Malacological and Parasitological Surveys on Ethiopian Rift Valley Lakes: Implications for Control and Elimination of Snail-Borne Diseases

High prevalence of intestinal schistosomiasis in school-age children in the villages adjacent to Lake Chamo in the southern Rift Valley of Ethiopia

BACKGROUND

The prevalence of intestinal schistosomiasis remains a challenge despite government efforts to eliminate the disease. This study aims to assess the prevalence of intestinal schistosomiasis in school-age children living in the villages surrounding Lake Chamo in southern Rift Valley of Ethiopia.

METHODOLOGY/PRINCIPAL FINDINGS

A cross-sectional study was conducted from January to July 2023 in Shele Mela Kebele in Gamo Zone, South Ethiopia. Stool samples were collected from 597 school-age children. A singe Kato-Katz for helminths and formalin-ether concentration technique for protozoan parasites were used to process the samples. The intensity of Schistosoma mansoni infection among school-age children was determined by counting the number of eggs per gram of stool. Of the 597 children screened, 52.3% (95% CI: 48.4.4-56.4) were positive for Schistosoma mansoni. These findings showed that 55% of the infections were light, 30.3% were moderate, and 14.7% were heavy. The mean egg count of S. mansoni parasites was 182.1 eggs per gram. The prevalence of other intestinal parasites (Hymenolopis nana, Ascaris lumbricoides, Hookworm, Taenia species, Giardia lamblia and Entamoeba histolytica) was found to be 7.7% (46/597). The overall prevalence of S. mansoni co-infection with other intestinal parasites was 5.0% (30/597). Specifically, the co-infection rates were 1.5% for A. lumbricoides, 1.3% for H. nana, 1.0% for Taenia species, 0.2% for Hookworm, 0.2% for E. histolytica, and 0.2% for G. lamblia.

CONCLUSIONS/SIGNIFICANCES

The study showed a high rate of S. mansoni infection among school-age children. This calls for immediate action, such as school-based deworming, to protect these children from the disease and reduce the burden. Further research is needed to understand the transmission of the infection by the intermediate host.

Abundance, Distribution, and Diversity of Freshwater Snail and Prevalences of Their Infection by Cercaria of Fasciola gigantica and Schistosoma spp at Mayo-Vreck River, Far North Region of Cameroon

Malacological and parasitological studies were conducted from April 2020 to March 2021 to determine the abundance and distribution of molluscs and cercariae of Schistosoma spp and Fasciola gigantica. Collected molluscs are exposed to strong light to induce cercarial release. Mollusc densities were higher at station 1 (Gamak) than in station 8 (Patakai), with Bellamya unicolor and Biomphalaria pfeifferi more abundant and Bulinus truncatus, B. tropicus, and B. globosus less abundant. The overall prevalence of cercariae (19.87%) is higher in station 3 (Yaye orchard), station 9 (Gougni), station 4 (Madiogo), station 5 (Madiogo pasture), and station 6 (Ziam 3). It varies significantly between 15.76% in station 8 and 25.77% in station 3, between 8.48% in B. truncatus and 25.53% in B. globosus, and between 19.27% for cercariae of Schistosoma spp and 21.60% for those of F. gigantica. Cercarial emissions in L. natalensis and B. pfeifferi were higher in hot and cold dry seasons; on the other hand, cercarial emissions in B. globosus were higher in hot dry seasons (31.48%) and rainy seasons (23.38%). Emissions of cercariae from S. haematobium are related to areas of human activity and defecation, while those of F. gigantica in L. natalensis, Schistosoma haematobium in B. tropicus, and S. mansoni in B. pfeifferi are related to grazing areas. Mayo-Vreck is a site that favors the endemicity of fascioliasis and human schistosomiasis.

Seasonal and environmental dynamics of intra-urban freshwater habitats and their influence on the abundance of Bulinus snail host of Schistosoma haematobium in the Tiko endemic focus, Mount Cameroon region

