Snail Control Strategies for Reduction of Schistosomiasis Transmission

Utilisation of Oxford Nanopore sequencing to generate six complete gastropod mitochondrial genomes as part of a biodiversity curriculum

High-throughput sequencing has enabled genome skimming approaches to produce complete mitochondrial genomes (mitogenomes) for species identification and phylogenomics purposes. In particular, the portable sequencing device from Oxford Nanopore Technologies (ONT) has the potential to facilitate hands-on training from sampling to sequencing and interpretation of mitogenomes. In this study, we present the results from sampling and sequencing of six gastropod mitogenomes (Aplysia argus, Cellana orientalis, Cellana toreuma, Conus ebraeus, Conus miles and Tylothais aculeata) from a graduate level biodiversity course. The students were able to produce mitogenomes from sampling to annotation using existing protocols and programs. Approximately 4 Gb of sequence was produced from 16 Flongle and one MinION flow cells, averaging 235 Mb and N50 = 4.4 kb per flow cell. Five of the six 14.1-18 kb mitogenomes were circlised containing all 13 core protein coding genes. Additional Illumina sequencing revealed that the ONT assemblies spanned over highly AT rich sequences in the control region that were otherwise missing in Illumina-assembled mitogenomes, but still contained a base error of one every 70.8-346.7 bp under the fast mode basecalling with the majority occurring at homopolymer regions. Our findings suggest that the portable MinION device can be used to rapidly produce low-cost mitogenomes onsite and tailored to genomics-based training in biodiversity research.

Three reasons why expanded use of natural enemy solutions may offer sustainable control of human infections

1. Many infectious pathogens spend a significant portion of their life cycles in the environment or in animal hosts, where ecological interactions with natural enemies may influence pathogen transmission to people. Yet, our understanding of natural enemy opportunities for human disease control is lacking, despite widespread uptake and success of natural enemy solutions for pest and parasite management in agriculture. 2. Here we explore three reasons why conserving, restoring, or augmenting specific natural enemies in the environment could offer a promising complement to conventional clinical strategies to fight environmentally mediated pathogens and parasites. (1) Natural enemies of human infections abound in nature, largely understudied and undiscovered. (2) Natural enemy solutions could provide ecological options for infectious disease control where conventional interventions are lacking. And, (3) Many natural enemy solutions could provide important co-benefits for conservation and human well-being. 3. We illustrate these three arguments with a broad set of examples whereby natural enemies of human infections have been used or proposed to curb human disease burden, with some clear successes. However, the evidence base for most proposed solutions is sparse, and many opportunities likely remain undiscovered, highlighting opportunities for future research.

Molecular Characterization of Rotifers and Their Potential Use in the Biological Control of Biomphalaria

Background

Schistosomiasis is one of the most important tropical parasitic diseases worldwide. Biomphalaria straminea, the intermediate host of Schistosoma mansoni, has invaded and spread to Southern China since 1974 and may pose enormous threats to public health. Controlling intermediate host snails is an effective strategy in schistosomiasis intervention. However, the only effective chemical molluscicide, niclosamide, currently recommended by WHO may cause environmental pollution, loss of biodiversity, and high costs. Thus, to counter intermediate hosts, a sustainable and environmentally friendly tool is urgently needed. Here, we conducted field investigations to collect and identify a potential snail competitor rotifer and evaluated its molluscicide effect.

Results

In this study, we collected two samples of rotifers from Shenzhen. We found both red and black phenotypic B. straminea snails at the sampling sites. We identified the rotifer population as a species of the genus Philodina according to the amplification and phylogenetic analysis results of coxI gene. We found that rotifer exposure did not significantly affect the hatching rate of B. straminea eggs but promoted the killing of juvenile snails. Meanwhile, rotifer exposure did not significantly alter the fecundity of B. straminea quantified by the number of eggs per egg mass, the number of egg masses per snail, and the number of eggs per snail; but the snails exposed to rotifers showed lower fecundity performance than the control snails. Importantly, rotifer exposure could significantly affect the development of juvenile B. straminea, showing a smaller shell diameter of the exposed snails than that of the control snails. In addition, rotifer exposure affected the life span of B. straminea snails, showing a 16.61% decline in the average life span. After rotifer exposure, the S. mansoni-infected B. straminea snails died significantly faster than those without rotifer exposure. Similar findings were observed in S. mansoni-infected Biomphalaria glabrata snails. These results implied that rotifer exposure significantly promoted the mortality of S. mansoni-infected B. straminea and B. glabrata.

Conclusions

Our study demonstrated the potential molluscicide effect of rotifers on intermediate hosts under laboratory conditions. Our findings may provide new insights into the development of biocontrol strategies for snail-borne disease transmission.

Molluscicides against the snail-intermediate host of Schistosoma: a review

Schistosomiasis, a neglected tropical disease (NTD), is one of the most prevalent parasitoses in the World. Certain freshwater snail species are the intermediate host in the life cycle of schistosome species. Controlling snails employing molluscicides is an effective, quick, and convenient intervention strategy to prevent the spread of Schistosoma species in endemic regions. Advances have been made in developing both synthetic molluscicides and molluscicides derived from plants. However, at present, the development of molluscicides is not adapted to the actual demand for snails and schistosoma controlling. We undertake a systematic review of exploitation and application of synthetic molluscicides and molluscicides derived from plants to combat intermediate host snails. The detailed molluscicidal activity, structure–activity relationship, structural feature, and possible mechanism of some molluscicides are also highlighted, which may afford an important reference for the design of new, more effective molluscicides with low environmental impact and realize the aim of controlling schistosome at transmission stages.

