnamese H. taichui can be attributed to their geographic isolation with no interconnected river basins. To test the hypothesis that interconnected river basins allow gene flow between otherwise isolated populations of H. taichui, as previously demonstrated for another trematode, Opisthorchis viverrini, we compared the genetic structures of seven populations of H. taichui from various localities in the lower Mekong Basin, in Thailand and Laos, with those in Vietnam, using the mitochondrial cytochrome c oxidase subunit 1 (COX1) gene. To determine the gene flow between these H. taichui populations, we calculated their phylogenetic relationships, genetic distances and haplotype diversity. Each population showed very low nucleotide diversity at this locus. However, high levels of genetic differentiation between the populations indicated very little gene flow. A phylogenetic analysis divided the populations into four clusters that correlated with the country of origin. The negligible gene flow between the Thai and Laos populations, despite sharing the Mekong Basin, caused us to reject our hypothesis. Our data suggest that the distribution of H. taichui populations was incidentally associated with national borders.

A Novel Toll-Like Receptor (TLR) Influences Compatibility between the Gastropod Biomphalaria glabrata, and the Digenean Trematode Schistosoma mansoni

Schistosomiasis, a devastating disease caused by parasitic flatworms of the genus Schistosoma, affects over 260 million people worldwide especially in tropical and sub-tropical regions. Schistosomes must undergo their larval development within specific species of snail intermediate hosts, a trait that is shared among almost all digenean trematodes. This unique and long-standing host-parasite relationship presents an opportunity to study both the importance of conserved immunological features in novel immunological roles, as well as new immunological adaptations that have arisen to combat a very specific type of immunological challenge. While it is well supported that the snail immune response is important for protecting against schistosome infection, very few specific snail immune factors have been identified and even fewer have been functionally characterized. Here, we provide the first functional report of a snail Toll-like receptor, which we demonstrate as playing an important role in the cellular immune response of the snail Biomphalaria glabrata following challenge with Schistosoma mansoni. This TLR (BgTLR) was identified as part of a peptide screen of snail immune cell surface proteins that differed in abundance between B. glabrata snails that differ in their compatibility phenotype to challenge by S. mansoni. The S. mansoni-resistant strain of B. glabrata (BS-90) displayed higher levels of BgTLR compared to the susceptible (M-line) strain. Transcript expression of BgTLR was found to be very responsive in BS-90 snails when challenged with S. mansoni, increasing 27 fold relative to β-actin (non-immune control gene); whereas expression in susceptible M-line snails was not significantly increased. Knockdown of BgTLR in BS-90 snails via targeted siRNA oligonucleotides was confirmed using a specific anti-BgTLR antibody and resulted in a significant alteration of the resistant phenotype, yielding patent infections in 43% of the normally resistant snails, which shed S. mansoni cercariae 1-week before the susceptible controls. Our results represent the first functional characterization of a gastropod TLR, and demonstrate that BgTLR is an important snail immune receptor that is capable of influencing infection outcome following S. mansoni challenge.

Genetic diversity and population structure of Synthesium pontoporiae (Digenea, Brachycladiidae) linked to its definitive host stocks, the endangered Franciscana dolphin, Pontoporia blainvillei (Pontoporiidae) off the coast of Brazil and Argentina

Pontoporia blainvillei (Gervais and d'Orbigny, 1844) is an endangered small cetacean endemic to South America with four Franciscana Management Areas (FMA) recognized as different population stocks. The role of the intestinal parasite Synthesium pontoporiae (Digenea: Brachycladiidae) as a possible biological marker to differentiate P. blainvillei stocks was evaluated using nuclear and mitochondrial DNA markers. Internal transcribed sequence 1 and 2 (ITS1 and ITS2) regions of S. pontoporiae did not show intraspecific variability. The mitochondrial NADH dehydrogenase subunit 3 (ND3) and cytochrome oxidase subunit I (COI) gene sequences suggested lack of population structure in S. pontoporiae and population expansion. The apparent panmixia of S. pontoporiae may be due to the high mobility of one or more of its intermediary hosts. Alternatively, it may be due to the small sample size. This result is incongruent with the previously proposed FMA.

