Microsatellite typing reveals strong genetic structure of Schistosoma mansoni from localities in Kenya

Population Genetic Structure and Hybridization of Schistosoma haematobium in Nigeria

Background: Schistosomiasis is a major poverty-related disease caused by dioecious parasitic flatworms of the genus Schistosoma with a health impact on both humans and animals. Hybrids of human urogenital schistosome and bovine intestinal schistosome have been reported in humans in several of Nigeria’s neighboring West African countries. No empirical studies have been carried out on the genomic diversity of Schistosoma haematobium in Nigeria. Here, we present novel data on the presence and prevalence of hybrids and the population genetic structure of S. haematobium. Methods: 165 Schistosoma-positive urine samples were obtained from 12 sampling sites in Nigeria. Schistosoma haematobium eggs from each sample were hatched and each individual miracidium was picked and preserved in Whatman® FTA cards for genomic analysis. Approximately 1364 parasites were molecularly characterized by rapid diagnostic multiplex polymerase chain reaction (RD-PCR) for mitochondrial DNA gene (Cox1 mtDNA) and a subset of 1136 miracidia were genotyped using a panel of 18 microsatellite markers. Results: No significant difference was observed in the population genetic diversity (p > 0.05), though a significant difference was observed in the allelic richness of the sites except sites 7, 8, and 9 (p < 0.05). Moreover, we observed two clusters of populations: west (populations 1−4) and east (populations 7−12). Of the 1364 miracidia genotyped, 1212 (89%) showed an S. bovis Cox1 profile and 152 (11%) showed an S. haematobium cox1 profile. All parasites showed an S. bovis Cox1 profile except for some at sites 3 and 4. Schistosoma miracidia full genotyping showed 59.3% of the S. bovis ITS2 allele. Conclusions: This study provides novel insight into hybridization and population genetic structure of S. haematobium in Nigeria. Our findings suggest that S. haematobium x S. bovis hybrids are common in Nigeria. More genomic studies on both human- and animal-infecting parasites are needed to ascertain the role of animals in schistosome transmission.

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.

Genetic evidence for the role of non-human primates as reservoir hosts for human schistosomiasis

Background

Schistosomiasis is a chronic parasitic disease, that affects over 207 million people and causes over 200,000 deaths annually, mainly in sub-Saharan Africa. Although many health measures have been carried out to limit parasite transmission, significant numbers of non-human primates such as Chlorocebus aethiops (Ch. aethiops) (vervet) and Papio anubis (baboon) are infected with S. mansoni, notably in Ethiopia, where they are expected to have potentially significant implications for transmission and control efforts.

Objective

The objective of this study was to assess and compare the genetic diversity and population structure of S. mansoni isolates from human and non-human primates free-ranging in close proximity to villages in selected endemic areas of Ethiopia.

Methods

A cross-sectional study was conducted in three transmission sites: Bochesa, Kime and Fincha. A total of 2,356 S. mansoni miracidia were directly isolated from fecal specimens of 104 hosts (i.e. 60 human hosts and 44 non-human primates). We performed DNA extraction and PCR amplification using fourteen microsatellite loci.

Results

At population scale we showed strong genetic structure between the three sample sites. At the definitive host scale, we observed that host factors can shape the genetic composition of parasite infra-populations. First, in male patients, we observed a positive link between parasite genetic diversity and the age of the patients. Second, we observed a difference in genetic diversity which was high in human males, medium in human females and low in non-human primates (NHPs). Finally, whatever the transmission site no genetic structure was observed between human and non-human primates, however, there appears to be little barriers, if any, host specificity of the S. mansoni populations with cross-host infections.

Conclusion

Occurrence of infection of a single host with multiple S. mansoni strains and inter- and intra-host genetic variations was observed. Substantial genetic diversity and gene flow across the S. mansoni population occurred at each site and non-human primates likely play a role in local transmission and maintenance of infection. Therefore, public health and wildlife professionals should work together to improve disease control and elimination strategies.