Contrasting physiological responses between invasive sea lamprey and non-target bluegill in response to acute lampricide exposure

Control of invasive sea lamprey (Petromyzon marinus) in the Laurentian Great Lakes of North America uses lampricides, which consist of 3-trifluoromethyl-4-nitrophenol (TFM) and niclosamide. Lampricides are thought to inhibit aerobic energy synthesis, with TFM having a relatively greater selective action against lampreys. While the toxicity and physiological effects of TFM are known, the impacts associated with exposure to niclosamide and TFM:niclosamide mixtures are poorly characterized in fishes. Therefore, focusing on energy metabolism, we quantified the physiological responses of larval sea lamprey and bluegill (Lepomis macrochirus), a non-target, native species. Exposures consisted of each lampricide alone (TFM at the species-specific 24 h LC₁₀; niclosamide at 1.5% of the mixture's TFM concentration) or a mixture of the two (larval sea lamprey at TFM 24 h LC₁₀ + 1.5% niclosamide; bluegill at sea lamprey's TFM 24 h LC_99.9 + 1.5% niclosamide) for 24 h. Tissues (brain, skeletal muscle, and liver) were sampled at 6, 12, and 24 h of exposure and assayed for concentrations of ATP, phosphocreatine, glycogen, lactate, and glucose and tissue lampricide levels. In larval sea lamprey, TFM had little effect on brain and skeletal muscle, but niclosamide resulted in a depletion of high energy substrates in both tissues. Mixture-exposed lamprey showed depletion of high energy substrates, accumulation of lactate, and high mortality rates. Bluegill were largely unaffected by toxicant exposures. However, bluegill liver showed lower glycogen and lactate under all three toxicant exposures suggesting increased metabolic turnover. Bluegill also had lower concentrations of TFM and niclosamide in their tissues when compared to lamprey. Our results indicate that lampricide toxicity in sea lamprey larvae is mediated through a depletion of high energy substrates because of impaired aerobic ATP synthesis. We also confirmed that non-target bluegill showed high tolerance to lampricide exposure, an effect potentially mediated through a high detoxification capacity relative to lampreys.

Approaches in scaling up schistosomiasis intervention towards transmission elimination in Africa: Leveraging from the Chinese experience and lessons

Schistosoma japonicum, differs from the African species including S. mansoni and S. haematobium, is a zoonotic parasite as it infects both human and animals including domestic ruminant animals such as cattle and animals from the wild. Considering China's success story in the elimination of schistosomiasis, the China-Africa collaboration on schistosomaisis elimination in Africa is an important cooperative health development initiative. This review examines the importance of China-Africa collaboration on schistosomiasis elimination using effective surveillance-response intervention strategy as the platform to effectively drive the elimination of schistosomiasis in Africa. Three conclusions were made after reviewing the similarity and differences in schistososmiasis control programmes between China and African continent as follows: (i) Politically, China's lessons is that leveraging on the integrated control strategies and the recognition that schistosomiasis is a public health problem which prompted the interest of government in China. It is necessary for African leaders and governments to recognize schistosomiasis as a public health challenge that must be given serious attention in terms of funding and setting up frameworks to complement control efforts. (ii) Technically, efficient monitoring and surveillance system mechanism will facilitate contextual and effective management of schistosomiasis elimination across different environment, and African programme managers should embrace the use of appropriate diagnostic tools to guide treatment strategies at different thresholds of schistosomiasis control. (iii) Strategically, effective control of snail intermediate hosts and precision mapping of snail distribution should be prioritized for successful schistosomiasis elimination in Africa.

Parasite associations predict infection risk: incorporating co-infections in predictive models for neglected tropical diseases

BACKGROUND

Schistosomiasis and infection by soil-transmitted helminths are some of the world's most prevalent neglected tropical diseases. Infection by more than one parasite (co-infection) is common and can contribute to clinical morbidity in children. Geostatistical analyses of parasite infection data are key for developing mass drug administration strategies, yet most methods ignore co-infections when estimating risk. Infection status for multiple parasites can act as a useful proxy for data-poor individual-level or environmental risk factors while avoiding regression dilution bias. Conditional random fields (CRF) is a multivariate graphical network method that opens new doors in parasite risk mapping by (i) predicting co-infections with high accuracy; (ii) isolating associations among parasites; and (iii) quantifying how these associations change across landscapes.

METHODS

We built a spatial CRF to estimate infection risks for Ascaris lumbricoides, Trichuris trichiura, hookworms (Ancylostoma duodenale and Necator americanus) and Schistosoma mansoni using data from a national survey of Rwandan schoolchildren. We used an ensemble learning approach to generate spatial predictions by simulating from the CRF's posterior distribution with a multivariate boosted regression tree that captured non-linear relationships between predictors and covariance in infection risks. This CRF ensemble was compared against single parasite gradient boosted machines to assess each model's performance and prediction uncertainty.

RESULTS

Parasite co-infections were common, with 19.57% of children infected with at least two parasites. The CRF ensemble achieved higher predictive power than single-parasite models by improving estimates of co-infection prevalence at the individual level and classifying schools into World Health Organization treatment categories with greater accuracy. The CRF uncovered important environmental and demographic predictors of parasite infection probabilities. Yet even after capturing demographic and environmental risk factors, the presences or absences of other parasites were strong predictors of individual-level infection risk. Spatial predictions delineated high-risk regions in need of anthelminthic treatment interventions, including areas with higher than expected co-infection prevalence.

CONCLUSIONS

Monitoring studies routinely screen for multiple parasites, yet statistical models generally ignore this multivariate data when assessing risk factors and designing treatment guidelines. Multivariate approaches can be instrumental in the global effort to reduce and eventually eliminate neglected helminth infections in developing countries.

Biomphalaria glabrata immunity: Post-genome advances

The freshwater snail, Biomphalaria glabrata, is an important intermediate host in the life cycle for the human parasite Schistosoma mansoni, the causative agent of schistosomiasis. Current treatment and prevention strategies have not led to a significant decrease in disease transmission. However, the genome of B. glabrata was recently sequenced to provide additional resources to further our understanding of snail biology. This review presents an overview of recently published, post-genome studies related to the topic of snail immunity. Many of these reports expand on findings originated from the genome characterization. These novel studies include a complementary gene linkage map, analysis of the genome of the B. glabrata embryonic (Bge) cell line, as well as transcriptomic and proteomic studies looking at snail-parasite interactions and innate immune memory responses towards schistosomes. Also included are biochemical investigations on snail pheromones, neuropeptides, and attractants, as well as studies investigating the frontiers of molluscan epigenetics and cell signaling were also included. Findings support the current hypotheses on snail-parasite strain compatibility, and that snail host resistance to schistosome infection is dependent not only on genetics and expression, but on the ability to form multimeric molecular complexes in a timely and tissue-specific manner. The relevance of cell immunity is reinforced, while the importance of humoral factors, especially for secondary infections, is supported. Overall, these studies reflect an improved understanding on the diversity, specificity, and complexity of molluscan immune systems.