Genetic diversity, multiplicity of infection and population structure of Schistosoma mansoni isolates from human hosts in Ethiopia

Background

Human intestinal schistosomiasis caused by Schistosoma mansoni and urinary schistosomiasis caused by Schistosoma haematobium are endemic in Ethiopia. Although schistosomes look morphologically uniform, there is variation in infectivity, egg productivity and virulence due to variation in their genetic make. Knowing the genetic diversity and population structure of S. mansoni isolates will enable to understand and consider the possible variability in terms of infectivity, egg productivity and virulence.

Methods

Between 2010 and 2011, genetic diversity and population structure of Schistosoma mansoni isolates from four endemic areas of Ethiopia was assessed using previously published 11 polymorphic microsatellite loci. Miracidia were hatched from eggs of S. mansoni collected from stools of human subjects residing in Kemissie, Wondo Genet, Ziway and Sille-Elgo villages. DNA was extracted from single miracidium and PCR was run following standard protocol. Allelic polymorphism and population genetic structure was analyzed using different software.

Result

At a population level (i.e. different villages), the mean number of alleles per locus, allelic richness, expected heterozygosity in Hardy–Weinberg equilibrium and pairwise F_ST values ranged from 8.5 to 11.5, 3.46–20.8, 0.66–0.73 and 3.57–13.63 %, respectively. All analyzes on population genetic structure reveals strong genetic structuration corresponding to the four sampled villages. At infrapopulation level (i.e. different hosts) the mean number of alleles per locus, allelic richness, expected heterozygosity in Hardy–Weinberg equilibrium and F_IS values ranged from 3.09 to 7.55, 1–1.96, 0.59–0.73 and 0.1763–0.4989, respectively. Mean estimated genetically unique adult worm pairs within hosts ranged from 66 to 92 % revealing the occurrence of infection of a single host with genetically unique multiple S. mansoni strains. The data also indicated the occurrence of genetic variation within inter- and intra-hosts.

Conclusion

High level of genetic diversity and significant population differentiation characterized the S. mansoni isolates of Ethiopia. These results are quite different from previous studies demonstrating that it is difficult to generalize schistosome transmission patterns because epidemiological situation tends to vary. These are important factors to be considered in relation with morbidity, drug resistance or vaccine development.

Genotypic variation in host response to infection affects parasite reproductive rate

Parasite fitness is largely influenced by a variation in host response due to the host's genetic background. Here we investigated the impact of host genotype on pathogen success in the snail vector of its castrating parasite, Schistosoma mansoni. We infected five inbred lines of Biomphalaria glabrata with two infection doses and followed their growth, reproductive output and parasite production throughout the course of infection. There was no difference in resistance to infection among inbred lines, but lines varied in their responses to infection and the numbers of parasites produced. Snails did not compensate for castration by increasing their fecundity during the early phase of infection (fecundity compensation). However, some lines were able to delay parasite shedding for up to 30 weeks, thus prolonging reproduction before the onset of castration. Here we propose this strategy as a novel defense against castrating pathogens in snails. Gigantism, a predicted outcome of castration due to energy reallocation, occurred early in infection (<15 weeks) and was not universal among the snail lines. Lines that did not show gigantism were also characterised by a high parasite production rate and low survivorship, perhaps indicating energy reallocation into parasite production and costly immune defense. We observed no differences in total parasite production among lines throughout the entire course of infection, although lines differed in their parasite reproductive rate. The average rate of parasite production varied among lines from 1300 to 2450 cercariae within a single 2h shedding period, resulting in a total production of 6981-29,509 cercariae over the lifetime of a single snail. Regardless of genetic background, snail size was a strong predictor of parasite reproduction: each millimetre increase in snail size at the time of the first shed resulted in up to 3500 more cercariae over the lifetime of the snail. The results of this study provide a detailed picture of variation in hosts' responses to infection and the resulting impacts on parasite fitness, further defining the intricacies of snail-schistosome compatibility.

