The distribution of Mekong schistosomiasis, past and future: Preliminary indications from an analysis of genetic variation in the intermediate host

Current advances in serological and molecular diagnosis of Schistosoma mekongi infection

Schistosomiasis, a neglected tropical disease, caused by blood flukes belonging to the genus Schistosoma; it persists as a public health problem in selected regions throughout Africa, South America, and Asia. Schistosoma mekongi, a zoonotic schistosome species endemic to the Mekong River in Laos and Cambodia, is one of the significant causes of human schistosomiasis along with S. japonicum, S. mansoni, S. haematobium and S. intercalatum. Since its discovery, S. mekongi infection has been highly prevalent in communities along the Mekong River. Although surveillance and control measures have shown success in recent years, more robust diagnostic tools are still needed to establish more efficient control and prevention strategies to achieve and sustain an elimination status. Diagnosis of S. mekongi infection still relies on copro-parasitological techniques, commonly made by Kato-Katz stool examination. Serological techniques such as enzyme-linked immunosorbent assay (ELISA) may also be applicable but in a limited setting. Targeted molecular and serological tools specific to the species, on the other hand, have been limited. This is due, in part, to the limited research and studies on the molecular biology of S. mekongi since genome information of this species has not yet been released. In this review, current advances, and gaps and limitations in the molecular and immunological diagnosis of S. mekongi are discussed.

Detection of Schistosoma mekongi DNA in Human Stool and Intermediate Host Snail Neotricula aperta via Loop-Mediated Isothermal Amplification Assay in Lao PDR

Schistosomiasis mekongi infection represents a public health concern in Laos and Cambodia. While both countries have made significant progress in disease control over the past few decades, eradication has not yet been achieved. Recently, several studies reported the application of loop-mediated isothermal amplification (LAMP) for detecting Schistosoma DNA in low-transmission settings. The objective of this study was to develop a LAMP assay for Schistosoma mekongi using a simple DNA extraction method. In particular, we evaluated the utility of the LAMP assay for detecting S. mekongi DNA in human stool and snail samples in endemic areas in Laos. We then used the LAMP assay results to develop a risk map for monitoring schistosomiasis mekongi and preventing epidemics. A total of 272 stool samples were collected from villagers on Khon Island in the southern part of Laos in 2016. DNA for LAMP assays was extracted via the hot-alkaline method. Following the Kato-Katz method, we determined that 0.4% (1/272) of the stool samples were positive for S. mekongi eggs, as opposed to 2.9% (8/272) for S. mekongi DNA based on the LAMP assays. Snail samples (n = 11,762) were annually collected along the riverside of Khon Island from 2016 to 2018. DNA was extracted from pooled snails as per the hot-alkaline method. The LAMP assay indicated that the prevalence of S. mekongi in snails was 0.26% in 2016, 0.08% in 2017, and less than 0.03% in 2018. Based on the LAMP assay results, a risk map for schistosomiasis with kernel density estimation was created, and the distribution of positive individuals and snails was consistent. In a subsequent survey of residents, schistosomiasis prevalence among villagers with latrines at home was lower than that among villagers without latrines. This is the first study to develop and evaluate a LAMP assay for S. mekongi detection in stools and snails. Our findings indicate that the LAMP assay is an effective method for monitoring pathogen prevalence and creating risk maps for schistosomiasis.

The causative agents of fascioliasis in animals and humans: Parthenogenetic Fasciola in Asia and other regions

Parthenogenetic Fasciola is the causative agent of fascioliasis in animals and humans and is widely distributed in Asian countries, such as Japan, South Korea, China, Vietnam, Thailand, the Philippines, Myanmar, Bangladesh, Nepal, and India. Parthenogenetic Fasciola geographically originated from central and eastern China, where it exists between the habitats of Fasciola hepatica and Fasciola gigantica; it likely appeared thousands of years ago following hybridization between F. hepatica and F. gigantica. Parthenogenetic Fasciola consists of diploids and triploids that possess nuclear genome of both F. hepatica and F. gigantica and mitochondrial genome of either F. hepatica or F. gigantica. Maternal parents of parthenogenetic Fasciola are either F. hepatica having Fh-C4 haplotype or F. gigantica having Fg-C2 haplotype in mitochondrial NADH dehydrogenase subunit 1 (ND1) nucleotide sequences. Parthenogenetic Fasciola flukes with the Fh-C4 haplotype have spread from China to South Korea and Japan, whereas the flukes with the Fg-C2 haplotype have not only spread to Korea and Japan but also southward to Vietnam, Thailand, the Philippines, Myanmar, Bangladesh, Nepal, and India. Parthenogenetic Fasciola can be distinguished from F. hepatica and F. gigantica using combinational DNA sequence analysis of nuclear phosphoenolpyruvate carboxykinase (pepck) and DNA polymerase delta (pold) along with mitochondrial ND1 markers. The establishment of parthenogenetic Fasciola is expected as follows: parthenogenetic diploids with the Fh-C4 and Fg-C2 haplotypes first appeared based on single or multiple interspecific hybridization events; subsequently, parthenogenetic triploids emerged via backcross events between the maternal parthenogenetic diploid and either paternal bisexual F. hepatica or F. gigantica. Parthenogenetic Fasciola diploids and triploids then survived for thousands of years by clonal parthenogenetic reproduction, and generated descendants with ND1 haplotypes, which were derived from the Fh-C4 and Fg-C2 due to nucleotide substitution. Thus, the emergence of parthenogenetic Fasciola may be due to extremely uncommon and accidental events. Parthenogenetic Fasciola should be treated as a new asexual hybrid species.