Schistosomiasis is a chronic disease caused by flukes (trematodes). The definitive host spectrum of schistosomes, whether human, non-human primates (NHPs) or other mammals, is highly dependent on the schistosome species concerned. Genetic diversity and population structure studies of S. mansoni have provided insights into the variation of natural populations. Understanding S. mansoni genetic diversity and population structure of isolates from human and non-human primate hosts living in close proximity showed the occurrence of infection of a single host with multiple S. mansoni strains and inter- and intra-host genetic variations. In this article, the researchers assert the fact that genetic approach reveals that parasites from the three different sites are independent. Thus, we could consider the three sites as geographical replicates showing the influence of NHPs in parasitic transmission in Ethiopia. This study provides insights into the epidemiology, genetic diversity and population structure of S. mansoni in human and non-human primates in Ethiopia, all of which are crucial for the control of schistosomiasis.

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.

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.

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.

No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area

BACKGROUND

Schistosomiasis is a debilitating neglected tropical disease that infects over 200 million people worldwide. To combat this disease, in 2012, the World Health Organization announced a goal of reducing and eliminating transmission of schistosomes. Current control focuses primarily on mass drug administration (MDA). Therefore, we monitored transmission of Schistosoma mansoni via fecal egg counts and genetic markers in a typical school based MDA setting to ascertain the actual impacts of MDA on the targeted schistosome population.

METHODS

For 4 years, we followed 67 children enrolled in a MDA program in Kenya. Infection status and egg counts were measured each year prior to treatment. For 15 of these children, for which there was no evidence of acquired resistance, meaning they became re-infected following each treatment, we collected microsatellite genotype data from schistosomes passed in fecal samples as a representation of the force of transmission between drug treatments. We genotyped a total of 4938 parasites from these children, with an average of 329.2 parasites per child for the entire study, and an average of 82.3 parasites per child per annual examination. We compared prevalence, egg counts, and genetic measures including allelic richness, gene diversity (expected heterozygosity), adult worm burdens and effective number of breeders among time points to search for evidence for a change in transmission or schistosome populations during the MDA program.

FINDINGS

We found no evidence of reduced transmission or schistosome population decline over the course of the program. Although prevalence declined in the 67 children as it did in the overall program, reinfection rates were high, and for the 15 children studied in detail, schistosome egg counts and estimated adult worm burdens did not decline between years 1 and 4, and genetic diversity increased over the course of drug treatment.

INTERPRETATION

School based control programs undoubtedly improve the health of individuals; however, our data show that in an endemic area, such a program has had no obvious effect on reducing transmission or of significantly impacting the schistosome population as sampled by the children we studied in depth. Results like these, in combination with other sources of information, suggest more integrated approaches for interrupting transmission and significantly diminishing schistosome populations will be required to achieve sustainable control.

Inbreeding within human Schistosoma mansoni: do host-specific factors shape the genetic composition of parasite populations?

The size, structure and distribution of host populations are key determinants of the genetic composition of parasite populations. Despite the evolutionary and epidemiological merits, there has been little consideration of how host heterogeneities affect the evolutionary trajectories of parasite populations. We assessed the genetic composition of natural populations of the parasite Schistosoma mansoni in northern Senegal. A total of 1346 parasites were collected from 14 snail and 57 human hosts within three villages and individually genotyped using nine microsatellite markers. Human host demographic parameters (age, gender and village of residence) and co-infection with Schistosoma haematobium were documented, and S. mansoni infection intensities were quantified. F-statistics and clustering analyses revealed a random distribution (panmixia) of parasite genetic variation among villages and hosts, confirming the concept of human hosts as 'genetic mixing bowls' for schistosomes. Host gender and village of residence did not show any association with parasite genetics. Host age, however, was significantly correlated with parasite inbreeding and heterozygosity, with children being more infected by related parasites than adults. The patterns may be explained by (1) genotype-dependent 'concomitant immunity' that leads to selective recruitment of genetically unrelated worms with host age, and/or (2) the 'genetic mixing bowl' hypothesis, where older hosts have been exposed to a wider variety of parasite strains than children. The present study suggests that host-specific factors may shape the genetic composition of schistosome populations, revealing important insights into host-parasite interactions within a natural system.