A likelihood approach to estimate the number of co-infections

The number of co-infections of a pathogen (multiplicity of infection or MOI) is a relevant parameter in epidemiology as it relates to transmission intensity. Notably, such quantities can be built into a metric in the context of disease control and prevention. Having applications to malaria in mind, we develop here a maximum-likelihood (ML) framework to estimate the quantities of interest at low computational and no additional costs to study designs or data collection. We show how the ML estimate for the quantities of interest and corresponding confidence-regions are obtained from multiple genetic loci. Assuming specifically that infections are rare and independent events, the number of infections per host follows a conditional Poisson distribution. Under this assumption, we show that a unique ML estimate for the parameter () describing MOI exists which is found by a simple recursion. Moreover, we provide explicit formulas for asymptotic confidence intervals, and show that profile-likelihood-based confidence intervals exist, which are found by a simple two-dimensional recursion. Based on the confidence intervals we provide alternative statistical tests for the MOI parameter. Finally, we illustrate the methods on three malaria data sets. The statistical framework however is not limited to malaria.

Can host ecology and kin selection predict parasite virulence?

Parasite virulence, or the damage a parasite does to its host, is measured in terms of both host costs (reductions in host growth, reproduction and survival) and parasite benefits (increased transmission and parasite numbers) in the literature. Much work has shown that ecological and genetic factors can be strong selective forces in virulence evolution. This review uses kin selection theory to explore how variations in host ecological parameters impact the genetic relatedness of parasite populations and thus virulence. We provide a broad overview of virulence and population genetics studies and then draw connections to existing knowledge about natural parasite populations. The impact of host movement (transporting parasites) and host resistance (filtering parasites) on the genetic structure and virulence of parasite populations is explored, and empirical studies of these factors using Plasmodium and trematode systems are proposed.

New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers

Significant numbers of pre-school children are infected with Schistosoma mansoni in sub-Saharan Africa and are likely to play a role in parasite transmission. However, they are currently excluded from control programmes. Molecular phylogenetic studies have provided insights into the evolutionary origins and transmission dynamics of S. mansoni, but there has been no research into schistosome molecular epidemiology in pre-school children. Here, we investigated the genetic diversity and population structure of S. mansoni in pre-school children and mothers living in lakeshore communities in Uganda and monitored for changes over time after praziquantel treatment. Parasites were sampled from children (<6 years) and mothers enrolled in the longitudinal Schistosomiasis Mothers and Infants Study at baseline and at 6-, 12- and 18-month follow-up surveys. 1347 parasites from 35 mothers and 45 children were genotyped by direct sequencing of the cytochrome c oxidase (cox1) gene. The cox1 region was highly diverse with over 230 unique sequences identified. Parasite populations were genetically differentiated between lakes and non-synonymous mutations were more diverse at Lake Victoria than Lake Albert. Surprisingly, parasite populations sampled from children showed a similar genetic diversity to those sampled from mothers, pointing towards a non-linear relationship between duration of exposure and accumulation of parasite diversity. The genetic diversity six months after praziquantel treatment was similar to pre-treatment diversity. Our results confirm the substantial genetic diversity of S. mansoni in East Africa and provide significant insights into transmission dynamics within young children and mothers, important information for schistosomiasis control programmes.

Many pre-school children in sub-Saharan Africa are infected with the parasite Schistosoma mansoni, which causes intestinal schistosomiasis. However, there has been no work published on the molecular epidemiology of Schistosoma in children under six or the role that these children play in parasite transmission. We analysed the genetic structure of parasite populations collected from mothers and young children living on the shores of Lakes Albert and Victoria in Uganda. Parasite populations were different at the two lakes indicating that there is little flow of parasite genes between the lakes. We were surprised to discover a large amount of genetic variation in parasites sampled from both children and mothers, suggesting that genetic variation is not directly related to duration of exposure to infested water. In addition, we found some evidence that young children are involved in S. mansoni transmission. The substantial genetic variation of S. mansoni in young children suggests that these parasites could be operating as a source of a variety of genetic traits, including drug susceptibility. Overall our findings offer significant insights into population genetics of S. mansoni in pre-school children and their mothers and provide important information for effective control of intestinal schistosomiasis.