First report of Schistosoma sinensium infecting Tupaia belangeri and Tricula sp. LF

Schistosoma sinensium belongs to the Asian Schistosoma and is transmitted by freshwater snails of the genus Tricula. Rodents are known definitive hosts of S. sinensium. In 2016, suspected schistosome eggs were found in the feces of the northern tree shrew (Tupaia belangeri) in a field in Lufeng County (latitude, 25°04'50″ N; longitude, 102°19'30″ E; altitude 1820 m), Yunnan Province, China. Morphological analysis suggested that the schistosome was S. sinensium. 18S, 12S and CO1 genes sequencing and phylogenetic analysis showed that this species had the highest similarity to and occupied the same evolutionary branch as S. sinensium from Mianzhu, Sichuan, China. Meanwhile, based on 16S and 28S rDNA sequencing and morphological identification, the snail intermediate host was identified as a species of Tricula, and was found in irrigation channels. Phylogeny indicated that Tricula sp. LF was a sister taxon to T. bambooensis, T. ludongbini. The S. sinensium was able to experimentally infect the captive-bred Tupaia belangeri, and Schistosoma eggs were recovered from all Tupaia belangeri exposed. In this study, we report the infection of Tupaia belangeri and Tricula sp. LF with S. sinensium in Lufeng, Yunnan, southwest China. These findings may improve our understanding of the host range, evolution, distribution, and phylogenetic position of S. sinensium.

Patients with severe schistosomiasis mekongi morbidity demonstrating ongoing transmission in Southern Lao People's Democratic Republic

Chronic infection with Schistosoma mekongi may result in severe hepatosplenic morbidity. We report on eight patients with severe morbidity due to S. mekongi infection. The patients were diagnosed, treated and followed-up between 2007 and 2010 in Khong district, Southern Lao People's Democratic Republic (Lao PDR), eight years after the end of a control intervention. S. mekongi control programmes aimed to prevent morbidity and mortality associated with infection. The patients were visited and interviewed annually. In addition, clinical and abdominal ultrasound examinations were performed and faecal and blood samples were examined. The patients' ages ranged from 6 to 66 years. Of the eight patients, three were children and five were adults. The four youngest patients (aged 6-27 years) significantly improved after praziquantel treatment. One patient (age 46 years) worsened between 2007 and 2010. Two patients died due to bleeding of the oesophageal varices. One patient was lost to follow-up. The leading clinical signs were ascites, splenomegaly, collateral veins on the abdomen and a poor general nutrition status. Ultrasonography disclosed advanced liver fibrosis patterns in all patients; in seven patients, fibrosis pattern E or F was revealed, as per the Niamey protocol (pattern A normal, pattern B to F pathological with increasing severity). Stool microscopy revealed that five patients were co-infected with hookworm and Opisthorchis viverrini. The youngest patient (aged 6 years) was born after the schistosomiasis control program had ended. From her severe morbidity, we can conclude that S. mekongi transmission was on-going in Khong district, and that even in areas with low S. mekongi transmission intensities, severe morbidity from schistosomiasis can develop quickly. Early diagnosis and treatment are imperative, and close monitoring is required.

Morbidity associated with Schistosoma mekongi and concurrent helminth infection in Lao People's Democratic Republic

In 2006, some 7 years after the halt of a multi-year schistosomiasis control programme, a study was conducted in the southern part of Lao People's Democratic Republic to determine the morbidity associated with Schistosoma mekongi and concurrent helminth infection. In two S. mekongi-endemic districts, Khong and Mounlapamok, 640 individuals aged 9-40 years were randomly selected. Two stool samples were obtained from each participant and subjected to the Kato-Katz technique for helminth diagnosis. Morbidity was assessed by clinical examination and abdominal ultrasonography. Multiple logistic regression analysis was employed to relate S. mekongi infection with morbidity indicators. In Khong district, S. mekongi prevalence was 13-fold higher than in Mounlapamok district (57.7% vs. 4.4%, p < 0.001). Likewise, we observed significantly more often palpated hepatomegaly (15.5% vs. 5.4%) and splenomegaly (9.6% vs. 0.4%). Abdominal ultrasonography revealed that, in Khong district, liver and spleen morbidity occurred more frequently (e.g., enlarged left liver lobe: 89.2% vs. 68.7%, p < 0.001). Single species infection with S. mekongi was associated with enlarged peri-portal vein (adjusted odds ratio (aOR) 47.7, 95% confidence interval (CI) 4.2-85.7, p = 0.002), as was S. mekongi-Opisthorchis viverrini co-infection (aOR 2.2, 95% CI 1.1-4.5, p = 0.020). In conclusion, our study shows that in 2006, several years after the halt of the Lao schistosomiasis control programme that emphasised preventive chemotherapy, S. mekongi infection again had reached high prevalence rates. Infection with S. mekongi was associated with substantial hepatosplenic morbidity. Schistosomiasis control measures were re-instigated in 2010. In view of the ultimate goal to eliminate schistosomiasis, rigorous surveillance and public health responses tailored to the social-ecological settings and long-term programme commitment are warranted.