Precision mapping of snail habitat provides a powerful indicator of human schistosomiasis transmission

Recently, the World Health Organization recognized that efforts to interrupt schistosomiasis transmission through mass drug administration have been ineffective in some regions; one of their new recommended strategies for global schistosomiasis control emphasizes targeting the freshwater snails that transmit schistosome parasites. We sought to identify robust indicators that would enable precision targeting of these snails. At the site of the world's largest recorded schistosomiasis epidemic-the Lower Senegal River Basin in Senegal-intensive sampling revealed positive relationships between intermediate host snails (abundance, density, and prevalence) and human urogenital schistosomiasis reinfection (prevalence and intensity in schoolchildren after drug administration). However, we also found that snail distributions were so patchy in space and time that obtaining useful data required effort that exceeds what is feasible in standard monitoring and control campaigns. Instead, we identified several environmental proxies that were more effective than snail variables for predicting human infection: the area covered by suitable snail habitat (i.e., floating, nonemergent vegetation), the percent cover by suitable snail habitat, and size of the water contact area. Unlike snail surveys, which require hundreds of person-hours per site to conduct, habitat coverage and site area can be quickly estimated with drone or satellite imagery. This, in turn, makes possible large-scale, high-resolution estimation of human urogenital schistosomiasis risk to support targeting of both mass drug administration and snail control efforts.

Mapping Schistosoma mansoni endemicity in Rwanda: a critical assessment of geographical disparities arising from circulating cathodic antigen versus Kato-Katz diagnostics

BACKGROUND

Schistosomiasis is a neglected tropical disease caused by Schistosoma parasites. Intervention relies on identifying high-risk regions, yet rapid Schistosoma diagnostics (Kato-Katz stool assays (KK) and circulating cathodic antigen urine assays (CCA)) yield different prevalence estimates. We mapped S. mansoni prevalence and delineated at-risk regions using a survey of schoolchildren in Rwanda, where S. mansoni is an endemic parasite. We asked if different diagnostics resulted in disparities in projected infection risk.

METHODS

Infection data was obtained from a 2014 Rwandan school-based survey that used KK and CCA diagnostics. Across 386 schools screened by CCA (N = 19,217). To allow for uncertainty when interpreting ambiguous CCA trace readings, which accounted for 28.8% of total test results, we generated two presence-absence datasets: CCA trace as positive and CCA trace as negative. Samples (N = 9,175) from 185 schools were also screened by KK. We included land surface temperature (LST) and the Normalized Difference Vegetation and Normalized Difference Water Indices (NDVI, NDWI) as predictors in geostatistical regressions.

FINDINGS

Across 8,647 children tested by both methods, prevalence was 35.93% for CCA trace as positive, 7.21% for CCA trace as negative and 1.95% for KK. LST was identified as a risk factor using KK, whereas NDVI was a risk factor for CCA models. Models predicted high endemicity in Northern and Western regions of Rwanda, though the CCA trace as positive model identified additional high-risk areas that were overlooked by the other methods. Estimates of current burden for children at highest risk (boys aged 5-9 years) varied by an order of magnitude, with 671,856 boys projected to be infected by CCA trace as positive and only 60,453 projected by CCA trace as negative results.

CONCLUSIONS

Our findings show that people in Rwanda's Northern, Western and capital regions are at high risk of S. mansoni infection. However, variation in identification of environmental risk factors and delineation of at-risk regions using different diagnostics likely provides confusing messages to disease intervention managers. Further research and statistical analyses, such as latent class analysis, can be used to improve CCA result classification and assess its use in guiding treatment regimes.

Identifying and Predicting the Geographical Distribution Patterns of Oncomelania hupensis

Schistosomiasis is a snail-borne parasitic disease endemic to the tropics and subtropics, whose distribution depends on snail prevalence as determined by climatic and environmental factors. Here, dynamic spatial and temporal patterns of Oncomelania hupensis distributions were quantified using general statistics, global Moran’s I, and standard deviation ellipses, with Maxent modeling used to predict the distribution of habitat areas suitable for this snail in Gong’an County, a severely affected region of Jianghan Plain, China, based on annual average temperature, humidity of the climate, soil type, normalized difference vegetation index, land use, ditch density, land surface temperature, and digital elevation model variables; each variable’s contribution was tested using the jackknife method. Several key results emerged. First, coverage area of O. hupensis had changed little from 2007 to 2012, with some cities, counties, and districts alternately increasing and decreasing, with ditch and bottomland being the main habitat types. Second, although it showed a weak spatial autocorrelation, changing negligibly, there was a significant east–west gradient in the O. hupensis habitat area. Third, 21.9% of Gong’an County’s area was at high risk of snail presence; and ditch density, temperature, elevation, and wetting index contributed most to their occurrence. Our findings and methods provide valuable and timely insight for the control, monitoring, and management of schistosomiasis in China.

Control of invasive sea lampreys using the piscicides TFM and niclosamide: Toxicology, successes & future prospects

The invasion of the Laurentian Great Lakes of North America by sea lampreys (Petromyzon marinus) in the early 20th century contributed to the depletion of commercial, recreational and culturally important fish populations, devastating the economies of communities that relied on the fishery. Sea lamprey populations were subsequently controlled using an aggressive integrated pest-management program which employed barriers and traps to prevent sea lamprey from migrating to their spawning grounds and the use of the piscicides (lampricides) 3-trifluoromethyl-4-nitrophenol (TFM) and niclosamide to eliminate larval sea lampreys from their nursery streams. Although sea lampreys have not been eradicated from the Great Lakes, populations have been suppressed to less than 10% of their peak numbers in the mid-1900s. The ongoing use of lampricides provides the foundation for sea lamprey control in the Great Lakes, one of the most successful invasive species control programs in the world. Yet, significant gaps remain in our understanding of how lampricides are taken-up and handled by sea lampreys, how lampricides exert their toxic effects, and how they adversely affect non-target invertebrate and vertebrates species. In this review we examine what has been learned about the uptake, handling and elimination, and the mode of TFM and niclosamide toxicity in lampreys and in non-target animals, particularly in the last 10 years. It is now clear that the mode of TFM toxicity is the same in non-target fishes and lampreys, in which TFM interferes with oxidative phosphorylation by the mitochondria leading to decreased ATP production. Vulnerability to TFM is related to abiotic factors such as water pH and alkalinity, which we propose changes the relative amounts of the bioavailable un-ionized form of TFM in the gill microenvironment. Niclosamide, which is also a molluscicide used to control snails in areas prone to schistosomiasis infections of humans, also likely works by uncoupling oxidative phosphorylation, but less is known about other aspects of its toxicology. The effects of TFM include reductions in energy stores, particularly glycogen and high energy phosphagens. However, non-target fishes readily recover from sub-lethal TFM exposure as demonstrated by the rapid restoration of energy stores and clearance of TFM. Although both TFM and niclosamide are non-persistent in the environment and critical for sea lamprey control, increasing public and institutional concerns about pesticides in the environment makes it imperative to explore other means of sea lamprey control. Accordingly, we also address possible "next-generation" strategies of sea lamprey control including genetic tools such as RNA interference and CRISPR-Cas9 to impair critical physiological processes (e.g. reproduction, digestion, metamorphosis) in lamprey, and the use of green chemistry to develop more environmentally benign chemical methods of sea lamprey control.