Environmental epidemiology of intestinal schistosomiasis and genetic diversity of Schistosoma mansoni infections in snails at Bugoigo village, Lake Albert

Intestinal schistosomiasis continues to be hyper-endemic in the fishing community of Bugoigo located on the eastern shore of Lake Albert, Uganda. Our study aimed to identify the factors that determine the local distribution and abundance of Biomphalaria, as well as infection(s) with Schistosoma mansoni inclusive of their genetic diversity. In addition, a DNA barcoding approach was taken to genotype schistosome cercariae, exploring the micro-epidemiology of infections. Over a 3-week period in June-July 2010, several hundred Biomphalaria spp. were collected, together with environmental information, from 10 selected sites, representative of both putative wave-exposed (n=5) and wave-sheltered shorelines (n=5). A Mann-Whitney U-test and a generalized linear model were used to assess associations with snail abundance and parasite infections across the shoreline. Levels of local wave action were recorded over the 19-day period using digital accelerometers. The general absence of wave action on the sheltered shoreline likely helped to raise and focalize other environmental parameters, such as water conductivity by lack of mixing, that foster transmission of intestinal schistosomiasis. Over the study period, a total of 10 infected snails were encountered and a selection of schistosome cercariae from each infected snail was harvested for analysis by DNA barcoding. In total, 91 DNA barcodes were generated with 15 unique barcode types identified. Of these, 4 barcodes had been found previously in Lake Albert and (or) Victoria, the remaining 11 were newly encountered here and described. The distribution of DNA barcodes across infected snails and sampled locations revealed a complicated spatial sub-structuring. By shedding new light on the fine-scale patterning of infections, DNA barcoding has revealed a rather heterogeneous landscape of cercariae, likely inclusive of multi-miracidial infections within the snail, which will in turn interplay with human water contact activities to shape the genetic diversity of worm populations within infected people.

Multi-clone infections and the impact of intraspecific competition on trematode colonies with a division of labour

A division of labour occurs in colonies of the trematode Philophthalmus sp. within their first intermediate hosts. Two castes exist: one which reproduces and one which does not reproduce. It has been hypothesized that the benefit of the non-reproductive caste is in competitive interactions. Evidence for this from past experiments with Philophthalmus sp. colonies has been contradictory: the non-reproductive caste appears to benefit the colony in some way but not necessarily by combating interspecific competitors. The aims of this study were to consider intraspecific competition as a possible cause of the division of labour in Philophthalmus sp. colonies. Results show that mixed genotype infections occur in Philophthalmus sp. infected hosts and thus intraspecific competition is likely. Furthermore, the total number of individuals per colony is reduced in mixed genotype infections, indicating that intraspecific competition reduces colony fitness. However, the results do not indicate that the division of labour in Philophthalmus sp. plays a role in competitive interactions as the ratio of small, non-reproductive to large, reproductive individuals is unaffected by the presence of intraspecific competition. This is the first study to identify and quantify intraspecific competition in Philophthalmus sp., and to assess its selective role in this species' division of labour.

Bilharzia: Pathology, Diagnosis, Management and Control

More than one billion people travel internationally each year and approximately 100 million to the tropics. Schistosomiasis is a neglected tropical disease caused by trematode blood flukes of the genus Schistosoma. It currently infects over 250 million people worldwide and results in approximately 25 million disability adjusted life years lost. Clinical manifestations depend on the affected organ. Subtle morbidities have also been documented including: growth retardation, anaemia and poor cognitive function in children. While schistosomiasis has been eradicated from Japan and significantly reduced in parts of China and Egypt, transmission in many other regions remains ongoing due to the wide-spread distribution of the intermediate snail host, poor sanitation, lack of health education and decreasing compliance to mass drug administration. Integrated control has significantly reduced the burden of disease in China but considerable financial capital is needed if similar results are to be duplicated elsewhere. Human vaccination is in various stages of development, and once found, will become an integral part of future control. This comprehensive review examines the epidemiology, pathology, diagnosis, clinical management, prevention and control of the disease.

Molecular assessment of trematode co-infection and intraspecific competition in molluscan intermediate hosts

In natural populations of the human parasite Schistosoma mansoni, parasite distribution among snail intermediate hosts is generally overdispersed, such that a small proportion of hosts harbor the majority of parasite genotypes. Within these few infected snails, researchers have found that it can be common for hosts to harbor multiple parasite genotypes, creating circumstances in which co-infecting parasites are faced with potential competition over limited host resources. Much theoretical modeling has focused on parasite competition, especially regarding the influence of co-infection on parasite exploitation strategy evolution. However, particularly in the case of intra-molluscan intermediate stages, empirical investigations of parasite-parasite competition have often hinged on the untested assumption that co-exposure produces co-infection. That is, infected hosts exposed to multiple strains have been assumed to harbor multiple strains, regardless of the true nature of the infection outcome. Here we describe a real-time quantitative PCR method to distinguish the conditions of multiple- versus single-strain infection, as well as quantify the relative larval output of co-infecting strains. We applied the method to an empirical investigation of intraspecific parasite competition between S. mansoni strains within the intermediate snail host Biomphalaria glabrata, assessing co-exposure's effects on parasite infectivity and productivity and the concomitant effects on host fitness. Overall, there was no effect of parasite co-infection on snail life history traits relative to single-strain infection. Parasite infectivity significantly increased as a result of increasing overall miracidial dose, rather than co-exposure, though strain-specific productivity was significantly reduced in co-infections in manner consistent with resource competition. Moreover, we show that less than half of infected, co-exposed hosts had patent co-infections and demonstrate the utility of this molecular tool for the study of trematode life history variation in molluscan hosts.

