A Cross Sectional Assessment on Distribution of Snail and Risk Factors of Schistosoma Mansoni Infection in Proximity to Water Contact Points in Gorgora Town, Western Dembia District, Northwest Ethiopia.

Background: Schistosomiasis is one of the great neglected tropical diseases with socio-economic and health problem worldwide. In Ethiopia numerous studies reported S. mansoni is high among school-age and preschool age children. Though intensive prevalence studies are conducted the snail distribution and infectivity status and human infection rate dynamics in Ethiopian context remains poorly understood. Thus the purpose of the current study was to assess distribution of snail and risk factors of Schistosoma mansoni infection in proximity to water contact points in Gorgora town, Western Dembia District, Northwest Ethiopia. Method: A Community based cross-sectional study was conducted in Gorgora Town from March to May 2020. A total of 385 study participants were selected by systematic random sampling technique. Stool sample was collected and examined by Kato-Katz technique. Malacological survey was done from sampling sites showing signs of human activity near Lake Tana shoreline. Live snails collected in plastic bucket containing water and weed were transported within four hours to the University of Gondar, Medical parasitology laboratory for identication and determination of infection. Data was entered with EPI Data version 4.4.2.1. Data analysis was carried out using SPSS version 20.0 and STATA version 15.0. p-value ≤ 0.05 was reported as statistically signicant. Spatial distribution analysis was done using ArcGIS system for Geographical Information System (GIS). Result: The overall prevalence of S. mansoni infection was 36.6% (CI: 32-41.9) with intensity of 30.5%, 27.0% and 42.6% for light, moderate, and heavy, respectively. Among the different fresh water snails collected on the basis of shell morphology 546(%) were Biomphalaria pfeifferi, 310(28.1%) were Bulinus spp, 101(9.1%) were Lymnaea, and 147(13.3%) were Bivalve. Schistosome infection in Biomphalaria spp. was 0.0%. Swimming frequency and proximity to water body were observed to be the most associated risk factor to S. mansoni infection. Conclusion: The study showed that S. mansoni is an ongoing health problem in Gorgora. Different fresh water snails with a potential of transmitting diseases


Background
Schistosomiasis is one of the great neglected and unconquered tropical diseases caused by the digenetic trematode Schistosoma (1). It is an important socio economic and public health problem worldwide that is mostly prevalent in many developing countries where poor sanitation and low access to safe drinking water is predominant (2). Schistosoma haematobium, Schistosoma japonicum, and Schistosoma mansoni are the three main species infecting humans and S. mekongi and S. intercalatum are restricted in certain geographical locations. Schistosoma mansoni and S. haematobium which causes intestinal and urinary schistosomiasis respectively, are the causes of African schistosomiasis (2,3). Transmission of schistosomiasis involves the release of ova from infected hos faces or urine depending on the species, hatching of ova that release miracidia which ends up in the infection of appropriate intermediate snail host. Human infection involves skin penetration by cercaria when coming in contact with water containing Schistosome-infected snails. Inside the human host, cercaria develop into sexually mature ova-laying worms in the hepatic portal system (4).
Freshwater snails play salient role in the ecology of fresh-water; serving as food for numerous other animals and feeds on vast amounts of algae and detritus. Whereas estimated 350 snail specie play signi cant role in public and veterinary health, many species of freshwater snail belonging to the family Planorbidae serve as intermediate hosts for a number of infections such as angiostrongyliasis, clonorchiasis, fascioliasis, fasciolopsiasis, opisthorchiasis, paragonimiasis and the most common, schistosomiasis in Africa, Asia and the Americas (5,6). Human Schistosoma parasites species are transmitted by a responsible intermediate hosts: Biomphalaria, Bulinus, and Oncomelania for S. mansoni, S. haematobium, and S. japonicum, respectively (7,8).
Schistosomiasis mostly affects poor and rural communities, particularly agricultural and shing populations. Women doing domestic chores in infested water, such as washing clothes, are also at risk and can develop female genital schistosomiasis. Inadequate hygiene and contact with infected water make children especially vulnerable to infection (11).
Mass drug administration with Praziquantel (PZQ) is the cornerstone in schistosomiasis control strategies in which extensive reliance on just one drug is scaling up and this leads to emergence of resistance due to selective pressure (12,13). Recent reports on the possible development of resistance and treatment failure to PZQ have generated much more unrest.
In Ethiopia, while S. mansoni is wide spread, S. hematobium is restricted to few areas. Although there are ample literatures on the distribution and infection status of schistosomiasis in Ethiopia, there is a need to conduct study on the distribution and infection status of snail intermediate hosts (3).Therefore, the availability of this information is important in designing effective disease control measures such as mollucicidal chemicals. Methods A Community based cross-sectional study was conducted from May 2020 to March 2020 in selected Schistosome endemic village in Gorgora Town of Western Dembia District Northwest, Ethiopia. Gorgora is a small road side town on the shore of Lake Tana, 818 Km away from Addis Ababa. The average temperature ranges from 29 °C to 31 °C. Annual rainfall ranges from 900 mm to 1038 mm, with Altitude of 1800 m to 1900 m above sea level and located at 10 0 17'N latitude and 37 0 26'E longitude. The population is estimated to be 2,500 with 2.89 km square area (16) The parasitological examination sample size was determined by using the single population proportion formula with the following assumptions: prevalence (p) of 50% to increase sample size, 95% con dence level, 5% margin of error, and a sample estimation correction formula was considered. Moreover, a 10% non-response rate was added and the nal sample size (n) was = 385.
A multi-stage sampling procedure was utilized to select study participants. At the rst stage, village was selected randomly by simple random sampling from three villages. At the second sampling stage, numbers of households included in each village was determined by proportional allocation according to the total number of households found in each village/kebeles. Then, systematic sampling method was employed to select the households. The sampling interval was calculated by dividing the total households to the number of households to be included in the sample for each village. The initial household was randomly selected by lottery method and the next households was selected at that interval. Whenever more than one eligible was found in the same selected household, only one of them was chosen using the lottery method. In case no eligible candidate was identi ed in a selected household, the next household was selected keeping the interval constant afterwards. In addition, a person in the selected households was enrolled based on inclusion criteria. Then each data collector conducted a house-to house survey. Each water point and study households was Geo-referenced using Garmin64.

