Distinct Genetic Diversity of Oncomelania hupensis,Intermediate Host of Schistosoma japonicum in Mainland China as Revealed by ITS Sequences

Background

Oncomelania hupensis is the unique intermediate host of Schistosoma japonicum, which causes schistosomiasis endemic in the Far East, and especially in mainland China. O. hupensis largely determines the parasite''s geographical range. How O. hupensis''s genetic diversity is distributed geographically in mainland China has never been well examined with DNA sequence data.

Methodology/Principal Findings

In this study we investigate the genetic variation among O. hupensis from different geographical origins using the combined complete internal transcribed spacer 1 (ITS1) and ITS2 regions of nuclear ribosomal DNA. 165 O. hupensis isolates were obtained in 29 localities from 7 provinces across mainland China: lake/marshland and hill regions in Anhui, Hubei, Hunan, Jiangxi and Jiangsu provinces, located along the middle and lower reaches of Yangtze River, and mountainous regions in Sichuan and Yunnan provinces. Phylogenetic and haplotype network analyses showed distinct genetic diversity and no shared haplotypes between populations from lake/marshland regions of the middle and lower reaches of the Yangtze River and populations from mountainous regions of Sichuan and Yunnan provinces. The genetic distance between these two groups is up to 0.81 based on Fst, and branch time was estimated as 2–6 Ma. As revealed in the phylogenetic tree, snails from Sichuan and Yunnan provinces were also clustered separately. Geographical separation appears to be an important factor accounting for the diversification of the two groups of O. hupensis in mainland China, and probably for the separate clades between snails from Sichuan and Yunnan provinces. In lake/marshland and hill regions along the middle and lower reaches of the Yangtze River, three clades were identified in the phylogenetic tree, but without any obvious clustering of snails from different provinces.

Conclusions

O. hupensis in mainland China may have considerable genetic diversity, and a more complex population structure than expected. It will be of significant importance to consider the genetic diversity of O. hupensis when assessing co-evolutionary interactions with S. japonicum.     

Ecological Model to Predict Potential Habitats of Oncomelania hupensis,the Intermediate Host of Schistosoma japonicum in the Mountainous Regions,China

Background

Schistosomiasis japonica is a parasitic disease that remains endemic in seven provinces in the People’s Republic of China (P.R. China). One of the most important measures in the process of schistosomiasis elimination in P.R. China is control of Oncomelania hupensis, the unique intermediate host snail of Schistosoma japonicum. Compared with plains/swamp and lake regions, the hilly/mountainous regions of schistosomiasis endemic areas are more complicated, which makes the snail survey difficult to conduct precisely and efficiently. There is a pressing call to identify the snail habitats of mountainous regions in an efficient and cost-effective manner.

Methods

Twelve out of 56 administrative villages distributed with O. hupensis in Eryuan, Yunnan Province, were randomly selected to set up the ecological model. Thirty out of the rest of 78 villages (villages selected for building model were excluded from the villages for validation) in Eryuan and 30 out of 89 villages in Midu, Yunnan Province were selected via a chessboard method for model validation, respectively. Nine-year-average Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST) as well as Digital Elevation Model (DEM) covering Eryuan and Midu were extracted from MODIS and ASTER satellite images, respectively. Slope, elevation and the distance from every village to its nearest stream were derived from DEM. Suitable survival environment conditions for snails were defined by comparing historical snail presence data and remote sensing derived images. According to the suitable conditions for snails, environment factors, i.e. NDVI, LST, elevation, slope and the distance from every village to its nearest stream, were integrated into an ecological niche model to predict O. hupensis potential habitats in Eryuan and Midu. The evaluation of the model was assessed by comparing the model prediction and field investigation. Then, the consistency rate of model validation was calculated in Eryuan and Midu Counties, respectively.

Results

The final ecological niche model for potential O. hupensis habitats prediction comprised the following environmental factors, namely: NDVI (≥ 0.446), LST (≥ 22.70°C), elevation (≤ 2,300 m), slope (≤ 11°) and the distance to nearest stream (≤ 1,000 m). The potential O. hupensis habitats in Eryuan distributed in the Lancang River basin and O. hupensis in Midu shows a trend of clustering in the north and spotty distribution in the south. The consistency rates of the ecological niche model in Eryuan and Midu were 76.67% and 83.33%, respectively.

Conclusions

The ecological niche model integrated with NDVI, LST, elevation, slope and distance from every village to its nearest stream adequately predicted the snail habitats in the mountainous regions.     