Fitness benefits of a division of labour in parasitic trematode colonies with and without competition

A reproductive division of labour has recently been discovered within polyembryonic colonies of two species of parasitic trematodes infecting snail hosts. In these colonies, one morph expands the colony through asexual reproduction while the other morph never reproduces. As in other polyembryonic species using a division of labour (parasitoid wasps, one species of sea anemone), the non-reproducing morph appears specialized for defense against competing colonies. In this study, we first assessed competition between Philophthalmus sp. (which possesses reproducing and non-reproducing morphs) and the most common co-infecting species, Maritrema novaezealandensis, by quantifying colony success within snail hosts. Colonies of either species that did not compete within their host were more successful (i.e., produced more transmission stages) than colonies that were competing in a shared host. Second, we cultured individuals of both species in vitro, alone or together, to study the interaction more closely and to measure any advantage obtained by the colony from the non-reproducing morphs. This was done by manipulating the presence and abundance of M. novaezealandensis as well as the presence of the non-reproducing 'defensive' morph. Philophthalmus sp. colonies with both reproducing and non-reproducing morphs but without M. novaezealandensis were most successful. This implies the non-reproducing morphs provide a fitness benefit to Philophthalmus sp. colonies even in the absence of competition, although the nature of this advantage remains unclear.

Variation in infectivity and aggressiveness in space and time in wild host-pathogen systems: causes and consequences

Variation in host resistance and in the ability of pathogens to infect and grow (i.e. pathogenicity) is important as it provides the raw material for antagonistic (co)evolution and therefore underlies risks of disease spread, disease evolution and host shifts. Moreover, the distribution of this variation in space and time may inform us about the mode of coevolutionary selection (arms race vs. fluctuating selection dynamics) and the relative roles of G × G interactions, gene flow, selection and genetic drift in shaping coevolutionary processes. Although variation in host resistance has recently been reviewed, little is known about overall patterns in the frequency and scale of variation in pathogenicity, particularly in natural systems. Using 48 studies from 30 distinct host-pathogen systems, this review demonstrates that variation in pathogenicity is ubiquitous across multiple spatial and temporal scales. Quantitative analysis of a subset of extensively studied plant-pathogen systems shows that the magnitude of within-population variation in pathogenicity is large relative to among-population variation and that the distribution of pathogenicity partly mirrors the distribution of host resistance. At least part of the variation in pathogenicity found at a given spatial scale is adaptive, as evidenced by studies that have examined local adaptation at scales ranging from single hosts through metapopulations to entire continents and - to a lesser extent - by comparisons of pathogenicity with neutral genetic variation. Together, these results support coevolutionary selection through fluctuating selection dynamics. We end by outlining several promising directions for future research.

Stopping schistosomes from 'monkeying-around' in chimpanzees

Ngamba Island Chimpanzee Sanctuary (NICS) in Lake Victoria, Uganda is currently home to 44 wild-borne, semi-captive chimpanzees. Despite regular veterinary health checks, it only came to light recently that many animals, and sanctuary staff, were naturally infected with Schistosoma mansoni. Indeed, local schistosome transmission appears firmly engrained for intermediate snail hosts can be found along almost the entirety of Ngamba's shoreline. Here, the epidemiology of infection is a dynamic interplay between human and chimpanzee populations, as revealed by genetic analyses of S. mansoni. In this review, our present understanding of this complex and evolving situation is discussed, alongside general disease control activities in Uganda, to highlight future interventions towards stopping schistosome morbidity and transmission within this conservation sanctuary setting.