Malacological survey
All sites which showed signs of human activity near the shoreline were selected for snail collection.
Intermediate snail hosts was surveyed both in the lake and in the stream at human water contact sites. In the likely habitats, observations was also made on physical characteristics such as vegetation abundance, turbidity, the nature of the substrate and stones. In addition, the meantime water contact activities were also surveyed(4).Snails were collected by scooping, hand picking using forceps and gloves and put in open plastic bucket that contained a small quantity of water and weeds. Thereafter, snails were transported to Medical Parasitology laboratory, College of Medicine and Health Sciences, University of Gondar for determination of infection and identi cation. Then, snails were examined for natural trematode infections by shedding method (18).Each snail was placed individually in the shedding vials with water (deionized water) and then exposed to electric light for about one hour. The cercariae shed by the snails was identi ed to the genus level by their tail morphology following WHO recommendation(5, 6).

Socio demography and risk factor assessment
Well-structured questionnaire based on the known risk factors for Schistosoma was prepared in Amharic and was translated later in English version. It was translated later into the Amharic, local language of the study area. The questionnaire addresses inhabitant's socio-demographic information and risk factors. A pretest was conducted and additional response categories was added based on the pretest ndings.

Parasitological Techniques
A single stool specimen of about 2 g was collected from each study participant using clean, dry and leak proof plastic container labeled with unique identi cation number. A portion of the sample was processed by Kato-Katz method using a template holding 41.7 mg of stool (19). Kato-Katz smears were processed in study sites and slides were transported and examined under a microscope at University of Gondar (UoG), Medical Parasitology Laboratory. A small portion of the stool samples was preserved in 10% formalin for repeating for formol-ether concentration techniques (19,20). Examination for hookworm was performed within one hour of stool collection and Kato-Katz slide preparation. The slides were left for 24 h to clear for easy visualization of S. mansoni and other helminthes eggs. After 24 h, two experienced laboratory technologists examined each slides independently. Whenever the result of the two laboratory technologists discorded, the third experienced laboratory technologist examined the slides. Results of the laboratory investigation was recorded on a format prepared for this purpose. Infection intensity of the S. mansoni and STHs was estimated by multiplying the total number of eggs counted by 24, which gives as the eggs per gram (epg) of stool. Besides, the species speci c classes of infection intensity with S. mansoni and STH were classi ed as light, moderate and heavy per the threshold set by WHO (21). Inhabitants who were found positive for intestinal parasites infection were treated with standard dose of anti helmintic treatment (22).

Evaluation of physicochemical parameters
The evaluation of the physico-chemical parameters of water bodies was carried out to a depth not exceeding 0.5 meters. Using conductivity (µS/cm) multi-parameter digital probe recorder (HANNA instruments). Then the tube was placed in the device for 3 minutes to obtain the value. Hydric potential (pH) was obtained using pH-meter (HANNA instruments) by immersion of a probe. The water temperature (°C) was measured by a owatch (HANNA instruments) equipped with a submerged propeller at a depth of 0.5 meters(7).