Urogenital schistosomiasis (UGS) caused by Schistosoma haematobium is endemic in the South West Region of Cameroon. An understanding of the abundance and distribution of the Bulinus snail, intermediate host can inform strategic snail control programmes at a local scale. This study investigated seasonal dynamics and environmental factors influencing occurrence and abundance of freshwater snail intermediate hosts in Tiko, a semi-urban endemic focus in the Mount Cameroon area. A longitudinal malacological field survey was conducted between December 2019 and December 2020 in the Tiko municipality. Snails were collected for one year monthly at 12 different human water contact sites along a stretch of the Ndongo stream using a standardized sampling technique. Freshwater snails were identified using shell morphological features. In addition, water temperature, pH, electrical conductivity, total dissolved solutes, salinity, water depth, width and flow velocity were measured, and vegetation cover as well as substrate type were determined. Bayesian regression models were used to identify the main environmental factors affecting the occurrence and abundance of Bulinus intermediate host. In total, 2129 fresh water snails were collected during the study period. Physa (51.4%) was the most abundant genus followed by Melanoides (28.6%) then, Bulinus (15.5%), Lymnaea (4.2%), Indoplanorbis (0.2%) and Potadoma (0.1%). Seasonality in abundance was significant in Bulinus sp as well as other genera, with greater numbers in the dry season (peaks between December and February). Water temperature, a rocky or sandy substrate type associated positively with Bulinus sp, meanwhile a higher water flow rate and medium vegetation negatively influenced the snail intermediate host population. These findings underscore the importance of timing behavioural and snail control interventions against schistosomiasis as well as increase vigilance of other trematode diseases in the study area. The continuous spread of planorbid snail hosts is a major concern.

Spatial and seasonal distribution of human schistosomiasis intermediate host snails and their interactions with other freshwater snails in 7 districts of KwaZulu-Natal province, South Africa

The spatial and seasonal distribution, abundance, and infection rates of human schistosomiasis intermediate host snails and interactions with other freshwater snails, water physicochemical parameters, and climatic factors was determined in this study. A longitudinal malacology survey was conducted at seventy-nine sites in seven districts in KwaZulu-Natal province between September 2020 and August 2021. Snail sampling was done simultaneously by two trained personnel for fifteen minutes, once in three months. A total of 15,756 snails were collected during the study period. Eight freshwater snails were found: Bulinus globosus (n = 1396), Biomphalaria pfeifferi (n = 1130), Lymnaea natalensis (n = 1195), Bulinus tropicus (n = 1722), Bulinus forskalii (n = 195), Tarebia granifera (n = 8078), Physa acuta (n = 1579), and Bivalves (n = 461). The infection rates of B. globosus and B. pfeifferi are 3.5% and 0.9%, respectively. In our study, rainfall, pH, type of habitats, other freshwater snails and seasons influenced the distribution, abundance, and infection rates of human schistosomiasis intermediate host snails (p-value < 0.05). Our findings provide useful information which can be adopted in designing and implementing snail control strategies as part of schistosomiasis control in the study area.

MALDI-TOF: A new tool for the identification of Schistosoma cercariae and detection of hybrids

Schistosomiasis is a neglected water-born parasitic disease caused by Schistosoma affecting more than 200 million people. Introgressive hybridization is common among these parasites and raises issues concerning their zoonotic transmission. Morphological identification of Schistosoma cercariae is difficult and does not permit hybrids detection. Our objective was to assess the performance of MALDI-TOF (Matrix Assistated Laser Desorption-Ionization–Time Of Flight) mass spectrometry for the specific identification of cercariae in human and non-human Schistosoma and for the detection of hybridization between S. bovis and S. haematobium. Spectra were collected from laboratory reared molluscs infested with strains of S. haematobium, S. mansoni, S. bovis, S. rodhaini and S. bovis x S. haematobium natural (Corsican hybrid) and artificial hybrids. Cluster analysis showed a clear separation between S. haematobium, S. bovis, S. mansoni and S. rodhaini. Corsican hybrids are classified with those of the parental strain of S. haematobium whereas other hybrids formed a distinct cluster. In blind test analysis the developed MALDI-TOF spectral database permits identification of Schistosoma cercariae with high accuracy (94%) and good specificity (S. bovis: 99.59%, S. haematobium 99.56%, S. mansoni and S. rodhaini: 100%). Most misidentifications were between S. haematobium and the Corsican hybrids. The use of machine learning permits to improve the discrimination between these last two taxa, with accuracy, F1 score and Sensitivity/Specificity > 97%. In multivariate analysis the factors associated with obtaining a valid identification score (> 1.7) were absence of ethanol preservation (p < 0.001) and a number of 2–3 cercariae deposited per well (p < 0.001). Also, spectra acquired from S. mansoni cercariae are more likely to obtain a valid identification score than those acquired from S. haematobium (p<0.001). MALDI-TOF is a reliable technique for high-throughput identification of Schistosoma cercariae of medical and veterinary importance and could be useful for field survey in endemic areas.