Compatibility polymorphism in snail/schistosome interactions: From field to theory to molecular mechanisms

Coevolutionary dynamics in host-parasite interactions potentially lead to an arms race that results in compatibility polymorphism. The mechanisms underlying compatibility have remained largely unknown in the interactions between the snail Biomphalaria glabrata and Schistosoma mansoni, one of the agents of human schistosomiasis. This review presents a combination of data obtained from field and laboratory studies arguing in favor of a matching phenotype model to explain compatibility polymorphism. Investigations focused on the molecular determinants of compatibility have revealed two repertoires of polymorphic and/or diversified molecules that have been shown to interact: the parasite antigens S. mansoni polymorphic mucins and the B. glabrata fibrinogen-related proteins immune receptors. We hypothesize their interactions define the compatible/incompatible status of a specific snail/schistosome combination. This line of thought suggests concrete approaches amenable to testing in field-oriented studies attempting to control schistosomiasis by disrupting schistosome-snail compatibility.

The molecular epidemiology of parasite infections: tools and applications

Molecular epidemiology, broadly defined, is the application of molecular genetic techniques to the dynamics of disease in a population. In this review, we briefly describe molecular and analytical tools available for molecular epidemiological studies and then provide an overview of how they can be applied to better understand parasitic disease. A range of new molecular tools have been developed in recent years, allowing for the direct examination of parasites from clinical or environmental samples, and providing access to relatively cheap, rapid, high throughput molecular assays. At the same time, new analytical approaches, in particular those derived from coalescent theory, have been developed to provide more robust estimates of evolutionary processes and demographic parameters from multilocus, genotypic data. To date, the primary application of molecular epidemiology has been to provide specific and sensitive identification of parasites and to resolve taxonomic issues, particularly at the species level and below. Population genetic studies have also been used to determine the extent of genetic diversity among populations of parasites and the degree to which this diversity is associated with different host cycles or epidemiologically important phenotypes. Many of these studies have also shed new light on transmission cycles of parasites, particularly the extent to which zoonotic transmission occurs, and on the prevalence and importance of mixed infections with different parasite species or intraspecific variants (polyparasitism). A major challenge, and one which is now being addressed by an increasing number of studies, is to find and utilize genetic markers for complex traits of epidemiological significance, such as drug resistance, zoonotic potential and virulence.

Fingerprint of Biomphalaria arabica, the intermediate host of Schistosoma mansoni in Saudi Arabia, using RAPD-PCR

In the time schistosomisis control programs are implemented in many countries, schistosomiasis continues to spread throughout the world. Among these control strategies is the vector control. Within this context, analysis of the genetic variability of the intermediate host snails is important because it allows identification of specific sequences of the genome of this mollusk related to determine their fingerprint. We investigated Biomphalaria arabica, which is found in Saudi Arabia, the intermediate host of Schistosoma mansoni infection. Genetic fingerprint was studied by RAPD-PCR using our own different random primers as well as published primers. The electrophoretic patterns resulting from amplification showed specific polymorphic markers of B. arabica. This information will be helpful in the identification of the snails and demonstrating that RAPD-PCR is an appropriate and efficient methodological approach for establishment of genetic barcode development.

Transmission dynamics of two strains of Schistosoma mansoni utilizing novel intermediate and definitive hosts

The intimate host-parasite relationship mandates adaptation to the genetic and phenotypic variability of their counterparts. Here, inbred and outcrossed strains of Schistosoma mansoni were challenged with "local" and "novel" intermediate and definitive hosts to examine effects of genetic variability and novelty on infection success and dynamics. Genetically distinct lines of Biomphalaria glabrata intermediate hosts exposed to inbred and outcrossed S. mansoni larvae were assessed for differences in both snail and parasite life-history parameters. Cercariae from each parasite-snail treatment were used to infect "local" and "novel" Mus musculus definitive hosts to assess parasite infectivity and fitness. Outcrossed parasites significantly reduced snail growth, were more productive, and induced greater host mortality than inbred parasites. Mouse strain did not influence parasite infectivity or reproduction, but parasite and snail host genetic background did, affecting both sex-specific infectivity and parasite productivity. Overall, genetic background of S. mansoni and its intermediate snail host altered life history traits and transmission dynamics of the parasite throughout its life cycle.