Using the Freshwater Health Index to Assess Hydropower Development Scenarios in the Sesan, Srepok and Sekong River Basin

Sustainable water resource management is a wicked problem, fraught with uncertainties, an indeterminate scope, and divergent social values and interests among stakeholders. To facilitate better management of Southeast Asia’s transboundary Sesan, Sekong and Srepok (3S) River basin, we used the Freshwater Health Index (FHI) to diagnose the basin’s current and likely future level of freshwater health. We used the conditions for December 2016 as a baseline, where Ecosystem Vitality and Ecosystem Services scored 66 and 80, respectively, out of a possible 100, whilst Governance & Stakeholders scored 43. Thus, the 3S provided a range of desired ecosystem services, but there were signs of environmental stress as well as undeveloped water governance systems and limited stakeholder engagement. We also modelled four hydropower development scenarios and found that increasing development reduced the scores of a subset of indicators. This compromised the future ability of the 3S basin’s ecosystem to provide its current range of services. The FHI helped identify data deficiencies, illuminated important social dynamics, made ecosystem–human–water dynamics more understandable to stakeholders, and examined the long-term dynamics of the basin.

Recent advances in Schistosoma mekongi ecology, transcriptomics and proteomics of relevance to snail control

Mekong schistosomiasis caused by Schistosoma mekongi is a public health problem that occurs along the border between southern Laos and northern Cambodia. Given its restricted distribution and low prevalence, eventual eradication via an effective control program can be expected to be successful. To achieve this goal detailed knowledge of its basic biology, molecular biology, biochemistry, and pathology is urgently required. In this regard, recent studies on transcriptome analysis of adult male and female S. mekongi worms, and proteome analysis of developmental stages have been reported and are discussed here. The biology, habitat, and distribution of the snail intermediate host Neotricula aperta, which are factors in disease transmission, are discussed in this review. These have initiated renewed interest in S. mekongi research and contributed promising data that will be utilized in the generation of effective control and prevention strategies.

Elimination of Schistosomiasis Mekongi from Endemic Areas in Cambodia and the Lao People's Democratic Republic: Current Status and Plans

The areas endemic for schistosomiasis in the Lao People’s Democratic Republic and in Cambodia were first reported 50 and 60 years ago, respectively. However, the causative parasite Schistosoma mekongi was not recognized as a separate species until 1978. The infection is distributed along a limited part of the Mekong River, regulated by the focal distribution of the intermediate snail host Neotricula aperta. Although more sensitive diagnostics imply a higher figure, the current use of stool examinations suggests that only about 1500 people are presently infected. This well-characterized setting should offer an exemplary potential for the elimination of the disease from its endemic areas; yet, the local topography, reservoir animals, and a dearth of safe water sources make transmission control a challenge. Control activities based on mass drug administration resulted in strong advances, and prevalence was reduced to less than 5% according to stool microscopy. Even so, transmission continues unabated, and the true number of infected people could be as much as 10 times higher than reported. On-going control activities are discussed together with plans for the future.

Population genetic structure and geographical variation in Neotricula aperta (Gastropoda: Pomatiopsidae), the snail intermediate host of Schistosoma mekongi (Digenea: Schistosomatidae)

Background

Neotricula aperta is the snail-intermediate host of the parasitic blood-fluke Schistosoma mekongi which causes Mekong schistosomiasis in Cambodia and the Lao PDR. Despite numerous phylogenetic studies only one DNA-sequence based population-genetic study of N. aperta had been published, and the origin, structure and persistence of N. aperta were poorly understood. Consequently, a phylogenetic and population genetic study was performed, with addition of new data to pre-existing DNA-sequences for N. aperta from remote and inaccessible habitats, including one new taxon from Laos and 505 bp of additional DNA-sequence for all sampled taxa,.

Principal findings

Spatial Principal Component Analysis revealed the presence of significant spatial-genetic clustering. Genetic-distance-based clustering indicated four populations with near perfect match to a priori defined ecogeographical regions. Spring-dwelling taxa were found to form an ecological isolate relative to other N. aperta. The poor dispersal capabilities suggested by spatial-genetic analyses were confirmed by Bayesian inference of migration rates. Population divergence time estimation implied a mid-Miocene colonisation of the present range, with immediate and rapid radiation in each ecogeographical region. Estimated effective population sizes were large (120–310 thousand).

