Behaviour change interventions for the control and elimination of schistosomiasis: A systematic review of evidence from low- and middle-income countries

BACKGROUND

For the last two decades, schistosomiasis control efforts have focussed on preventive treatment. The disease, however, still affects over 200 million people worldwide. Behaviour change (BC) interventions can strengthen control by interrupting transmission through modifying exposure behaviour (water contact) or transmission practices (open urination/defaecation); or through fostering treatment seeking or acceptance. This review examines these interventions to assess their effectiveness in modifying risk practices and affecting epidemiological trends.

METHODOLOGY/PRINCIPAL FINDINGS

A systematic multi-database literature search (PROSPERO CRD42021252368) was conducted for peer-reviewed publications released at any time before June 2021 assessing BC interventions for schistosomiasis control in low- and middle-income countries. 2,593 unique abstracts were identified, 66 were assigned to full text review, and 32 met all inclusion criteria. A typology of intervention models was outlined according to their use of behaviour change techniques and overarching rationale: health education (HEIs), social-environmental (SEIs), physical-environmental (PEIs), and incentives-centred interventions (ICIs). Available evidence does not allow to identify which BC approach is most effective in controlling risk behaviour to prevent schistosomiasis transmission. HEIs' impacts were observed to be limited by structural considerations, like infrastructure underdevelopment, economic obligations, socio-cultural traditions, and the natural environment. SEIs may address those challenges through participatory planning and implementation activities, which enable social structures, like governance and norms, to support BC. Their effects, however, appear context-sensitive. The importance of infrastructure investments was highlighted by intervention models. To adequately support BC, however, they require users' inputs and complementary services. Whilst ICIs reported positive impacts on treatment uptake, there are cost-effectiveness and sustainability concerns. Evaluation studies yielded limited evidence of independent epidemiological impacts from BC, due to limited use of suitable indicators and comparators. There was indicative evidence, however, that BC projects could sustain gains through treatment campaigns.

CONCLUSIONS/SIGNIFICANCE

There is a need for integrated interventions combining information provision, community-based planning, and infrastructure investments to support BC for schistosomiasis control. Programmes should carefully assess local conditions before implementation and consider that long-term support is likely needed. Available evidence indicates that BC interventions may contribute towards schistosomiasis control when accompanied by treatment activities. Further methodologically robust evidence is needed to ascertain the direct epidemiological benefits of BC.

Parasite Population Genetic Contributions to the Schistosomiasis Consortium for Operational Research and Evaluation within Sub-Saharan Africa

Analyses of the population genetic structure of schistosomes under the "Schistosomiasis Consortium for Operational Research and Evaluation" (SCORE) contrasting treatment pressure scenarios in Tanzania, Niger, and Zanzibar were performed to provide supplementary critical information with which to evaluate the impact of these large-scale control activities and guide how activities could be adjusted. We predicted that population genetic analyses would reveal information on a range of important parameters including, but not exclusive to, recruitment and transmission of genotypes, occurrence of hybridization events, differences in reproductive mode, and degrees of inbreeding, and hence, the evolutionary potential, and responses of parasite populations under contrasting treatment pressures. Key findings revealed that naturally high levels of gene flow and mixing of the parasite populations between neighboring sites were likely to dilute any effects imposed by the SCORE treatment arms. Furthermore, significant inherent differences in parasite fecundity were observed, independent of current treatment arm, but potentially of major impact in terms of maintaining high levels of ongoing transmission in persistent "biological hotspot" sites. Within Niger, naturally occurring Schistosoma haematobium/Schistosoma bovis viable hybrids were found to be abundant, often occurring in significantly higher proportions than that of single-species S. haematobium infections. By examining parasite population genetic structures across hosts, treatment regimens, and the spatial landscape, our results to date illustrate key transmission processes over and above that which could be achieved through standard parasitological monitoring of prevalence and intensity alone, as well as adding to our understanding of Schistosoma spp. life history strategies in general.

Snail-Related Contributions from the Schistosomiasis Consortium for Operational Research and Evaluation Program Including Xenomonitoring, Focal Mollusciciding, Biological Control, and Modeling

The Schistosomiasis Consortium for Operational Research and Evaluation (SCORE) was created in 2008 to answer questions of importance to program managers working to reduce the burden of schistosomiasis in Africa. In the past, intermediate host snail monitoring and control was an important part of integrated schistosomiasis control. However, in Africa, efforts to control snails have declined dramatically over the last 30 years. A resurgence of interest in the control of snails has been prompted by the realization, backed by a World Health Assembly resolution (WHA65.21), that mass drug administration alone may be insufficient to achieve schistosomiasis elimination. SCORE has supported work on snail identification and mapping and investigated how xenomonitoring techniques can aid in the identification of infected snails and thereby identify potential transmission areas. Focal mollusciciding with niclosamide was undertaken in Zanzibar and Côte d'Ivoire as a part of elimination studies. Two studies involving biological control of snails were conducted: one explored the association of freshwater riverine prawns and snail hosts in Côte d'Ivoire and the other assessed the current distribution of Procambarus clarkii, the invasive Louisiana red swamp crayfish, in Kenya and its association with snail hosts and schistosomiasis transmission. SCORE also supported modeling studies on the importance of snail control in achieving elimination and a meta-analysis of the impact of molluscicide-based snail control programs on human schistosomiasis prevalence and incidence. SCORE's snail control studies contributed to increased investment in building capacity, and specimens collected during SCORE research deposited in the Schistosomiasis Collections at the Natural History Museum (SCAN) will provide a valuable resource for the years to come.