Vertebrate host protective immunity drives genetic diversity and antigenic polymorphism in Schistosoma mansoni

Schistosomes are gonochoric blood parasites with a complex life cycle responsible for a disease of considerable medical and veterinary importance in tropical and subtropical regions. Understanding the evolution of schistosome genetic diversity is clearly of fundamental importance to interpreting schistosomiasis epidemiology and disease transmission patterns of this parasite. In this article, we investigated the putative role of the host immune system in the selection of male genetic diversity. We demonstrated the link between genetic dissimilarity and the protective effect among male worms. We then compared the proteomes of three male clones with different genotypes and differing by their capacity to protect against reinfection. The identified differences correspond mainly to antigens known or supposed to be involved in the induction of protective immunity. These results underline the role played by host immune system in the selection of schistosome genetic diversity that is linked to antigenic diversity. We discuss the evolutionary consequences in the context of schistosome infection.

Transmission of Schistosoma japonicum in marshland and hilly regions of China: parasite population genetic and sibship structure

The transmission dynamics of Schistosoma japonicum remain poorly understood, as over forty species of mammals are suspected of serving as reservoir hosts. However, knowledge of the population genetic structure and of the full-sibship structuring of parasites at two larval stages will be useful in defining and tracking the transmission pattern between intermediate and definitive hosts. S. japonicum larvae were therefore collected in three marshland and three hilly villages in Anhui Province of China across three time points: April and September-October 2006, and April 2007, and then genotyped with six microsatellite markers. Results from the population genetic and sibling relationship analyses of the parasites across two larval stages demonstrated that, within the marshland, parasites from cattle showed higher genetic diversity than from other species; whereas within the hilly region, parasites from dogs and humans displayed higher genetic diversity than those from rodents. Both the extent of gene flow and the estimated proportion of full-sib relationships of parasites between two larval stages indicated that the cercariae identified within intermediate hosts in the marshlands mostly came from cattle, whereas in the hilly areas, they were varied between villages, coming primarily from rodents, dogs or humans. Such results suggest a different transmission process within the hilly region from within the marshlands. Moreover, this is the first time that the sibling relationship analysis was applied to the transmission dynamics for S. japonicum.

Schistosoma japonicum involves two obligatory host stages, with asexual reproduction within a molluscan host and sexual reproduction within a mammalian host. Having over 40 species of mammals suspected of being potential reservoirs complicates the transmission patterns. Understanding the complex transmission patterns is further hampered by the ethical and logistical difficulty in sampling adult worms from mammalian hosts. However, the two free-swimming larval stages, cercariae (released from a mollusc and then infective to a mammal) and miracidia (hatched from eggs passed in a mammal's faeces, and then infective to a mollusc), are available, and elucidating the genetic composition of parasites at theses two stages could provide information of infection processes. Here we sampled cercariae during April 2006, miracidia during September-October 2006, and cercariae during April 2007 in three marshland and three hilly villages in Anhui Province of China, and, using microsatellite markers, analyzed the population genetic structure and, for the first time, the familial relationships of parasites at different stages. We found contrasting population structures of parasites, and host species-associated diversities and transmission patterns of parasites between and within two regions. Moreover, we demonstrate that the successful application of sibship analyses to infection process provides an alternative approach to the dissection of transmission dynamics.

Applying evolutionary genetics to schistosome epidemiology

We review how molecular markers and evolutionary analysis have been applied to the study of schistosome parasites, important pathogens that infect over 200 million people worldwide. Topics reviewed include phylogenetics and biogeography, hybridization, infection within snails, mating systems, and genetic structure. Some interesting generalizations include that schistosome species hybridize frequently and have switched definitive hosts repeatedly in evolutionary time. We show that molecular markers can be used to infer epidemiologically relevant processes such as spatial variation in transmission, or to reveal complex patterns of mate choice. Analysis of genetic structure data shows that transmission foci can be structured by watershed boundaries, habitat types, and host species. We also discuss sampling and analytical problems that arise when using larvae to estimate genetic parameters of adult schistosome populations. Finally, we review pitfalls in methodologies such as genotyping very small individuals, statistical methods for identifying clonemates or for identifying sibling groups, and estimating allele frequencies from pooled egg samples.