Conclusions

The strong spatial-genetic structure confirmed the poor dispersal capabilities of N. aperta—suggesting human-mediated reintroduction of disease to controlled areas as the primary reason for control failure. The isolation of the spring-dwelling taxa and ecogeographical structure suggests adaptation of sub-populations to different habitats; the epidemiological significance of this needs investigation. The large effective population sizes indicate that the high population densities observed in surveyed habitats are also present in inaccessible areas; affording great potential for recrudescence driven by animal-reservoir transmission in remote streams. Mid-Miocene colonisation implies heterochronous evolution of these snails and associated schistosomes and suggests against coevolution of snail and parasite. Heterochronicity favours ecological factors as shapers of host-parasite specificity and greater potential for escape from schistosomiasis control through host-switching.

The disease Mekong schistosomiasis poses a threat to the health of about 1.5 million people living near the Mekong river and its tributaries in Cambodia and Laos. It is a water-borne parasite transmitted by direct contact with water in which freshwater snails of the species Neotricula aperta live. Control of the snails is an effective approach to control of the parasite; however, because many suitable habitats for N. aperta occur in remote and inaccessible areas, knowledge of N. aperta population sizes and interconnectivity is insufficient for the design of effective snail control interventions. Although much of the region is difficult to survey by conventional means, population genetics can be used to estimate population structure and total size from small samples of accessible populations. The study added to existing data-sets, to give more population samples and longer DNA-sequences, together with improved analytical approaches to provide a better overview of N. aperta. The findings suggest that N. aperta in different kinds of habitats are also genetically different, with very low levels of migration between them; this genetic clustering is greater than expected from spatial distance alone. Further work is needed to determine if these different clusters vary in ability to transmit the parasite. The overall population size estimates were very large; thus suggesting that high snail population densities observed in accessible habitats are also characteristic of inaccessible populations—parasites are therefore more likely to return after disease control by immigration from remote areas. Finally, the timing of evolutionary events for snails and parasites was found to differ; this implies that the parasite may not be as strongly restricted to one species of snail as originally thought, which has implications for avoidance of parasite control by host-switching.

Comparison of novel and standard diagnostic tools for the detection of Schistosoma mekongi infection in Lao People's Democratic Republic and Cambodia

Background

Given the restricted distribution of Schistosoma mekongi in one province in Lao People’s Democratic Republic (Lao PDR) and two provinces in Cambodia, together with progress of the national control programmes aimed at reducing morbidity and infection prevalence, the elimination of schistosomiasis mekongi seems feasible. However, sensitive diagnostic tools will be required to determine whether elimination has been achieved. We compared several standard and novel diagnostic tools in S. mekongi-endemic areas.

Methods

The prevalence and infection intensity of S. mekongi were evaluated in 377 study participants from four villages in the endemic areas in Lao PDR and Cambodia using Kato-Katz stool examination, antibody detection based on an enzyme-linked immunosorbent assay (ELISA) and schistosome circulating antigen detection by lateral-flow tests. Two highly sensitive test systems for the detection of cathodic and anodic circulating antigens (CCA, CAA) in urine and serum were utilized.

Results

Stool microscopy revealed an overall prevalence of S. mekongi of 6.4% (one case in Cambodia and 23 cases in Lao PDR), while that of Opisthorchis viverrini, hookworm, Trichuris trichiura, Ascaris lumbricoides and Taenia spp. were 50.4%, 28.1%, 3.5%, 0.3% and 1.9%, respectively. In the urine samples, the tests for CCA and CAA detected S. mekongi infections in 21.0% and 38.7% of the study participants, respectively. In the serum samples, the CAA assay revealed a prevalence of 32.4%, while a combination of the CAA assay in serum and in urine revealed a prevalence of 43.2%. There was a difference between the two study locations with a higher prevalence reached in the samples from Lao PDR.

Conclusions

The CCA, CAA and ELISA results showed substantially higher prevalence estimates for S. mekongi compared to Kato-Katz thick smears. Active schistosomiasis mekongi in Lao PDR and Cambodia might thus have been considerably underestimated previously. Hence, sustained control efforts are still needed to break transmission of S. mekongi. The pivotal role of highly sensitive diagnostic assays in areas targeting elimination cannot be overemphasised.

Electronic supplementary material

The online version of this article (doi:10.1186/s40249-017-0335-x) contains supplementary material, which is available to authorized users.

China-Africa and China-Asia Collaboration on Schistosomiasis Control: A SWOT Analysis

Schistosomiasis, a disease caused by a trematode, parasitic worm, is a worldwide public health problem. In spite of great progress with regard to morbidity control, even elimination of this infection in recent decades, there are still challenges to overcome in sub-Saharan Africa and endemic areas in Southeast Asia. Regarded as one of the most successful countries with respect to schistosomiasis control, The People's Republic of China has accumulated considerable experience and learnt important lessons in various local settings that could benefit schistosomiasis control in other endemic countries. Based on an analysis of conceived strengths, weaknesses, opportunities and threats (SWOT) of potential collaborative activities with regard to schistosomiasis in Africa and Asia, this article addresses the importance of collaborative efforts and explores the priorities that would be expected to facilitate the transfer of Chinese experience to low- and middle-income countries in Africa and Asia.