Association of riverine prawns and intermediate host snails and correlation with human schistosomiasis in two river systems in south-eastern Côte d'Ivoire

The current emphasis of schistosomiasis control is placed on preventive chemotherapy using praziquantel. However, reinfection may occur rapidly in the absence of complementary interventions. Recent studies from Senegal suggest that predatory prawns might feed on intermediate host snails and thus impact on schistosomiasis transmission. We designed a study with four repeated cross-sectional surveys pertaining to prawns and snails, coupled with a single cross-sectional parasitological survey among humans. We assessed for potential associations between the presence/density of prawns and snails and correlation with Schistosoma infection in a composite sample of school-aged children and adults. The study was carried out between October 2015 and December 2016 in 24 villages located near the Agnéby and Mé coastal river systems in south-eastern Côte d'Ivoire. At each site, snails and prawns were collected, and in each village, 150 individuals were subjected to stool and urine examination for the diagnosis of Schistosoma mansoni and S. haematobium. We found peaks of relative abundance of intermediate host snails in the villages of the Agnéby River system, while predatory prawns were predominantly recorded in the Mé River system. A negative association was observed between intermediate host snail densities and riverine prawns; however, no pattern was found between this trend in the predator-prey relationship and the prevalence of human schistosomiasis.

Treading the Path towards Genetic Control of Snail Resistance to Schistosome Infection

Schistosomiasis remains the most important tropical snail-borne trematodiasis that threatens many millions of human lives. In achieving schistosomiasis elimination targets, sustainable control of the snail vectors represents a logical approach. Nonetheless, the ineffectiveness of the present snail control interventions emphasizes the need to develop new complementary strategies to ensure more effective control outcomes. Accordingly, the use of genetic techniques aimed at driving resistance traits into natural vector populations has been put forward as a promising tool for integrated snail control. Leveraging the Biomphalaria-Schistosoma model system, studies unraveling the complexities of the vector biology and those exploring the molecular basis of snail resistance to schistosome infection have been expanding in various breadths, generating many significant discoveries, and raising the hope for future breakthroughs. This review provides a compendium of relevant findings, and without neglecting the current existing gaps and potential future challenges, discusses how a transgenic snail approach may be adapted and harnessed to control human schistosomiasis.

Discovery of the pyridylphenylureas as novel molluscicides against the invasive snail Biomphalaria straminea, intermediate host of Schistosoma mansoni

BACKGROUND

The snail Biomphalaria straminea is one of the intermediate hosts of Schistosoma mansoni. Biomphalaria straminea is also an invasive species, known for its strong capability on peripheral expansion, long-distance dispersal and colonization. Using molluscicides to control snail populations is an important strategy to interrupt schistosomiasis transmission and to prevent the spread of the invasive species. In this study, a series of pyridylphenylurea derivatives were synthesized as potential molluscicides. Their impact on adult snails and egg masses was evaluated. Acute toxicity to fish of the derivatives was also examined to assess their effect on non-target organisms. The preliminary mechanisms of action of the derivatives were studied by enzyme activity assays.

RESULTS

The representative compounds, 1-(4-chlorophenyl)-3-(pyridin-3-yl)urea (compound 8) and 1-(4-bromophenyl)-3-(pyridin-3-yl)urea (compound 9), exhibited strong molluscicidal activity against adult snails with LD₅₀ values of 0.50 and 0.51 mg/l and potent inhibitory effects on snail egg hatchability with IC₅₀ values of 0.05 and 0.09 mg/l. Notably, both compounds showed good target specificity with potent molluscicidal capability observed in snails, but very low toxicity to local fishes. Furthermore, the exposure of compounds 8 and 9 significantly elevated the enzyme activities of acid phosphatase and nitric oxide synthase of the snails, while no significant change was recorded in the activities of alkaline phosphatase, acetylcholine esterase and superoxide dismutase.

CONCLUSION

The results suggested that compounds 8 and 9 of pyridylphenylurea derivatives could be developed as promising molluscicide candidates for snail control.

Differentiating snail intermediate hosts of Schistosoma spp. using molecular approaches: fundamental to successful integrated control mechanism in Africa

Background

Snail intermediate hosts play active roles in the transmission of snail-borne trematode infections in Africa. A good knowledge of snail-borne diseases epidemiology particularly snail intermediate host populations would provide the necessary impetus to complementing existing control strategy.

Main body

This review highlights the importance of molecular approaches in differentiating snail hosts population structure and the need to provide adequate information on snail host populations by updating snail hosts genome database for Africa, in order to equip different stakeholders with adequate information on the ecology of snail intermediate hosts and their roles in the transmission of different diseases. Also, we identify the gaps and areas where there is need for urgent intervention to facilitate effective integrated control of schistosomiasis and other snail-borne trematode infections.

Conclusions

Prioritizing snail studies, especially snail differentiation using molecular tools will boost disease surveillance and also enhance efficient schistosomaisis control programme in Africa.

Electronic supplementary material

The online version of this article (10.1186/s40249-018-0401-z) contains supplementary material, which is available to authorized users.