Genetic diversity and population structure of Schistosoma mansoni within human infrapopulations in Mwea, central Kenya assessed by microsatellite markers

A recently developed high-throughput technique that allows multi-locus microsatellite analysis of individual miracidia of Schistosoma mansoni was used to assess the levels of genetic diversity and population structure in 12 infrapopulations of the parasite, each infrapopulation derived from an infected school child from the Mwea area, central Kenya. The mean number of alleles per locus was in the range 8.22-10.22, expected heterozygosity in Hardy-Weinberg equilibrium was 0.68-0.70, and pairwise F(ST) values ranged from 0.16% to 3.98% for the 12 infrapopulations. Although the genetic diversity within each infrapopulation of S. mansoni in this area was generally high, low levels of genetic structure were observed, suggestive of high levels of gene flow among infrapopulations. Private alleles were found in 8 of the 12 infrapopulation, the highest number of private alleles recorded per infrapopulation was 3. Our data suggest that the level of gene flow among infrapopulations of S. mansoni in Mwea is extremely high, thus providing opportunity for spread of rare alleles, including those that may confer character traits such as drug resistance and virulence.

Parasite genetic differentiation by habitat type and host species: molecular epidemiology of Schistosoma japonicum in hilly and marshland areas of Anhui Province, China

Schistosoma japonicum, a parasite of significant public health importance in parts of China and Southeast Asia, is a true generalist pathogen with over 40 species of mammals suspected as definitive host reservoirs. In order to characterize levels of parasite gene flow across host species and identify the most important zoonotic reservoirs, S. japonicum larvae (miracidia) were sampled from a range of definitive host species in two contrasting habitat types within Anhui Province, China: a low-lying marshland region, and a hilly region, where animal reservoir populations may be predicted to differ substantially. Miracidia samples were genotyped using seven multiplexed microsatellite markers. Hierarchical F-statistics and clustering analyses revealed substantial geographical structuring of S. japonicum populations within Anhui, with strong parasite genetic differentiation between habitat types. Within most villages, there was very little or no parasite genetic differentiation among host species, suggesting frequent S. japonicum gene flow, and thus also transmission, across species. Moreover, the data provide novel molecular evidence that rodents and dogs are potentially very important infection reservoirs in hilly regions, in contrast to bovines in the marshland regions. The parasite genetic differentiation between habitat types might therefore be associated with contrasting host reservoirs. The high levels of parasite gene flow observed across host species in sympatric areas have important implications for S. japonicum control, particularly in hilly regions where control of infection among wild rodent populations could be challenging.

Successful parasitism of vector snail Biomphalaria glabrata by the human blood fluke (trematode) Schistosoma mansoni: a 2009 assessment

Schistosomiasis, caused by infections by human blood flukes (Trematoda), continues to disrupt the lives of over 200,000,000 people in over 70 countries, inflicting misery and precluding the individuals' otherwise reasonable expectations of productive lives. Infection requires contact with freshwater in which infected snails (the intermediate hosts of schistosomes) have released cercariae larvae. Habitats suitable for the host snails continue to expand as a consequence of water resource development. No vaccine is available, and resistance has emerged towards the single licensed schistosomicide drug. Since human infections would cease if parasite infections in snails were prevented, efforts are being made to discover requirements of intra-molluscan development of these parasites. Wherever blood flukes occur, naturally resistant conspecific snails are present. To understand the mechanisms used by parasites to ensure their survival in immunocompetent hosts, one must comprehend the interior defense mechanisms that are available to the host. For one intermediate host snail (Biomphalaria glabrata) and trematodes for which it serves as vector, molecular genetic and proteomic surveys for genes and proteins influencing the outcomes on infections are yielding lists of candidates. A comparative approach drawing on data from studies in divergent species provides a robust basis for hypothesis generation to drive decisions as to which candidates merit detailed further investigation. For example, reactive oxygen and nitrogen species are known mediators or effectors in battles between infectious agents and their hosts. An approach targeting genes involved in relevant pathways has been fruitful in the Schistosoma mansoni -- B. glabrata parasitism, leading to discovery of a functionally relevant gene set (encoding enzymes responsible for the leukocyte respiratory burst) that associates significantly with host resistance phenotype. This review summarizes advances in the understanding of strategies used by both this trematode parasite and its molluscan host to ensure their survival.

Effects of laboratory culture on compatibility between snails and schistosomes

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.