Differential susceptibility of male and female Oncomelania hupensis quadrasi infected with Schistosoma japonicum

Laboratory-reared male and female Oncomelania hupensis quadrasi were exposed to infection with Schistosoma japonicum. In three of four trials, infection rates in female snails (30.5–68.0%) were higher than in males (8.6–28.6%). Higher cercarial production in females may be accounted for by male-female size difference, and presumably by the availability of more nutrients in females as a result of ovarian tissue breakdown. Male and female snails were individually exposed to 20 miracidia, fixed 3–72 h post-exposure and serially sectioned to check for encapsulated and nonencapsulated primary sporocysts (ps). In 23 female (65.2%) and 12 male (39.3%) snails, ps were all unencapsulated. Six male snails (17.6%) showed 100% ps encapsulation. Among the 12 females and 16 males with both negative and positive host-tissue response, encapsulated ps in females varied from 7.1 to 50.0% and 20.0 to 71.4% in males. Distribution of encapsulated and non-encapsulated ps in both sexes suggests that males are more resistant to infection by their encapsulation reaction. Whether resistance in O. h. quadrasi to S. japonicum is genetically determined warrants further study.     

Ultrastructure of the daughter sporocyst and developing cercaria of Schistosoma japonicum in experimentally infected snails, Oncomelania hupensis hupensis

Ultrastructure of daughter sporocysts and cercariae of Schistosoma japonicum were studied 2 and 4 months after infection of Oncomelania hupensis hupensis. The body walls of daughter sporocysts are similar at all infectious stages. They consist of an external syncytial tegument on a basement membrane, and an internal cellular subtegument surrounding a body cavity containing developing cercariae. The cercariae embryos develop 2 months after infection from germinal balls in the brood chamber of the daughter sporocyst. They are at first enveloped by a primitive epithelium rising from the daughter sporocyst. Four months after infection, the cercariae were almost fully developed and the primitive epithelium had degenerated. The body wall of the cercaria consists of a thin tegument covered by a surface coat of fibrous material and connected to the subtegumental cells by cytoplasmic processes. The matrix of the tegument contains numerous dense bodies, vacuoles, and spines. Two types of sensory structures - uniciliated and multiciliated - are found at the anterior tip of the cercaria. There are five pairs of penetration gland cells of two distinct types differentiated by the morphology of secretory granules. Flame cells are found in both daughter sporocysts and in cercariae. The cilia of the flame cells are characterized by the typical 9 and 2 cilium pattern.     

Malic dehydrogenase patterns in different strains of Schistosoma japonicum and subspecies of Oncomelania hupensis.

Malic dehydrogenase (MDH) of the Formosan Philippine and Indonesian Strains of Schistosoma japonicum and 3 subspecies of snails, Oncomelania hupensis formosana, O. h. quadrasi, and O. h. lindoensis were identified by disc electrophoresis with 10.5% acrylamide gel columns using tris-malic buffer. Male worm extracts of S. japonicum demonstrated 3, 2, and 2 clear MDH patterns and female worm extracts showed patterns of 2, 1 and 1 for the Formosan, Philippine and Indonesian strains of the parasite, respectively. MDH patterns were the same for all 3 subspecies of snails; one major and one minor band being found in all.     

Induction of resistance in Oncomelania hupensis nosophora against Schistosoma japonicum, but not against Paragonimus ohirai, using irradiated miracidia

Oncomelania hupensis nosophora snails sensitized with X-irradiated Schistosoma japonicum miracidia demonstrated resistance against a following challenge infection with non-irradiated homologous miracidia. The resistance in O. h. nosophora against S. japonicum was acquired within 1 day of sensitization, and it was strongest in a group challenged at an interval of 3 days. The resistance persisted for at least 4 weeks. Histological examinations revealed amoebocyte accumulation around the challenged S. japonicum sporocysts. On the other hand, when O. h. nosophora sensitized by exposure to X-irradiated P. ohirai or S. japonicum miracidia were subsequently challenged with normal P. ohirai miracidia, no resistance was observed, although they expressed the resistance against heterologous S. japonicum infection.     