Molluscicidal activity of Manilkara subsericea (Mart.) dubard on Biomphalaria glabrata (Say, 1818)

Schistosomiasis is promoted for species from Schistosoma genus affecting over 200 million people worldwide. Molluscicides are an efficient method to control this disease, being able to reduce intermediate host snail Biomphalaria glabrata number. In function of resistance cases using niclosamide, natural products are promisors to discover new drugs. Manilkara subsericea is endemic to Brazilian sandbanks of Rio de Janeiro State and wide ranges of biological activities. However, there is no studies evaluating its effects as molluscicidal agent. We tested crude extract from leaves of M. subsericea molluscicidal action, as well it ethyl-acetate fraction and isolated substances against B. glabrata. M. subsericea leaves crude extract and ethyl acetate fraction induced 80±4.13% and 86.66±4.59% mortality of adult snails at concentrations of 250ppm after 96h, and their LD₅₀ values were 118.7±1.62 and 23.41±1.15ppm respectively. Isolated substances from M. subsericea were also considered active. Quercetin, myricetin and ursolic acid, at concentration of 100ppm (96h), were able to induce mortality levels of 100%, 80% and 53.33%, respectively. Our results suggest that M. subsericea can be considered promising as a molluscicide agent.

Apropos: critical analysis of molluscicide application in schistosomiasis control programs in Brazil

Schistosomiasis is a snail-transmitted infectious disease affecting over 200 million people worldwide. Snail control has been recognized as an effective approach to interrupt the transmission of schistosomiasis, since the geographic distribution of this neglected tropical disease is determined by the presence of the intermediate host snails. In a recent Scoping Review published in Infectious Diseases of Poverty, Coelho and Caldeira performed a critical review of using molluscicides in the national schistosomiasis control programs in Brazil. They also described some chemical and plant-derived molluscicides used in China. In addition to the molluscicides described by Coelho and Caldeira, a large number of chemicals, plant extracts and microorganisms have been screened and tested for molluscicidal actions against Oncomelania hupensis, the intermediate host of Schistosoma japonicum in China. Here, we presented the currently commercial molluscicides available in China, including 26% suspension concentrate of metaldehyde and niclosamide (MNSC), 25% suspension concentrate of niclosamide ethanolamine salt (SCNE), 50% niclosamide ethanolamine salt wettable powder (WPN), 4% niclosamide ethanolamine salt dustable powder (NESP), 5% niclosamide ethanolamine salt granule (NESG) and the plant-derived molluscicide "Luowei". These molluscicides have been proved to be active against O. hupensis in both laboratory and endemic fields, playing an important role in the national schistosomiasis control program of China. Currently, China is transferring its successful experiences on schistosomiasis control to African countries. The introduction of Chinese commercial molluscicides to Africa, with adaptation to local conditions, may facilitate the progress towards the elimination of schisosomiasis in Africa.

Implications of Changing Temperatures on the Growth, Fecundity and Survival of Intermediate Host Snails of Schistosomiasis: A Systematic Review

Climate change has been predicted to increase the global mean temperature and to alter the ecological interactions among organisms. These changes may play critical roles in influencing the life history traits of the intermediate hosts (IHs). This review focused on studies and disease models that evaluate the potential effect of temperature rise on the ecology of IH snails and the development of parasites within them. The main focus was on IH snails of schistosome parasites that cause schistosomiasis in humans. A literature search was conducted on Google Scholar, EBSCOhost and PubMed databases using predefined medical subject heading terms, Boolean operators and truncation symbols in combinations with direct key words. The final synthesis included nineteen published articles. The studies reviewed indicated that temperature rise may alter the distribution, optimal conditions for breeding, growth and survival of IH snails which may eventually increase the spread and/or transmission of schistosomiasis. The literature also confirmed that the life history traits of IH snails and their interaction with the schistosome parasites are affected by temperature and hence a change in climate may have profound outcomes on the population size of snails, parasite density and disease epidemiology. We concluded that understanding the impact of temperature on the growth, fecundity and survival of IH snails may broaden the knowledge on the possible effects of climate change and hence inform schistosomiasis control programmes.

The population genetic structure of Biomphalaria choanomphala in Lake Victoria, East Africa: implications for schistosomiasis transmission

Background

The freshwater snail Biomphalaria acts as the intermediate host of Schistosoma mansoni, a globally important human parasite. Understanding the population structure of intermediate host species can elucidate transmission dynamics and assist in developing appropriate control methods.

Methods

We examined levels of population genetic structure and diversity in 29 populations of Biomphalaria choanomphala collected around the shoreline of Lake Victoria in Uganda, Kenya and Tanzania, where S. mansoni is hyper-endemic. Molecular markers were utilized to estimate the degree to which snail populations are genetically differentiated from one another.

Results

High levels of snail genetic diversity were found coupled with evidence of geographically-determined population structure but low levels of local inbreeding. The data are consistent with an effect of schistosome infection on population structure of intermediate host snails, but other factors, such as habitat and historical demographic changes, could also be important determinants of the degree of population genetic structure in Biomphalaria choanomphala.

Conclusions

The low stratification of populations and high genetic diversity indicates potentially less local compatibility with intermediate snail populations than previously theorized, and highlights the importance of coordinated parasite control strategies across the region.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0524-4) contains supplementary material, which is available to authorized users.

History of echinostomes (Trematoda)

Echinostomatidae (Trematoda) is the largest family within the class Trematoda. Members of this family have been studied for many years in relation to their utility as basic research models in biodiversity and systematics and also as experimental models in parasitology since they offer many advantages. Echinostomes have contributed significantly to numerous developments in many areas studied by parasitologists and experimental biologists. In this review, we examine the history of the echinostomebased studies from the beginnings to the present. For this purpose, we have divided the history of echinostomes into four periods (i.e. 18(th) and 19(th) centuries, first half of the 20(th) century, second half of the 20(th) century and the late 20(th) and 21(th) century) according to the types of studies performed in each of them. Moreover, we also briefly review the history of echinostome infections in humans.

A benzoic acid derivative and flavokawains from Piper species as schistosomiasis vector controls

The search of alternative compounds to control tropical diseases such as schistosomiasis has pointed to secondary metabolites derived from natural sources. Piper species are candidates in strategies to control the transmission of schistosomiasis due to their production of molluscicidal compounds. A new benzoic acid derivative and three flavokawains from Piper diospyrifolium, P. cumanense and P. gaudichaudianum displayed significant activities against Biomphalaria glabrata snails. Additionally, "in silico" studies were performed using docking assays and Molecular Interaction Fields to evaluate the physical-chemical differences among the compounds in order to characterize the observed activities of the test compounds against Biomphalaria glabrata snails.