Schistosoma mekongi cathepsin B and its use in the development of an immunodiagnosis

Schistosomiasis mekongi is one of the most important human parasitic diseases caused by Schistosoma mekongi in South-east Asia. The endemic area is the Mekong River sub-region from Laos to Cambodia. This parasite also infects dogs and pigs which are its alternative host species. Currently, the lack of reliable rapid diagnosis makes it difficult to monitor the infection and spreading of the disease. In this study, we screened the antigens of the parasite with sera of infected mice using Western blotting and identified proteins of interest with LC-MS/MS to obtain potential candidate proteins for diagnostic development. This assay yielded 2 immunoreactive bands at molecular masses of 31 and 22kDa. The 31kDa protein was the major band identified as cathepsin B, and its gene was cloned to obtain a full cDNA sequence (SmekCatB). The cDNA consisted of 1123bp and its longest reading frame encoded for 342 amino acids with some putative post translation modifications. The recombinant SmekCatB (rSmekCatB) with hexahistidine tag at the C-terminus was expressed in Escherichia coli and purified by Ni-NTA resin under denaturing conditions. The rSmekCatB reacted with sera of S. mekongi-infected mice. Indirect ELISA using rSmekCatB as the antigen to detect mouse antibodies, revealed a sensitivity of 91.67% for schistosomiasis mekongi and the specificity of 100%. Our data suggested that SmekCatB is one of the most promising parasitic antigens that could be used for the diagnosis of S. mekongi infection.

Comparative Phylogenetic Studies on Schistosoma japonicum and Its Snail Intermediate Host Oncomelania hupensis: Origins, Dispersal and Coevolution

BACKGROUND

Schistosoma japonicum causes major public health problems in China and the Philippines; this parasite, which is transmitted by freshwater snails of the species Oncomelania hupensis, causes the disease intestinal schistosomiasis in humans and cattle. Researchers working on Schistosoma in Africa have described the relationship between the parasites and their snail intermediate hosts as coevolved or even as an evolutionary arms race. In the present study this hypothesis of coevolution is evaluated for S. japonicum and O. hupensis. The origins and radiation of the snails and the parasite across China, and the taxonomic validity of the sub-species of O. hupensis, are also assessed.

METHODOLOGY/PRINCIPAL FINDINGS

The findings provide no evidence for coevolution between S. japonicum and O. hupensis, and the phylogeographical analysis suggests a heterochronous radiation of the parasites and snails in response to different palaeogeographical and climatic triggers. The results are consistent with a hypothesis of East to West colonisation of China by Oncomelania with a re-invasion of Japan by O. hupensis from China. The Taiwan population of S. japonicum appears to be recently established in comparison with mainland Chinese populations.

CONCLUSIONS/SIGNIFICANCE

The snail and parasite populations of the western mountain region of China (Yunnan and Sichuan) appear to have been isolated from Southeast Asian populations since the Pleistocene; this has implications for road and rail links being constructed in the region, which will breach biogeographical barriers between China and Southeast Asia. The results also have implications for the spread of S. japonicum. In the absence of coevolution, the parasite may more readily colonise new snail populations to which it is not locally adapted, or even new intermediate host species; this can facilitate its dispersal into new areas. Additional work is required to assess further the risk of spread of S. japonicum.

Reprint of "An overview of freshwater snails in Asia with main focus on Vietnam"

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.

A new population and habitat for Neotricula aperta in the Mekong river of northeastern Thailand: a DNA sequence-based phylogenetic assessment confirms identifications and interpopulation relationships

Neotricula aperta (Gastropoda: Pomatiopsidae), the snail intermediate host of Schistosoma mekongi, is found in Cambodia, Laos, and Thailand. We update information on the distribution of this species in the Mekong River and its tributary, the Mun River, in Thailand. DNA sequences of a portion of the mitochondrial cytochrome c oxidase subunit 1 were obtained from N. aperta collected from different locations and used to confirm species and strain identities. Specimens of the β-strain were found in the Mun River, whereas specimens of the γ-strain were found in the Mekong River. The γ-strain (with molecular confirmation of identity) is newly reported from Nong Khai Province, where it occurred in a habitat novel for this species: under paving slabs instead of under natural bed rocks, where agal aufwuchs is extensively located on the islet in the middle of the Mekong River. The new location is approximate 400 km upstream from the nearest previously known site for this species.

An overview of freshwater snails in Asia with main focus on Vietnam

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.