Comparative studies of the defense mechanism against Schistosoma japonicum of schistosome-susceptible and -resistant Oncomelania nosophora

The amphibious snail Oncomelania nosophora is an intermediate host of Schistosoma japonicum. Previously we reported that there are two strains of the snail, one resistant and one susceptible to a Mindoro, the Philippines, strain of S. japonicum. The resistant snails were collected from Nirasaki and susceptible snails from Kisarazu, Japan. To determine early cellular responses in the two snail strains, we examined histologic alterations in the snails for up to 18 h after the initial exposure to miracidia. In both strains, the penetrating miracidia were distributed in the foot, mantle, gills, heart, stomach, and kidney, and the mean number of penetrating miracidia was similar in both strains. After penetration, snail hemocytes migrated toward the larvae, and by 12 h after exposure, substantial numbers of penetrated larvae were surrounded and encapsulated by hemocytes. The percentage of larvae encapsulated by hemocytes during 12-18 h after the exposure was significantly higher in the resistant Nirasaki strain (60.9+/-19.8%) than in the susceptible Kisarazu strain (42.3+/-15.0%). In a few snails of the Nirasaki strain, all the larvae found were encapsulated by hemocytes. The differences in hemocyte responses between the two strains may explain the susceptibility of the snails to schistosome larvae.     

An integrated approach to identify distribution of Oncomelania hupensis, the intermediate host of Schistosoma japonicum, in a mountainous region in China

The aim of this study is to better understand ecological variability related to the distribution of Oncomelania hupensis, the snail intermediate host of Schistosoma japonicum, and predict the spatial distribution of O. hupensis at the local scale in order to develop a more effective control strategy for schistosomiasis in the hilly and mountainous regions of China. A two-pronged approach was applied in this study consisting of a landscape pattern analysis complemented with Bayesian spatial modelling. The parasitological data were collected by cross-sectional surveys carried out in 11 villages in 2006 and mapped based on global positioning system (GPS) coordinates. Environmental surrogates and landscape metrics were derived from remotely-sensed images and land-cover/land-use classification data. Bayesian non-spatial and spatial models were applied to investigate the variation of snail density in relation to environmental surrogates and landscape metrics at the local scale. A Bayesian spatial model, validated by the deviance information criterion (DIC), was found to be the best-fitting model. The mean shape index (MSI) and Shannon's evenness indexes (SEI) were significantly associated with snail density. These findings suggest that decreasing the heterogeneity of the landscape can reduce snail density. A prediction maps were generated by the Bayesian model together with environmental surrogates and landscape metrics. In conclusion, the risk areas of snail distribution at the local scale can be identified using an integrated approach with landscape pattern analysis supported by remote sensing and GIS technologies, as well as Bayesian modelling.     

Molecular Phylogeography of Oncomelania Lindoensis (Gastropoda: Pomatiopsidae), the Intermediate Host of Schistosoma Japonicum in Sulawesi

Oncomelania lindoensis from Lake Lindu, Sulawesi, was characterizedfor genetic variation at 21 allozyme loci and compared withO. hupensis (China) and O. quadrasi (Philippines). Geneticdistances and interpopulation patterns of allele-sharing pointto a closer relationship between Sulawesi and the Philippines(Nei's unbiased genetic distances (D) averaged 0.50) than betweenSulawesi and China (D= 0.79). These data, coupled with a considerationof the geographic distribution of the genus, support the hypothesisthat the Sulawesi Oncomelaniaoriginated by avian-facilitated colonizationfrom the Philippines about two million years ago. Oncomelania from Sulawesi were originally described as subspecificallydistinct: Oncomelania hupensis lindoensis. However, the allopatricdistribution, unique alleles at five loci, and significant geneticdistances from congeners in Mindanao and elsewhere in the Philippinessuggest that this taxon should be distinguished as a full specieswithin the Oncomelania hupensis species group, namely: O. lindoensisDavis & Carney 1973. Comparison with published data on variationwithin quadrasi and in three Chinese subspecies of hupensisshowed that D values increase with taxonomic level in this speciesgroup. D averaged 0.15 (0–0.26) within Chinese subspeciesand 0.04 (0–0.13) within the Philippines, but was 0.30(0.20–0.45) between Chinese subspecies, and 0.48–0.80between the three species (hupensis, quadrasi and lindoensis).The genotypic cluster species concept and these multilocus geneticdistances can be used to help define species and subspeciesin these medically important snails. (Received 14 May 1997; accepted 20 April 1998)     

Schistosoma japonicum and Paragonimus ohirai: antagonism between S. japonicum and P. ohirai in Oncomelania nosophora