Ovicidal effect of Piperaceae species on Biomphalaria glabrata, Schistosoma mansoni host

SUMMARY

Schistosomiasis is a neglected disease with public health importance in tropical and subtropical regions. An alternative to the disease control is the use of molluscicides to eliminate or reduce the intermediate host snail population causing a reduction of transmission in endemic regions. In this study nine extracts from eight Piperaceae species were evaluated against Biomphalaria glabrata embryos at blastula stage. The extracts were evaluated in concentrations ranging from 100 to 10 mg/L. Piper crassinervium and Piper tuberculatum extracts were the most active (100% of mortality at 20 mg/L and 30 mg/L respectively).

Time to set the agenda for schistosomiasis elimination

It is time to raise global awareness to the possibility of schistosomiasis elimination and to support endemic countries in their quest to determine the most appropriate approaches to eliminate this persistent and debilitating disease. The main interventions for schistosomiasis control are reviewed, including preventive chemotherapy using praziquantel, snail control, sanitation, safe water supplies, and behaviour change strategies supported by information, education and communication (IEC) materials. Differences in the biology and transmission of the three main Schistosoma species (i.e. Schistosoma haematobium, S. mansoni and S. japonicum), which impact on control interventions, are considered. Sensitive diagnostic procedures to ensure adequate surveillance in areas attaining low endemicity are required. The importance of capacity building is highlighted. To achieve elimination, an intersectoral approach is necessary, with advocacy and action from local communities and the health community to foster cooperative ventures with engineers, the private sector, governments and non-governmental organizations specialized in water supply and sanitation. Examples of successful schistosomiasis control programmes are reviewed to highlight what has been learnt in terms of strategy for control and elimination. These include St. Lucia and other Caribbean islands, Brazil and Venezuela for S. mansoni; Saudi Arabia and Egypt for both S. mansoni and S. haematobium; Morocco, Tunisia, Algeria, Mauritius and the Islamic Republic of Iran for S. haematobium; Japan and the People's Republic of China for S. japonicum. Additional targets for elimination or even eradication could be the two minor human schistosome species S. guineenisis and S. intercalatum, which have a restricted distribution in West and Central Africa. The examples show that elimination of schistosomiasis is an achievable and desirable goal requiring full integration of preventive chemotherapy with the tools of transmission control. An agenda for the elimination of schistosomiasis would aim to identify the gaps in knowledge, and define the tools, strategies and guidelines that will help national control programmes move towards elimination, including an internationally accepted mechanism that allows verification/confirmation of elimination.

Molluscicidal activities of six species of Bignoniaceae from north–eastern Brazil, as measured againstBiomphalaria glabrataunder laboratory conditions

The molluscicidal profile and brine-shrimp bio-activity of the ethanolic extracts of plants from the Bignoniaceae family were determined. The six extracts investigated were of the stems of Melloa quadrivalvis and Tabebuia aurea, and whole plants of Adenocalymma comosum, Arrabidaea parviflora, Cuspidaria argentea and Clytostoma binatum. When tested in the laboratory, with Biomphalaria glabrata as the test snail, all six extracts gave median lethal concentrations (9-54 microg/ml) that fell well below the upper threshold, of 100 mug/ml, set for a potential molluscicide by the World Health Organization.

Synthesis of Ginkgolic Acid Analogues and Evaluation of Their Molluscicidal Activity

Based on the molluscicidal activity of ginkgolic acids (GAs) isolated from Ginkgo biloba L, a series of Z/E isomers of GA analogues were prepared and evaluated for their molluscicidal activities against the host snail Oncomelania hupensis. The results and analysis of the structure-activity relationship revealed that the E-isomers showed better molluscicidal activities than their Z-isomers. Molluscicidal activities decreased with the shortening of the alkenyl chain lengths.

Bioactivities of glycoalkaloids and their aglycones from Solanum species

Potatoes, tomatoes, and aubergines are all species of the Solanum genus and contain a vast array of secondary metabolites including calystegine alkaloids, phenolic compounds, lectins, and glycoalkaloids. Glycoalkaloids have been the subject of many literature papers, occur widely in the human diet, and are known to induce toxicity. Therefore, from a food safety perspective further information is required regarding their analysis, toxicity, and bioavailability. This is especially important in crop cultivars derived from wild species to prevent glycoalkaloid-induced toxicity. A comprehensive review of the bioactivity of glycoalkaloids and their aglycones of the Solanum species, particularly focused on comparison of their bioactivities including their anticancer, anticholesterol, antimicrobial, anti-inflammatory, antinociceptive, and antipyretic effects, toxicity, and synergism of action of the principal Solanum glycoalkaloids, correlated to differences of their individual molecular structures is presented.

Structure-Molluscicidal Activity Relationships of Acylphloroglucinols from Ferns

The molluscicidal activity of 12 phloroglucinol derivatives previously isolated from Elaphoglossum piloselloides, E. gayanum, E. yungense, and E. lindbergii, as well as 3 known acylphloroglucinols, now reported from an Argentine collection of Dryopteris wallichiana, was evaluated against the schistosomiasis vector snail Biomphalaria peregrina. Molluscicidal effects were analyzed and compared with those previously observed for 4 acylphloroglucinols from E. piloselloides and their corresponding peracetylated derivatives, in order to draw structure-activity relationships. The most active compounds were the prenylated desaspidins elaphogayanin B and elaphopilosins A and B (LD50 = 1.90, 2.90, and 0.94 ppm, respectively), together with the only evaluated prenylated para-aspidin, elaphopilosin C (LD50 = 2.15 ppm). Quantitative structure-activity relationships (QSAR) were studied by means of a semiempirical method (PM3) for the 24 natural phloroglucinol derivatives included in this paper. The descriptor molecular volume was found to have good correlation with the observed molluscicidal activity (r2 = 0.77). The derived equation can be considered useful to predict the molluscicidal activity of bi and tricyclic acylphloroglucinols. The QSAR analysis showed that there is an optimum volume for high activity, probably related to the size of a receptor's active site. Bigger molecules display lower activity.