A phylogeny for the pomatiopsidae (Gastropoda: Rissooidea): a resource for taxonomic, parasitological and biodiversity studies

BACKGROUND

The Pomatiopsidae are reported from northern India into southern China and Southeast Asia, with two sub-families, the Pomatiopsinae (which include freshwater, amphibious, terrestrial and marine species) and the freshwater Triculinae. Both include species acting as intermediate host for species of the blood-fluke Schistosoma which cause a public health problem in East Asia. Also, with around 120 species, triculine biodiversity exceeds that of any other endemic freshwater molluscan fauna. Nevertheless, the origins of the Pomatiopsidae, the factors driving such a diverse radiation and aspects of their co-evolution with Schistosoma are not fully understood. Many taxonomic questions remain; there are problems identifying medically relevant species. The predicted range is mostly unsurveyed and the true biodiversity of the family is underestimated. Consequently, the aim of the study was to collect DNA-sequence data for as many pomatiopsid taxa as possible, as a first step in providing a resource for identification of epidemiologically significant species (by non-malacologists), for use in resolving taxonomic confusion and for testing phylogeographical hypotheses.

RESULTS

The evolutionary radiation of the Triculinae was shown to have been rapid and mostly post late Miocene. Molecular dating indicated that the radiation of these snails was driven first by the uplift of the Himalaya and onset of a monsoon system, and then by late-Pliocene global warming. The status of Erhaia as Anmicolidae is supported. The genera Tricula and Neotricula are shown to be non-monophyletic and the tribe Jullieniini may be polyphyletic (based on convergent characters). Triculinae from northern Vietnam could be derived from Gammatricula of Fujian/Yunnan, China.

CONCLUSIONS

The molecular dates and phylogenetic estimates in this study are consistent with an Australasian origin for the Pomatiopsidae and an East to West radiation via Oligocene Borneo-Philippines island hopping to Japan and then China (Triculinae arising mid-Miocene in Southeast China), and less so with a triculine origin in Tibet. The lack of monophyly in the medically important genera and indications of taxonomic inaccuracies, call for further work to identify epidemiologically significant taxa (e.g., Halewisia may be potential hosts for Schistosoma mekongi) and highlight the need for surveys to determine the true biodiversity of the Triculinae.

A population growth trend analysis for Neotricula aperta, the snail intermediate host of Schistosoma mekongi, after construction of the Pak-Mun dam

Background

The Pak-Mun dam is a controversial hydro-power project on the Mun River in Northeast Thailand. The dam is sited in a habitat of the freshwater snail Neotricula aperta, which is the intermediate host for the parasitic blood-fluke Schistosoma mekongi causing Mekong schistosomiasis in humans in Cambodia and Laos. Few data are available which can be used to assess the effects of water resource development on N. aperta. The aim of this study was to obtain data and to analyze the possible impact of the dam on N. aperta population growth.

Methodology/Principal Findings

Estimated population densities were recorded for an N. aperta population in the Mun River 27 km upstream of Pak-Mun, from 1990 to 2011. The Pak-Mul dam began to operate in 1994. Population growth was modeled using a linear mixed model expression of a modified Gompertz stochastic state-space exponential growth model. The N. aperta population was found to be quite stable, with the estimated growth parameter not significantly different from zero. Nevertheless, some marked changes in snail population density were observed which were coincident with changes in dam operation policy.

Conclusions/Significance

The study found that there has been no marked increase in N. aperta population growth following operation of the Pak-Mun dam. The analysis did indicate a large and statistically significant increase in population density immediately after the dam came into operation; however, this increase was not persistent. The study has provided the first vital baseline data on N. aperta population behavior near to the Pak-Mun dam and suggests that the operation policy of the dam may have an impact on snail population density. Nevertheless, additional studies are required for other N. aperta populations in the Mun River and for an extended time series, to confirm or refine the findings of this work.

There is much controversy over the effects of water resource development on the transmission of schistosomiasis in the lower Mekong Basin. Impact assessments are urgently required because there are currently 12 such projects planned in the region. The key to understanding the effects of impoundment is the impact on the snail intermediate host, which, in the case of Mekong schistosomiasis, is Neotricula aperta. Surprisingly, we have almost no data on N. aperta population trends nor on the impact of dams. To address this, the present work focused on a population near the Pak-Mun dam in Thailand. The analysis suggested that N. aperta populations were not growing significantly over the study period (1990–2011), but that the dam may have affected a spike in population density immediately after its completion. The study also revealed changes in density that were coincident on changes in operation of the dam; suggesting that keeping the dam open might lower snail population densities. This is the first scientific assessment of the impact of the Pak-Mun dam on N. aperta and suggests that dams of this kind may affect snail population density. The study also indicates an urgent need for additional independent observations and continuing regular surveys.