Antagonistic interactions between Schistosoma japonicum and Paragonimus ohirai were examined in the snail host, Oncomelania nosophora. When P. ohirai-infected snails were exposed to S. japonicum miracidia at intervals of 4 to 18 weeks post-first exposure, only a few snails (0-7%) were found to be superinfected with S. japonicum sporocysts. Sporocysts were fewer in number than those of single infected controls. Mature S. japonicum cercariae were not observed. Furthermore, when the snails were examined at intervals of 14 to 18 weeks post-second exposure, neither sporocysts nor cercariae of S. japonicum were found. On the other hand, when the snails were exposed to miracidia of S. japonicum and P. ohirai simultaneously, they were easily infected with both parasites. At 26 weeks after simultaneous exposure, however, the infection rate of S. japonicum was significantly lower than that of controls. In contrast, when S. japonicum-infected snails were exposed to P. ohirai miracidia, they were superinfected with P. ohirai, although the infection rate was somewhat lower than that of controls. These results indicate the existence of antagonism between S. japonicum and P. ohirai in O. nosophora. Furthermore, P. ohirai was dominant over S. japonicum in the antagonistic interactions in this snail host.     

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.     

Three Gorges Dam: Impact of Water Level Changes on the Density of Schistosome-Transmitting Snail Oncomelania hupensis in Dongting Lake Area,China

BackgroundSchistosomiasis remains an important public health issue in China and worldwide. Oncomelania hupensis is the unique intermediate host of schistosoma japonicum, and its change influences the distribution of S. japonica. The Three Gorges Dam (TGD) has substantially changed the ecology and environment in the Dongting Lake region. This study investigated the impact of water level and elevation on the survival and habitat of the snails.MethodsData were collected for 16 bottomlands around 4 hydrological stations, which included water, density of living snails (form the Anxiang Station for Schistosomiasis Control) and elevation (from Google Earth). Based on the elevation, sixteen bottomlands were divided into 3 groups. ARIMA models were built to predict the density of living snails in different elevation areas.ResultsBefore closure of TGD, 7 out of 9 years had a water level beyond the warning level at least once at Anxiang hydrological station, compared with only 3 out of 10 years after closure of TGD. There were two severe droughts that happened in 2006 and 2011, with much fewer number of flooding per year compared with other study years. Overall, there was a correlation between water level changing and density of living snails variation in all the elevations areas. The density of living snails in all elevations areas was decreasing after the TGD was built. The relationship between number of flooding per year and the density of living snails was more pronounced in the medium and high elevation areas; the density of living snails kept decreasing from 2003 to 2014. In low elevation area however, the density of living snails decreased after 2003 first and turned to increase after 2011. Our ARIMA prediction models indicated that the snails would not disappear in the Dongting Lake region in the next 7 years. In the low elevation area, the density of living snails would increase slightly, and then stabilize after the year 2017. In the medium elevation region, the change of the density of living snails would be more obvious and would increase till the year 2020. In the high elevation area, the density of living snails would remain stable after the year 2015.ConclusionThe TGD influenced water levels and reduced the risk of flooding and the density of living snails in the study region. Based on our prediction models, the density of living snails in all elevations tends to be stabilized. Control of S. japonica would continue to be an important task in the study area in the coming decade.     

Histamine Immunoreactive Elements in the Central and Peripheral Nervous Systems of the Snail,Biomphalaria spp., Intermediate Host for Schistosoma mansoni

Histamine appears to be an important transmitter throughout the Animal Kingdom. Gastropods, in particular, have been used in numerous studies establishing potential roles for this biogenic amine in the nervous system and showing its involvement in the generation of diverse behaviours. And yet, the distribution of histamine has only previously been described in a small number of molluscan species. The present study examined the localization of histamine-like immunoreactivity in the central and peripheral nervous systems of pulmonate snails of the genus Biomphalaria. This investigation demonstrates immunoreactive cells throughout the buccal, cerebral, pedal, left parietal and visceral ganglia, indicative of diverse regulatory functions in Biomphalaria. Immunoreactivity was also present in statocyst hair cells, supporting a role for histamine in graviception. In the periphery, dense innervation by immunoreactive fibers was observed in the anterior foot, perioral zone, and other regions of the body wall. This study thus shows that histamine is an abundant transmitter in these snails and its distribution suggest involvement in numerous neural circuits. In addition to providing novel subjects for comparative studies of histaminegic neurons in gastropods, Biomphalaria is also the major intermediate host for the digenetic trematode parasite, which causes human schistosomiasis. The study therefore provides a foundation for understanding potential roles for histamine in interactions between the snail hosts and their trematode parasites.     