Oxamniquine resistance alleles are widespread in Old World Schistosoma mansoni and predate drug deployment

Do mutations required for adaptation occur de novo, or are they segregating within populations as standing genetic variation? This question is key to understanding adaptive change in nature, and has important practical consequences for the evolution of drug resistance. We provide evidence that alleles conferring resistance to oxamniquine (OXA), an antischistosomal drug, are widespread in natural parasite populations under minimal drug pressure and predate OXA deployment. OXA has been used since the 1970s to treat Schistosoma mansoni infections in the New World where S. mansoni established during the slave trade. Recessive loss-of-function mutations within a parasite sulfotransferase (SmSULT-OR) underlie resistance, and several verified resistance mutations, including a deletion (p.E142del), have been identified in the New World. Here we investigate sequence variation in SmSULT-OR in S. mansoni from the Old World, where OXA has seen minimal usage. We sequenced exomes of 204 S. mansoni parasites from West Africa, East Africa and the Middle East, and scored variants in SmSULT-OR and flanking regions. We identified 39 non-synonymous SNPs, 4 deletions, 1 duplication and 1 premature stop codon in the SmSULT-OR coding sequence, including one confirmed resistance deletion (p.E142del). We expressed recombinant proteins and used an in vitro OXA activation assay to functionally validate the OXA-resistance phenotype for four predicted OXA-resistance mutations. Three aspects of the data are of particular interest: (i) segregating OXA-resistance alleles are widespread in Old World populations (4.29–14.91% frequency), despite minimal OXA usage, (ii) two OXA-resistance mutations (p.W120R, p.N171IfsX28) are particularly common (>5%) in East African and Middle-Eastern populations, (iii) the p.E142del allele has identical flanking SNPs in both West Africa and Puerto Rico, suggesting that parasites bearing this allele colonized the New World during the slave trade and therefore predate OXA deployment. We conclude that standing variation for OXA resistance is widespread in S. mansoni.

It has been argued that drug resistance is unlikely to spread rapidly in helminth parasites infecting humans. This is based, at least in part, on the premise that resistance mutations are rare or absent within populations prior to treatment, and take a long time to reach appreciable frequencies because helminth parasite generation time is long. This argument is critically dependent on the starting frequency of resistance alleles–if high levels of “standing variation” for resistance are present prior to deployment of treatment, resistance may spread rapidly. We examined frequencies of oxamniquine resistance alleles present in Schistosoma mansoni from Africa and the Middle East where oxamniquine has seen minimal use. We found that oxamniquine resistance alleles are widespread in the Old World, ranging from 4.29% in the Middle East to 14.91% in East African parasite populations. Furthermore, we show that resistance alleles from West African and the Caribbean schistosomes share a common origin, suggesting that these alleles travelled to the New World with S. mansoni during the transatlantic slave trade. Together, these results demonstrate extensive standing variation for oxamniquine resistance. Our results have important implications for both drug treatment policies and drug development efforts, and demonstrate the power of molecular surveillance approaches for guiding helminth control.

Longitudinal survey on the distribution of Biomphalaria sudanica and B. choanomophala in Mwanza region, on the shores of Lake Victoria, Tanzania: implications for schistosomiasis transmission and control

BACKGROUND

Schistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live. The two intermediate host species of Schistosoma mansoni in the Mwanza region are Biomphalaria sudanica, found on the banks of the lakes, and B. choanomphala, found in the lake itself. There are few longitudinal surveys documenting changing abundance and differential transmission patterns of these Biomphalaria snails across seasons and years. We undertook 15 field surveys at 26 sites over four years to determine the parameters that influence Biomphalaria abundance, presence of S. mansoni-shedding snails and impact of schistosomiasis treatment interventions on transmission potential in the Mwanza region.

RESULTS

Statistical analysis revealed seasonal difference in the abundance of B. sudanica with the highest number of snails found in the dry season (Kruskal-Wallis χ ² = 37.231, df = 3, P < 0.0001). Water measurements were not associated with B. sudanica abundance; however, high levels of rainfall did have a negative effect on B. sudanica [coefficient effect -0.1405, 95% CI (-0.2666, -0.0144)] and B. choanomphala abundance [coefficient effect -0.4388, 95% CI (-0.8546, -0.0231)] potentially due to inundation of sites "diluting" the snails and influencing collection outcome. Biomphalaria sudanica snails were found at all sites whereas B. choanomphala were far more focal and only found in certain sites. Shedding Biomphalaria did not show any variation between dry and rainy seasons; however, a decrease in shedding snails was observed in year 4 of the study.