Global stressors and the global decline of amphibians: tipping the stress immunocompetency axis

There is a widespread consensus that the earth is experiencing a mass extinction event and at the forefront are amphibians, the most threatened of all vertebrate taxa. A recent assessment found that nearly one-third (32%, 1,856 species) of the world's amphibian species are threatened. Amphibians have existed on the earth for over 300 million years, yet in just the last two decades there have been an alarming number of extinctions, nearly 168 species are believed to have gone extinct and at least 2,469 (43%) more have populations that are declining. Infectious diseases have been recognized as one major cause of worldwide amphibian population declines. This could be the result of the appearance of novel pathogens, or it could be that exposure to environmental stressors is increasing the susceptibility of amphibians to opportunistic pathogens. Here I review the potential effects of stressors on disease susceptibility in amphibians and relate this to disease emergence in human and other wildlife populations. I will present a series of case studies that illustrate the role of stress in disease outbreaks that have resulted in amphibian declines. First, I will examine how elevated sea-surface temperatures in the tropical Pacific since the mid-1970s have affected climate over much of the world and could be setting the stage for pathogen-mediated amphibian declines in many regions. Finally, I will discuss how the apparently rapid increase in the prevalence of amphibian limb deformities is linked to the synergistic effects of trematode infection and exposure to chemical contaminants.

Molluscicidal Polyphenols from Species of Fucaceae

The aqueous fractions of the dry methanol extracts (500 ppm) of sixty marine algal species were screened for molluscicidal activity against Biomphalaria glabrata, which is an important host of the bilharzia-causing Schistosoma species. The majority of the extracts tested were inactive at the concentration used, but those of Fucus serratus, F. vesiculosus, Pelvetia canaliculata, Ascophyllum nodosum, Halidrys siliquosa, Bifurcaria bifurcata, Dictyota dichotoma and Halopithys incurva all showed significant molluscicidal activity. Treatment with polyvinylpolypyrrolidone (PVPP) either removed or drastically reduced the activity of the extracts of F. serratus, F. vesiculosus, P. canaliculata, A. nodosum, Halidrys siliquosa and Halopithys incurva, which suggested that the active compounds in the extracts of these species were polyphenolic in nature. The active extracts of the other two seaweed species did not appear to be affected by treatment with PVPP. Dialysis of the active extracts against distilled water separated them into high and low molecular weight fractions. In the case of the two Fucus species, P. canaliculata and A. nodosum, the activity resided in the high molecular weight fraction, whereas with all the other species, the activity was found in the low molecular weight fraction. 1H NMR spectroscopic examination of the active extracts confirmed that the molluscicidal components of the extracts of the Fucus species, P. canaliculata and A. nodosum were high molecular weight polyphenols.

Signal transduction in larval trematodes: putative systems associated with regulating larval motility and behaviour

The multi-host lifestyle of parasitic trematodes necessitates their ability to communicate with their external environment in order to invade and navigate within their hosts' internal environment. Through recent EST and genome sequencing efforts, it has become clear that members of the Trematoda possess many of the elaborate signal transduction systems that have been delineated in other invertebrate model systems like Drosophila melanogaster and Caenorhabditis elegans. Gene homologues representing several well-described signal receptor families including receptor tyrosine kinases, receptor serine tyrosine kinases, G protein-coupled receptors and elements of their downstream signalling systems have been identified in larval trematodes. A majority of this work has focused on the blood flukes, Schistosoma spp. and therefore represents a narrow sampling of the diverse digenean helminth taxon. Despite this fact and given the substantial evidence supporting the existence of such signalling systems, the question then becomes, how are these systems employed by larval trematodes to aid them in interpreting signals received from their immediate environment to initiate appropriate responses in cells and tissues comprising the developing parasite stages? High-throughput, genome-wide analysis tools now allow us to begin to functionally characterize genes differentially expressed throughout the development of trematode larvae. Investigation of the systems used by these parasites to receive and transduce external signals may facilitate the creation of technologies for achieving control of intramolluscan schistosome infections and also continue to yield valuable insights into the basic mechanisms regulating motility and behaviour in this important group of helminths.

New 1,2,3,4-tetrahydro-1-aza-anthraquinones and 2-aminoalkyl compounds from norlapachol with molluscicidal activity

New nitrogen derivatives from norlapachol, including four new diastereomeric 1,2,3,4-tetrahydro-1-aza-anthraquinones obtained from the Prins cyclization on suitable aminoacetaldehyde dimethylacetal derivatives with formic acid, were found to exhibit molluscicidal activity against Biomphalaria glabrata. These derivatives showed low to medium LC(50) values, similar to those reported previously for the homologous series of nitrogen derivatives of lapachol. The toxicity profile against Artemia salina was also determined for all compounds.

Synthesis and evaluation of molluscicidal and larvicidal activities of some novel enaminones derived from 4-hydroxyquinolinones: part IX

A series of 10 3-(hetarylaminomethylene)quinolinediones, 12 3-(substituted aminopropenoyl)-4-hydroxyquinolinones, and 10 3-(substituted aminomethylene-5-oxo-pyrazolinyl)-4-hydroxyquinolinones were synthesized as novel enaminones derived from 3-(un)substituted 4-hydroxyquinolin-2(1H)-ones in 72-94% yields and assayed for their molluscicidal activities against Biomphalaria alexandrina and Lymnaea natalensis snails. Some of the tested enaminones presented high molluscicidal activities (LC(50)20ppm). The new compounds showed more potency against hatchability of B. alexandrina egg masses, the infection rate and prepatent period of the snails. In addition, these derivatives revealed potential larvicidal effects (100% mortality) on both miracidia and cercariae of Schistosoma mansoni at reduced exposure time. The selected active derivatives were examined against Daphnia magna and their nontoxic effect at all sublethal, lethal, and higher concentrations suggests that these compounds can play an important role as molluscicides and larvicides with environmental safe properties.

Molluscicidal activity of synthetic lapachol amino and hydrogenated derivatives

A series of new amino derivatives and a new partially hydrogenated derivative of the natural naphthoquinone lapachol were assayed for molluscicidal activity against Biomphalaria glabrata. These derivatives showed low to medium LC(50) values, and a 3.1 microg/mL value for the most potent derivative of the series. The toxicity is in agreement with the decrease of polar character of the tested compounds.

Molluscicidal effect of nicotinanilide and its intermediate compounds against a freshwater snail Lymnaea luteola, the vector of animal schistosomiasis

The molluscicidal effect of nicotinanilide was evaluated and compared with niclosamide (2',5-dichloro-4'-nitrosalicylanilide, ethanolamide salt) against different stages of the freshwater snail Lymnaea luteola i.e., eggs, immature, young mature, and adults. Calculated values of lethal concentrations (LC50 and LC90) showed that both nicotinanilide and niclosamide as toxic against eggs, immature, and adults. The young mature stage of the snails was comparatively more tolerant to both molluscicides than the other stages. The toxicity of the intermediate compounds of nicotinanilide against the young mature stage of the snails showed them as ineffective. The mortality pattern of the snails exposed to LC90 concentration of these molluscicides showed niclosamide to kill faster (within 8 to 9 h) than nicotinanilide (26 to 28 h). In view of the above studies it may be concluded that both molluscicides are toxic against all the stages of the L. luteola snails.