南水北调工程对日本血吸虫中间宿主湖北钉螺(Oncomelania hupensis)分布的影响

中国南水北调工程是缓解北方缺水和生态环境恶化的重大战略性工程.从长江下游引水北上的南水北调东线工程,因其引水口处在有日本血吸虫中间宿主湖北钉螺孳生的江苏省江都市,并且输水干线将穿过我国钉螺分布区及其最北点的宝应县(北纬33°15′)进入钉螺非分布区.为探讨因南水北调将钉螺带到北方钉螺非分布区后,钉螺能否生存与繁殖并形成新有螺区,研究沿工程输水干线在江苏徐州(北纬34°21′)和山东济宁(北纬35°23′)现场设点,采用螺笼放养定量观察法,对北移钉螺生存繁殖力及其子代钉螺的适应性作了为期8a的纵向观察;采用组织学、组织化学、酶组织化学及电镜技术,观察了北移钉螺生殖腺变化.结果:北移钉螺及其子代在徐州存活时间不超过8a,在济宁存活时间不超过1.5a;钉螺北移3～6个月后,其雌、雄生殖腺均呈萎缩状,相关代谢酶和组化成分含量出现异常变化.研究认为,南水北调若将钉螺移至北纬33°15′～34°21′地区,钉螺虽然能存活一定时间,但其繁殖力逐年下降,种群呈逐渐消亡趋势;若移至北纬35°23′以北地区,钉螺则难以存活.因而,随南水北调移至北纬33°15′以北地区的钉螺难以正常生存繁殖形成新的有螺区.     

Impact of Changing Water Levels and Weather on <Emphasis Type="Italic">Oncomelania hupensis hupensis</Emphasis> Populations,the Snail Host of <Emphasis Type="Italic">Schistosoma japonicum</Emphasis>, Downstream of the Three Gorges Dam

Increasing evidence indicates that dams impact riverine ecosystems and human diseases. Poyang Lake, one of the largest schistosomiasis endemic environments in China, will change due to the construction of the Yangtze River Three Gorges Dam. We assess changes in Oncomelania hupensis hupensis, the snail host for Schistosoma japonicum, in response to changing water levels and weather from 1998 to 2002. In the 5 years following the major flooding of Poyang Lake in 1998, seasonal water levels have gradually decreased, concomitant with decreases in mean and variance of fall snail densities. Nonlinear relationships suggest that the highest spring density is associated with current, 2-, and 3-month prior temperatures of 18°, 9.1°, and 5.8°C, while the highest fall density is associated with 2- and 3-month prior water levels of 17 and 18 m, respectively. This suggests that lower, more stable water levels downstream of the dam may result in a reduction in mean fall densities and their variance. However, additional data are needed to determine whether snail populations that are typically destroyed by seasonal floods may live longer in more stable environments created by the dam.     

The Effect of Simulating Different Intermediate Host Snail Species on the Link between Water Temperature and Schistosomiasis Risk

Introduction

A number of studies have attempted to predict the effects of climate change on schistosomiasis risk. The importance of considering different species of intermediate host snails separately has never previously been explored.

Methods

An agent-based model of water temperature and Biomphalaria pfeifferi population dynamics and Schistosoma mansoni transmission was parameterised to two additional species of snail: B. glabrata and B. alexandrina.

Results

Simulated B. alexandrina populations had lower minimum and maximum temperatures for survival than B. pfeifferi populations (12.5–29.5°C vs. 14.0–31.5°C). B. glabrata populations survived over a smaller range of temperatures than either B. pfeifferi or B. alexandrina (17.0°C–29.5°C). Infection risk peaked at 16.5°C, 25.0°C and 19.0°C respectively when B. pfeifferi, B. glabrata and B. alexandrina were simulated. For all species, infection risk increased sharply once a minimum temperature was reached.

Conclusions

The results from all three species suggest that infection risk may increase dramatically with small increases in temperature in areas at or near the currents limits of schistosome transmission. The effect of small increases in temperature in areas where schistosomiasis is currently found will depend both on current temperatures and on the species of snail acting as intermediate host(s) in the area. In most areas where B. pfeifferi is the host, infection risk is likely to decrease. In cooler areas where B. glabrata is the host, infection risk may increase slightly. In cooler areas where B. alexandrina is the host, infection risk may more than double with only 2°C increase in temperature. Our results show that it is crucial to consider the species of intermediate host when attempting to predict the effects of climate change on schistosomiasis.