Praziquantel treatment decreases Schistosoma mansoni genetic diversity in experimental infections

BACKGROUND

Schistosomiasis has a considerable impact on public health in many tropical and subtropical areas. In the new world, schistosomiasis is caused by the digenetic trematode Schistosoma mansoni. Chemotherapy is the main measure for controlling schistosomiasis, and the current drug of choice for treatment is praziquantel (PZQ). Although PZQ is efficient and safe, its repetitive large-scale use in endemic areas may lead to the selection of resistant strains. Isolates less susceptible to PZQ have been found in the field and selected for in the laboratory. The impact of selecting strains with a decreased susceptibility phenotype on disease dynamics and parasite population genetics is not fully understood. This study addresses the impact of PZQ pressure on the genetics of a laboratory population by analyzing frequency variations of polymorphic genetic markers.

METHODOLOGY

Infected mice were treated with increasing PZQ doses until the highest dose of 3 × 300 mg/Kg was reached. The effect of PZQ treatment on the parasite population was assessed using five polymorphic microsatellite markers. Parasitological and genetic data were compared with those of the untreated control. After six parasite generations submitted to treatment, it was possible to obtain a S. mansoni population with decreased susceptibility to PZQ. In our experiments we also observed that female worms were more susceptible to PZQ than male worms.

CONCLUSIONS

The selective pressure exerted by PZQ led to decreased genetic variability in S. mansoni and increased endogamy. The understanding of how S. mansoni populations respond to successive drug pressure has important implications on the appearance and maintenance of a PZQ resistance phenotype in endemic regions.

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.

Population genetic structure of Schistosoma mansoni and Schistosoma haematobium from across six sub-Saharan African countries: implications for epidemiology, evolution and control

We conducted the first meta-analysis of ten Schistosoma haematobium (one published and nine unpublished) and eight Schistosoma mansoni (two published and six unpublished) microsatellite datasets collected from individual schistosome-infected school-children across six sub-Saharan Africa countries. High levels of genetic diversity were documented in both S. haematobium and S. mansoni. In S. haematobium populations, allelic richness did not differ significantly between the ten schools, despite widely varying prevalences and intensities of infection, but higher levels of heterozygote deficiency were seen in East than in West Africa. In contrast, S. mansoni populations were more diverse in East than West African schools, but heterozygosity levels did not vary significantly with geography. Genetic structure in both S. haematobium and S. mansoni populations was documented, at both a regional and continental scale. Such structuring might be expected to slow the spread to new areas of anti-schistosomal drug resistance should it develop. There was, however, limited evidence of genetic structure at the individual host level, which might be predicted to promote the development or establishment of drug resistance, particularly if it were a recessive trait. Our results are discussed in terms of their potential implications for the epidemiology and evolution of schistosomes as well as their subsequent control across sub-Saharan Africa.

Drug resistance in vectorborne parasites: multiple actors and scenarios for an evolutionary arms race

Drug-resistant pathogens emerge faster than new drugs come out of drug discovery pipelines. Current and future drug options should therefore be better protected, requiring a clear understanding of the factors that contribute to the natural history of drug resistance. Although many of these factors are relatively well understood for most bacteria, this proves to be more complex for vectorborne parasites. In this review, we discuss considering three key models (Plasmodium, Leishmania and Schistosoma) how drug resistance can emerge, spread and persist. We demonstrate a multiplicity of scenarios, clearly resulting from the biological diversity of the different organisms, but also from the different modes of action of the drugs used, the specific within- and between-host ecology of the parasites, and environmental factors that may have direct or indirect effects. We conclude that integrated control of drug-resistant vectorborne parasites is not dependent upon chemotherapy only, but also requires a better insight into the ecology of these parasites and how their transmission can be impaired.

Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre- and post-praziquantel treatment in Tanzania

Recent shifts in global health policy have led to the implementation of mass drug administration (MDA) for neglected tropical diseases. Here we show how population genetic analyses can provide vital insights into the impact of such MDA on endemic parasite populations. We show that even a single round of MDA produced a genetic bottleneck with reductions in a range of measures of genetic diversity of Schistosoma mansoni. Phylogenetic analyses and indices of population differentiation indicated that schistosomes collected in the same schools in different years were more dissimilar than those from different schools collected within either of the study's 2 years, in addition to distinguishing re-infection from non-clearance (that might indicate putatively resistant parasites) from within those children infected at both baseline and follow-up. Such unique results illustrate the importance of genetic monitoring and examination of long lived multi-cellular parasites such as these under novel or increased chemotherapeutic selective pressures.

Molecular epidemiology and phylogeography of Schistosoma mansoni around Lake Victoria

Intestinal schistosomiasis continues to be a major public health problem in sub-Saharan Africa, and is endemic in communities around Lake Victoria. Interest is growing in the molecular evolution and population genetic structure of Schistosoma mansoni and we describe a detailed analysis of the molecular epidemiology and phylogeography of S. mansoni from Lake Victoria. In total, 388 cytochrome oxidase 1 (COI) sequences were obtained from 25 sites along the Ugandan, Tanzanian and Kenyan shorelines of Lake Victoria, and 122 unique barcodes were identified; 9 corresponded to previously discovered barcodes from Lakes Victoria and Albert. A subset of the data, composed of COI sequences from miracidia from 10 individual children, was used for population genetics analyses; these results were corroborated by microsatellite analysis of 4 isolates of lab-passaged adult worms. Overall, 12 barcodes were found to be shared across all 3 countries, whereas the majority occurred singly and were locally restricted. The population genetics analyses were in agreement in revealing high diversity at the level of the human host and negligible population structuring by location. The lack of correlation between genetic distance and geographical distance in these data may be attributed to the confounding influence of high intra-individual diversity as well as human migration between communities.

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.

Genetic diversity of schistosomes and snails: implications for control

Molecular approaches are providing new insights into the genetic diversity of schistosomes and their intermediate snail hosts. For instance, molecular tools based on the polymerase chain reaction are being developed for the diagnosis of schistosomiasis and the detection of prepatent schistosome infections in snails at transmission sites. Robust phylogenies of the different species of Schistosoma, Bulinus and Biomphalaria have been determined and novel methods are available to identify the different and cryptic taxa involved. Microsatellite analyses and mitochondrial DNA sequencing methods have been developed and are contributing to a better understanding of the genetic structure of both schistosome and snail populations. New sampling procedures to capture DNA of eggs and larval stages of schistosomes in field situations are facilitating more detailed and ethically advantageous studies on parasite heterogeneity. Knowledge of the genetic diversity of schistosome and snail populations adds a further dimension to the monitoring and surveillance of disease, and the implementation of new molecular-based approaches will be of increasing importance in helping to assess the impact of schistosomiasis control strategies.

Genetic structure of Schistosoma mansoni in western Kenya: The effects of geography and host sharing