Genetics of anti-parasite resistance in invertebrates

This review summarizes and compares available data on genetic and molecular aspects of resistance in four well-described invertebrate host-parasite systems: snail-schistosome, mosquito-malaria, mosquito-filarial worm, and Drosophila-wasp associations. It underlies that the major components of the immune reaction, such as hemocyte proliferation and/or activation, and production of cytotoxic radicals are common to invertebrate hosts. Identifying genes responsible for naturally occurring resistance will then be helpful to understand the mechanisms of invertebrate immune defenses and to determine how virulence factors are used by parasites to overcome host resistance. Based on these four well-studied models, invertebrate resistance appears as generally determined by one major locus or a few loci, displaying at least partial dominance. Interestingly, specificity of resistance is highly variable and would involve processes other than simple recognition mechanisms. Finally, resistance was shown to be generally costly but is nevertheless observed at high frequencies in many natural populations, suggesting a high potential for host parasite coevolution.

The mitochondrial genome of Biomphalaria glabrata (Gastropoda: Basommatophora), intermediate host of Schistosoma mansoni

The complete mitochondrial (Mt) genome of the gastropod Biomphalaria glabrata, a major intermediate host for the human parasite Schistosoma mansoni, was sequenced. The circular genome, the first determined from a basommatophoran snail, is AT rich (74.6%) and the smallest Mt genome (13,670 nucleotides [nt]) characterized from mollusks to date. Sequences from 2 B. glabrata strains, M-line and 1742, differed by only 18 nt. Phylogenetic analysis of 16S and ND1 sequences confirmed the Brazilian ancestry of both B. glabrata strains. Gene predictions indicated 22 transfer RNA, 12S and 16S ribosomal RNA (rRNA), and 13 protein-encoding genes, as is typical for metazoans. Of the mollusk Mt genomes currently known, the gene order was most similar to that of stylommatophoran gastropods, concordant with the monophyly of pulmonate gastropods. Screening of GenBank (expressed sequence tags database [dbEST]) with the Mt sequence identified 108 entries from B. glabrata as Mt-derived sequences, including 12S and 16S rRNA sequences. Moreover, 11 sequences originating from the Mt genome of B. glabrata were identified among EST entries ascribed to intramolluskan stages of S. mansoni. The availability of this Mt sequence will facilitate further molecular investigations into the biology of Biomphalaria sp. and interactions between this intermediate host and intramolluskan stages of S. mansoni.

Synthesis and evaluation of the molluscicidal activity of the 5,6-dimethyl-dihydro-pyran-2,4-dione and 6-substituted analogous

Five dihydro-piran-2,4-diones, including 5,6-dimethyl-dihydro-piran-2,4-dione one of the intermediates of the synthesis of caloverticilic acid, were synthesized and submitted to molluscicidal bioassay. The compound's yields varied from moderate to good (42%- 80%) and were achieved through the preparation of the dianion of ethyl acetoacetate, reaction with and aldehyde followed by hydrolysis of the ester (NaOH, H(2)O, 2 h, T.A.) and lactonization in acidic medium (HCl, 0 degrees C). The 5,6-dimethyl-dihydro-piran-2,4-dione and three analogous dihydro-piran-2,4-diones 6-substituted,-phenyl, (4-methoxy-phenyl), and -propenyl, showed significant activities against the Biomphalaria glabrata egg masses, while the analogous 6-(3,4-dimethoxy-phenyl) was inactive as molluscicide. This activity is reported for the first time, extending the range of biological activities of this group.

Biology of the schistosome genus Trichobilharzia

Trichobilharzia is the largest genus within the family Schistosomatidae, covering over 40 species of avian parasites. To clarify the existing confusion in the systematics of the genus, we recommend combining knowledge of life cycles and developmental stages, snail/bird hosts, cytogenetical and molecular data together with morphological criteria for the characterization of particular species. The high specificity of Trichobilharzia for the intermediate host is a likely reflection of the ability to avoid the internal defence of specific snails. The spectrum of final hosts (birds) seems to be much wider. The infection of birds--trichobilharziasis--may lead to considerable tissue injuries, caused by eggs of the parasite or migration of immature/mature worms through the body. Most Trichobilharzia (visceral species) migrate through the viscera of the host, but nasal species display a neurotropic mode of migration. Due to a low specificity of penetrating cercariae, mammals (including humans) can be attacked. This leads to cercarial dermatitis, predominantly in sensitized hosts. Experimental infections indicate that Trichobilharzia never mature in an incompatible (mammalian) host. However, not all cercariae and schistosomula are necessarily trapped and eliminated in the skin, and parasites may migrate throughout the viscera and the nervous system of mammals. These findings suggest that the pathogenicity of Trichobilharzia may have been underestimated in the past and health risks associated with trichobilharziasis need to be studied further.

Genetic markers between Biomphalaria glabrata snails susceptible and resistant to Schistosoma mansoni infection

The analysis of the genetic variability related to susceptibility to Schistosoma mansoni infection in the vector of the genus Biomphalaria is important in terms of a better understanding of the epidemiology of schistosomiasis itself, the possible pathological implications of this interaction in vertebrate hosts, and the formulation of new strategies and approaches for disease control. In the present study, the genetic variability of B. glabrata strains found to be resistant or susceptible to S. mansoni infection was investigated using DNA amplification by random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR). The amplification products were analyzed on 8% polyacrylamide gel and stained with silver. We selected 10 primers, since they have previously been useful to detect polymorphism among B. glabrata and/or B. tenagophila. The results showed polymorphisms with 5 primers. Polymorphic bands observed only in the susceptible strain. The RAPD-PCR methodology represents an adequate approach for the analysis of genetic polymorphisms. The understanding of the genetic polymorphisms associated to resistance may contribute to the future identification of genomic sequences related to the resistance/susceptibility of Biomphalaria to the larval forms of S. mansoni and to the development of new strategies for the control of schistosomiasis.