CONCLUSIONS

Biomphalaria sudanica is uniformly present in the Mwanza region whereas B. choanomphala is far more focal. Seasonality plays a role for B. sudanica abundance, likely due to its habitat preference on the banks of the lake, but not for B. choanomphala. The decrease in shedding Biomphalaria abundance in Year 4 could be linked to ongoing schistosomiasis treatment efforts in the neighbouring human populations. The highest number of shedding Biomphalaria was observed at sites with high levels of human movement. Prioritising snail control at such sites could greatly reduce transmission in these high-risk areas.

DNA 'barcoding' of Schistosoma mansoni across sub-Saharan Africa supports substantial within locality diversity and geographical separation of genotypes

Schistosoma mansoni is a widespread human helminth and causes intestinal schistosomiasis in 54 countries, mainly across Africa but also in Madagascar, the Arabian Peninsula and the neotropics. The geographical range of this parasite relies on the distribution of certain species of freshwater pulmonate snails of the genus Biomphalaria. Whilst S. mansoni is known to exhibit high population diversity the true extent of this diversity is still to be fully elucidated as sampling of this taxon progressively accrues. Here a DNA 'barcoding' approach is taken using sequence analysis of a 450bp region within the mitochondrial cox1 gene to assess the genetic diversity within a large number of S. mansoni larval stages collected from their natural human hosts across sub-Saharan Africa. Five hundred and sixty one individual parasite samples were examined from 22 localities and 14 countries. Considerable within-species diversity was found with 120 unique haplotypes splitting geographically into five discrete lineages. The highest diversity was found in East Africa with samples forming three of the five lineages. Less diversity was found in the Far and Central Western regions of Africa with haplotypes from the New World showing a close affinity to the Far Western African S. mansoni populations supporting the hypothesis of a colonisation of South America via the West African slave trade. The data are discussed in relation to parasite diversity and disease epidemiology.

Efficacy and safety of two closely spaced doses of praziquantel against Schistosoma haematobium and S. mansoni and re-infection patterns in school-aged children in Niger

The aim of this study was to assess the efficacy and safety of two closely spaced doses of praziquantel (PZQ) against Schistosoma haematobium and S. mansoni infection in school-aged children, and to characterise re-infection patterns over a 12-month period. The study was carried out in five villages in western Niger: Falmado, Seberi and Libore (single S. haematobium infection foci), and Diambala and Namarigoungou (mixed S. haematobium-S. mansoni infection foci). Parasitological examinations consisted of triplicate urine filtrations and triplicate Kato-Katz thick smears at each visit. Two 40mg/kg oral doses of PZQ were administered 3 weeks apart. Adverse events were monitored within 4h after dosing by the survey team and 24h after treatment using a questionnaire. Our final study cohort comprised 877 children who were infected with either S. haematobium, or S. mansoni, or both species concurrently and received both doses of PZQ. Follow-up visits were conducted 6 weeks, 6 months and 12 months after the first dose of PZQ. At baseline, the geometric mean (GM) infection intensity of S. haematobium ranged from 3.6 (Diambala) to 30.3eggs/10ml of urine (Falmado). The GM infection intensity of S. mansoni ranged from 86.7 (Diambala) to 151.4eggs/g of stool (Namarigoungou). Adverse events were reported by 33.0% and 1.5% of the children after the first and second doses of PZQ, respectively. We found cure rates (CRs) in S. haematobium-infected children 3 weeks after the second dose of PZQ ranging between 49.2% (Falmado) and 98.4% (Namarigoungou) and moderate-to-high egg reduction rates (ERRs) (71.4-100%). Regarding S. mansoni, only moderate CRs and ERRs were found (51.7-58.8% in Diambala, 55.2-60.2% in Namarigoungou). Twelve months post-treatment, prevalence rates approached pre-treatment levels, but infection intensities remained low. In conclusion, PZQ, given in two closely spaced doses, is efficacious against S. haematobium, but the low ERR observed against S. mansoni raises concern about mounting PZQ tolerance.

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.

The impact of single versus mixed Schistosoma haematobium and S. mansoni infections on morbidity profiles amongst school-children in Taveta, Kenya

Two schistosome species--Schistosoma haematobium and S. mansoni--with two very different pathological profiles (urogenital versus intestinal), are responsible for the majority of human schistosomiasis infections across sub-Saharan Africa. The aim of this study was to determine whether coinfections have an impact on species-specific morbidity measures when compared to single species infections. Children from two neighbouring schools in Taveta, Kenya were grouped by infection status, i.e. uninfected, single species infections or coinfected. Clinical examination of the liver and spleen by palpation was performed and urinary albumin levels were recorded at baseline and at 12 months after praziquantel administration. Additional ultrasonographic profiles of the children's liver, spleen and bladder were incorporated at follow-up. It was found that S. haematobium-associated urogenital morbidity was lower in the coinfected group relative to single S. haematobium infections, even when infection intensities were taken into account. We also observed an association between S. haematobium infection and liver (intestinal-associated) morbidity regardless of coinfections. The findings reported here suggest that further research should be performed on the impact of S. haematobium infections on liver morbidity as well as to determine the impact of mixed schistosome species infections on human morbidity outcomes across different endemic settings.