We examined the spatial structure of Schistosoma mansoni, a parasite of humans, from natural infections at two levels: across the Lake Victoria basin of Kenya and among snail hosts. Using 20 microsatellite markers we examined geographic patterns of relatedness and population structure of cercariae and found weak, but significant structure detected by some, but not all analyses. We hypothesise structure created by aggregations of clonal individuals or adherence of hosts to local transmission sites is eroded by high amounts of gene flow in the region. This finding also supports previous hypotheses concerning the evolution of drug resistance in the region. Intrasnail dynamics were investigated in the context of aggregation and kin selection theory to determine how relatedness and also sex influence host sharing and host exploitation. Cercarial production did not differ significantly between snails with one or two genotypes suggesting that mixed infections resulted in decreased individual fitness and provides a framework for reproductive competition. Coinfection patterns in snails were independent of parasite relatedness indicating that schistosomes were not aggregated according to their relatedness and that kin selection was not influencing host sharing. Additionally, host exploitation in coinfections (measured by cercarial production) was not negatively correlated with relatedness, as predicted by classical models due to increased competition and thus exploitation when parasites are unrelated. Because of the low levels of relatedness within the population, schistosomes may rarely encounter close relatives and kin selection mechanisms that influence the distribution of individuals within snails or the virulence mode of the parasites may simply have not evolved.

A new approach to characterize populations of Schistosoma mansoni from humans: development and assessment of microsatellite analysis of pooled miracidia

OBJECTIVES

To develop and assess a microsatellite technique to characterize populations of Schistosoma mansoni from humans.

METHODS

For each of five patients, we calculated the allele count and frequency at 11 loci for several pools of miracidia (50 and 100), and compared these to population values, determined by amplifying microsatellites from 186 to 200 individual miracidia per patient.

RESULTS

We were able to detect up to 94.5% of alleles in pools. Allele count and frequency strongly and significantly correlated between singles and pools; marginally significant differences (P < 0.05) were detected for one patient (pools of 50) for allele frequencies and for two patients (pools of 100) for allele counts. Kato-Katz egg counts and number of alleles per pool did not co-vary, indicating that further direct comparisons of the results from these two techniques are needed.

CONCLUSIONS

Allele counts and frequency profiles from pooling provide important information about infection intensity and complexity, beyond that obtained from traditional methods. Although we are not advocating use of pooling to replace individual genotyping studies, it can potentially be useful in certain applications as a rapid and cost effective screening method for studies of S. mansoni population genetics, or as a more informative way to quantify and characterize human worm populations.

Evolutionary concepts in predicting and evaluating the impact of mass chemotherapy schistosomiasis control programmes on parasites and their hosts

Schistosomiasis is a parasitic disease of significant medical and veterinary importance in many regions of the world. Recent shifts in global health policy have led towards the implementation of mass chemotherapeutic control programmes at the national scale in previously 'neglected' countries such as those within sub-Saharan Africa. Evolutionary theory has an important role to play in the design, application and interpretation of such programmes. Whilst celebrating the rapid success achieved to date by such programmes, in terms of reduced infection prevalence, intensity and associated human morbidity, evolutionary change in response to drug selection pressure may be predicted under certain circumstances, particularly in terms of the development of potential drug resistance, evolutionary changes in parasite virulence, transmission and host use, and/or competitive interactions with co-infecting pathogens. Theoretical and empirical data gained to date serve to highlight the importance of careful monitoring and evaluation of parasites and their hosts whenever and wherever chemotherapy is applied and where parasite transmission remains.

Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village

The digenean trematode Schistosoma mansoni is responsible for chronic schistosomiasis worldwide, and in Brazil alone an estimated 35 million people are at risk. To evaluate epidemiological patterns among human definitive hosts, we assessed genetic diversity and population subdivision of S. mansoni infrapopulations in human hosts from the highly endemic village of Virgem das Graças in the state of Minas Gerais, Brazil. We believe this is the largest such survey to date. Genetic diversity of parasites, measured over eight polymorphic microsatellite loci, was relatively high and standard measures of inbreeding indicated that the population was panmictic. Furthermore, there was no significant isolation-by-distance of parasite infrapopulations, and measures of population subdivision indicated significant but low to moderate levels of population differentiation. We conclude that patients within this village sample from a broad range of schistosome genetic diversity and effectively act as "genetic mixing bowls" for the parasites. These results contrast with those previously observed in the Brazilian village of Melqui ades and thus provide the opportunity for comparisons of environmental and epidemiological differences that are likely to influence host-parasite coevolution and parasite transmission.