Effects of Land-Use and Environmental Factors on Snail Distribution and Trematode Infection in Ethiopia

Freshwater snails are intermediate hosts for several snail-borne diseases affecting humans and animals. Understanding the distribution of snail intermediate hosts and their infection status is very important to plan and implement effective disease prevention and control interventions. In this study, we determined the abundance, distribution, and trematode infection status of freshwater snails in two agro-ecological zones of Ethiopia. We sampled snails from 13 observation sites and examined them for trematode infections using a natural cercarial shedding method. A redundancy analysis (RDA) was used to examine the relationship between snail abundance and environmental variables. Overall, a total of 615 snails belonging to three species were identified. Lymnea natalensis and Bulinus globosus were the dominant snail species, representing 41% and 40% of the total collection, respectively. About one-third of the total snail population (33%) shed cercariae. The cercariae species recorded were Xiphidiocercaria, Brevifurcate apharyngeate distome (BAD), Echinostome, and Fasciola. Snail species were found in high abundance in aquatic habitats located in the agricultural landscape. Therefore, land-use planning and protection of aquatic habitats from uncontrolled human activities and pollution can be considered as important strategies to prevent and control the spread of snail-borne diseases in the region.

Parasitological and malacological surveys to identify transmission sites for Schistosoma mansoni in Gomma District, south-western Ethiopia

Schistosomiasis is a neglected tropical disease that disproportionately affects the poorest people in tropical and subtropical countries. It is a major parasitic disease causing considerable morbidity in Ethiopia. Despite significant control efforts, schistosomiasis transmission is still widespread in many rural areas of the country. The aim of this study was to determine the prevalence and intensity of intestinal schistosomiasis among schoolchildren, as well as to identify schistosomiasis transmission sites in Gomma District, southwestern Ethiopia. Between October 2018 and September 2019, cross-sectional parasitological and malacological surveys were conducted in the study area. The study comprised 492 school-children aged 6 to 15 years old from four primary schools in Gomma District. To identify and quantify eggs of Schistosoma mansoni from the children, stool specimens were collected and processed using double Kato-Katz thick smears. Water bodies adjacent to human settlements in the study area were surveyed for snail intermediate hosts of S. mansoni. Morphological identification of collected snails was conducted, followed by examining their infection status using a dissecting microscope. The overall prevalence of S. mansoni infection was 73.8% (95%CI: 69.9-77.7%) and 41.6% of them had moderate-to-heavy infections. The prevalence of S. mansoni infection differed considerably by age group, with the older age groups (12-15) having a higher prevalence than the younger age groups (6-11) (p < 0.001). The prevalence of infection also varied significantly among schools; Dedo Ureche had the highest prevalence (86.9%) (p = 0.034), while Goga Kilole had relatively the lowest prevalence of S. mansoni infection (59.6%) (p = 0.003). A total of 1463 Biomphalaria pfeifferi snails were collected from 11 survey sites throughout the study area, with 357 (24.4%) of the snails shedding schistosomes cercariae. Despite intensified efforts to scale up mass drug administration in Ethiopia, this study reported high levels of S. mansoni infection among schoolchildren and snail intermediate hosts in rural communities in Gomma. Such a high infection rate warrants pressing needs for targeted and integrated interventions to control the disease in the area.

Schistosomiasis Control in Ethiopia: The Role of Snail Mapping in Endemic Communities

Introduction: Schistosomiasis, a neglected tropical disease (NTD), remains a public health problem in Ethiopia. Freshwater snails, acting as intermediate hosts, release cercariae, the infectious parasite, into the water, which penetrate human skin that encounters infested waters. The objective of this study was to map snail abundance along rivers and study its association with schistosomiasis infection in communities using these rivers. Materials and Methods: A cross-sectional study was carried out at 20 river sites in Mizan Aman city administration, Bench Sheko zone, South West Ethiopia Peoples (SWEP) region, Ethiopia, to study the distribution of host snails and transmission sites for intestinal schistosomiasis. This study used a quantitative database consisting of data on the prevalence of infected snails, the characteristics of rivers and riverbanks, and the prevalence of schistosomiasis in the community, based on stool samples collected from community members near the sampling sites. Results: Aquatic snails were found in 11 of the 20 sites sampled. A total of 598 snails was collected, including Biomphalaria pfeifferi, Biomphalaria sudanica, Radix natalensis and Bulinus globosus species; the most abundant species was Biomphalaria pfeifferi. Stool samples were collected from 206 community members from all 20 sites. Forty-one (19.9%) were positive for Schistosoma mansoni. A positive correlation was found between the presence of snails and positive stool samples (r = 0.60, p = 0.05) and between the presence of infected snails and the prevalence of infection (r = 0.64, p = 0.03). Locations with muddy riverbanks were associated with the presence of snails (r = 0.81, p < 0.001). Conclusions: These results emphasize the importance of mapping snails for the control of schistosomiasis by defining hotspots of infection and identifying factors associated with the presence of infected snails. The results support the need for a continuous mapping of snails and the introduction of snail control as a major element for the successful control of schistosomiasis in endemic communities.