Observations on Neotricula aperta (Gastropoda: Pomatiopsidae) population densities in Thailand and central Laos: implications for the spread of Mekong schistosomiasis

Background

The snail Neotricula aperta transmits Mekong schistosomiasis in southern Laos and Cambodia, with about 1.5 million people at risk of infection. Plans are under consideration for at least 12 hydroelectric power dams on the lower Mekong river and much controversy surrounds predictions of their environmental impacts. Unfortunately, there are almost no ecological data (such as long term population trend studies) available for N. aperta which could be used in impact assessment. Predictions currently assume that the impacts will be the same as those observed in Africa (i.e., a worsening of the schistosomiasis problem); however, marked ecological differences between the snails involved suggest that region specific models are required. The present study was performed as an initial step in providing data, which could be useful in the planning of water resource development in the Mekong. Snail population density records were analyzed for populations close to, and far downstream of, the Nam Theun 2 (NT2) project in Laos in order to detect any changes that might be attributable to impoundment.

Results

The population immediately downstream of NT2 and that sampled 400 km downstream in Thailand both showed a long term trend of slow growth from 1992 to 2005; however, both populations showed a marked decline in density between 2005 and 2011. The decline in Thailand was to a value significantly lower than that predicted by a linear mixed model for the data, whilst the population density close to NT2 fell to undetectable levels in 2011 from densities of over 5000 m^-2 in 2005. The NT2 dam began operation in 2010.

Conclusions

The impact of the NT2 dam on N. aperta population density could be more complex than first thought and may reflect the strict ecological requirements of this snail. There was no indication that responses of N. aperta populations to dam construction are similar to those observed with Bulinus and Schistosoma haematobium in Africa, for example. In view of the present findings, more ecological data (in particular population density monitoring and surveillance for new habitats) are urgently required in order to understand properly the likely impacts of water resource development on Mekong schistosomiasis.

Development of a real-time PCR assay with fluorophore-labelled hybridization probes for detection of Schistosoma mekongi in infected snails and rat feces

Schistosoma mekongi, a blood-dwelling fluke, is a water-borne parasite that is found in communities along the lower Mekong River basin, i.e. Cambodia and Lao People's Democratic Republic. This study developed a real-time PCR assay combined with melting-curve analysis to detect S. mekongi in laboratory setting conditions, in experimentally infected snails, and in fecal samples of infected rats. The procedure is based on melting-curve analysis of a hybrid between an amplicon from S. mekongi mitochondrion sequence, the 260 bp sequence specific to S. mekongi, and specific fluorophore-labelled probes. This method could detect as little as a single cercaria artificially introduced into a pool of 10 non-infected snails, a single cercaria in filtered paper, and 2 eggs inoculated in 100 mg of non-infected rat feces. All S. mekongi-infected snails and fecal samples from infected rats were positive. Non-infected snails, non-infected rat feces, and genomic DNA of other parasites were negative. The method gave high sensitivity and specificity, and could be applied as a fast and reliable tool for cercarial location in water environments in endemic areas and for epidemiological studies and eradication programmes for intermediate hosts.

Zoonotic schistosomiasis in non-human primates: past, present and future activities at the human–wildlife interface in Africa

Schistosomiasis is one of the world's most widely distributed and prevalent parasitic diseases. Less widely recognized is that some species of Schistosoma, including several that commonly affect humans, also cause disease in other mammalian species; in particular, infections in non-human primates are known. With interest increasing in emerging zoonotic diseases, the status of schistosomiasis as a zoonotic infection is in need of re-appraisal, especially in light of advances in application of molecular screening and epidemiological tools where newly reported infections raise general animal welfare and conservation concerns. Focusing on Africa, this review provides a summary of the occurrence of schistosomiasis in non-human primates and discusses new ways in which surveillance for schistosomiasis should be integrated into more effective conservation management and disease control strategies. Emphasis is on the more common forms of human schistosomiasis, their clinical manifestations and epidemiological significance in terms of infection reservoir potential.

Genetic differentiation among populations of Brachytrupes portentosus (Lichtenstein 1796) (Orthoptera: Gryllidae) in Thailand and the Lao PDR: the Mekong River as a biogeographic barrier

The Mekong River is known to act as a boundary between a number of terrestrial and freshwater species, including various parasites and their intermediate hosts as well as endangered mammal species. Little information is available, however, on the genetic differentiation between terrestrial invertebrates to the east and the west of this wide river. The genetic diversity among eight natural populations of Brachytrupes portentosus (Lichtenstein, 1796) (Orthoptera: Gryllidae) collected from Thailand and the Lao People's Democratic Republic (PDR) were analyzed by multilocus enzyme electrophoresis. The allelic profiles of 20 enzymes encoding 23 loci were analyzed. An average of 41% fixed differences was detected between the populations from Thailand and Lao PDR, which are separated by the Mekong River. The percent fixed differences ranged between 4% and 26% within the populations from Thailand and between 4% and 22% within the populations from Lao PDR. A phenogram shows that the eight populations fell into two major clusters based on the Thai and Lao sampling sites. The genetic distance between the samples within Thailand and within Lao PDR was related to the distances between sampling areas. The genetic variability between populations of this cricket indicates that genetic relationships are influenced by a natural barrier as well as by the geographical distance between these allopatric populations.

Important helminth infections in Southeast Asia diversity, potential for control and prospects for elimination

Besides the 'big three'-HIV/AIDS, malaria and tuberculosis-there are a host of diseases that, by comparison, are truly neglected. These so-called neglected tropical diseases (NTDs), many of which caused by helminths, are intimately linked with poverty and are rampant where housing is poor; access to clean water and adequate sanitation is lacking; hygiene and nutrition is substandard and populations are marginalized and vulnerable. More than a billion people are affected by NTDs, mainly in remote rural and deprived urban settings of the developing world. An overview of papers published in two special thematic volumes of the Advances in Parasitology is provided here under the umbrella of current status of research and control of important helminth infections. A total of 25 comprehensive reviews are presented, which summarise the latest available data pertaining to the diagnosis, epidemiology, pathogenesis, prevention, treatment, control and eventual elimination of NTDs in Southeast Asia and neighbourhood countries. The focus of the first volume provides the current regional status of schistosomiasis, lymphatic filariasis, food-borne trematodiases, echinococcosis and cysticercosis/taeniasis, less common parasitic diseases that can cause epidemic outbreaks and helminth infections affecting the central nervous system. The second volume deals with the tools and strategies for control, including diagnostics, drugs, vaccines and cutting-edge basic research (e.g. the '-omics' sciences). Moreover, cross-cutting themes such as multiparasitism, social sciences, capacity strengthening, geospatial health technologies, health metrics and modelling the potential impact of climate change on helminthic diseases are discussed. Hopefully, these two volumes will become useful for researchers and, most importantly, disease control managers for integrated and sustainable control, rigorous monitoring and eventual elimination of NTDs in Southeast Asia and elsewhere.

Schistosoma mekongi in Cambodia and Lao People's Democratic Republic

Schistosomiasis found in communities along the Mekong River in Cambodia and Lao People's Democratic Republic (Lao PDR) is caused by the blood fluke Schistosoma mekongi. Early observations on patients in 1957 revealed severe intestinal and hepatosplenic disease. High mortality rates and widespread disease were reported from the provinces of northern Cambodia (Stung Treng and Kratié) and southern Lao PDR (Champasack) in the early 1970s and 1990s. Control programmes built around mass drug administration, with praziquantel, and combined with information and education campaigns, were carried out. In Cambodia, such programmes were started in 1995 in the endemic provinces and sustained until today; these efforts resolved the public health problem of schistosomiasis mekongi and led to a significant reduction in transmission. In Lao PDR, the interventions started in the late 1980s, but suffered several interruptions which permitted transmission to resume. Today, a number of small foci continue to show substantial prevalence rates. The snail intermediate host, Neotricula aperta, is present in the Mekong River and some of its tributaries. There is evidence that the snail might not yet have reached its full geographical distribution emphasising the need to sustain vigilance. New infections with S. mekongi occur in the endemic population and travellers alike. Comprehensive guidelines for the elimination of S. mekongi and bilateral efforts between Cambodia and Lao PDR are required.

The phylogeography of Indoplanorbis exustus (Gastropoda: Planorbidae) in Asia

BACKGROUND

The freshwater snail Indoplanorbis exustus is found across India, Southeast Asia, central Asia (Afghanistan), Arabia and Africa. Indoplanorbis is of economic importance in that it is responsible for the transmission of several species of the genus Schistosoma which infect cattle and cause reduced livestock productivity. The snail is also of medical importance as a source of cercarial dermatitis among rural workers, particularly in India. In spite of its long history and wide geographical range, it is thought that Indoplanorbis includes only a single species. The aims of the present study were to date the radiation of Indoplanorbis across Asia so that the factors involved in its dispersal in the region could be tested, to reveal potential historical biogeographical events shaping the phylogeny of the snail, and to look for signs that I. exustus might be polyphyletic.

RESULTS

The results indicated a radiation beginning in the late Miocene with a divergence of an ancestral bulinine lineage into Assam and peninsular India clades. A Southeast Asian clade diverged from the peninsular India clade late-Pliocene; this clade then radiated at a much more rapid pace to colonize all of the sampled range of Indoplanorbis in the mid-Pleistocene.

CONCLUSIONS

The phylogenetic depth of divergences between the Indian clades and Southeast Asian clades, together with habitat and parasitological differences suggest that I. exustus may comprise more than one species. The timescale estimated for the radiation suggests that the dispersal to Arabia and to Southeast Asia was facilitated by palaeogeographical events and climate change, and did not require human involvement. Further samples from Afghanistan, Africa and western India are required to refine the phylogeographical hypothesis and to include the African Recent dispersal.

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.

Travel-related schistosomiasis acquired in Laos

Twelve Israeli travelers acquired schistosomiasis in Laos during 2002-2008, and 7 of them had acute schistosomiasis. The patients were probably exposed to Schistosoma mekongi in southern Laos, an area known to be endemic for schistosomiasis. Four possibly were infected in northern Laos, where reports of schistosomiasis are rare.