Data Analysis and Interpretation
Data were double entered, coded, cleaned, and veri ed using EpiData Version 4.4.2.1 software (EpiData Association, Odense, Denmark). Data analysis was carried out using SPSS version 20.0 (IBM Corp. IBM SPSS Statistics for Windows, Armonk, NY, USA) and STATA version 15.0 (StataCorp. Statistical Software: College Station, TX 77845, USA). Different variables were described and characterized by frequency distribution (n (%)). Binary and Multivariable logistic regression analysis was done for associated risk factors for acquiring S. mansoni infection. Distribution analysis was done using ArcGIS system for Geographical Information System (GIS) for proximity analysis. The prevalence of schistosomiasis was computed as percentage (8).

Socio-demographic characteristics
A total of 385 study participants of which 189 (49.1%) males and 196 (50.9%) females were participated in the study ( Table 1). All the study participants were urban residents in Gorgora town. The mean age of  with S. mansoni. In bivariate analysis, risk factors associated with (p-value > 0.05) and those variables with (p-value < 0.25) were carried to multiple logistic regression to see independent effect of variables. The odds of getting S. mansoni infection among individuals swimming sometimes was 7.7 times higher than those they didn't have an exposure of swimming at Lake Tana (AOR: 7.7; 95% CI: 2.42-24.5; p = 0.001). The odds of getting S. mansoni infection among individuals swimming frequently was 10 times higher than those they didn't have an exposure of swimming at Lake Tana (AOR: 10.0; 95% CI: 3.19-36.6; p = 0.000). The odds of getting S. mansoni infection among individuals living 1 km away from the shoreline was 1.7 times higher than those who are living 1 km away from the shoreline at Lake Tana (AOR: 1.70, 95% CI:1.03-2.84; p = 0.039). The odds of getting S. mansoni infection among individuals Fishing and Farming was 7.9 times higher than those they didn't have an exposure of shing and farming at Lake Tana (AOR: 7.9, 95%CI: 2.15-29.1; p = 0.002) ( Table 3). respectively followed by 6(10.0%) and 6(10.0%) in adolescent 25-29 and 30-34years, respectively. The heavy infections to S. mansoni in males and females were 19(50.0%) and 19 (50.0%), respectively. The highest egg count was 3600 egg per gram of stool (Table 4).  Human infection rate in proximity to water point Out of 94(24.4%) who are living beyond 1-1.5 km from the shoreline of Lake Tana 15(10.6%) positive for S. mansoni of this 6(40.0%), 8(53.3%) and 1(6.7%) had frequently, sometimes and none swimming frequency respectively. S. mansoni intensity of infections in proximity to water point Out of 385/Two hundred ninety-one 75.6% participants lived in less than 1 km from the shoreline Lake Tana. From 291 participants lived in less than 1 km 81.6% of S. mansoni positive cases are found in this area while 18.4% of the infections occurred beyond 1 km distance (Table 7).   Physico-chemical parameters of the water points During snail collection the average water temperature, pH and conductivity was recorded as indicated in Table 10 below

Discussion
Schistosomiasis is the most prevalent helminthic infection in Ethiopia. Depending on various reasons, the prevalence of S. mansoni is ranging from less than 1% up to more than 90% in Ethiopia (9). In Western Dembia District, numerous studies conducted in different localities showed that the prevalence of S. mansoni were high among school-age children and preschool aged children. The present study was aimed to assess the distribution of snail and risk factors of S. mansoni in proximity to water contact points in Gorgora town.
The prevalence of intestinal schistosomiasis in the current study was 36.6% (CI: 32-41.9). This result is higher when compared to studies carried out in Western Kenya 16.3% (10), Kwale, 0.0% (11) (14). In Wondo Genet, Southern Ethiopia 42.4% (30) had high intensity of infection. This variation might be due to low level of education, poor hygienic practice and absence of control and preventive measures in the area. However, this nding was in contrast to a study carried out in Wolaita Zone, Southern Ethiopia, with an overall heavy intensity of infection of 56.4% (27). This variation might be due to infection rate and frequency of contact with cercaria contaminated water bodies.
In the current study S. mansoni was 7.7 times more likely to happen in study participants with the sometimes habit of contact to Lake Tana than those who had no habit of contact to Lake Tana. This is nding was positively associated when compared to studies carried out, in Western Kenya(10), higher than Wolaita Zone, Southern Ethiopia, (3.9 times) (27), while in Jimma Zone, South West Ethiopia children who have bathing habits in open water sources 8.8 times (19), southern Ethiopia, had shown that those who swum in the lake Hawassa are infected about 2.7 times more than those who do not (17), Sanja, northwest Ethiopia, had shown that those who swum in river about two times more than who do not (20), Jiga town, Northwest-Ethiopia, showed that habit of contact with river who were swum in river were 4.8 times (35). This variation might be due to immune status of participants, distance from the water source and frequency of contact with infected water which is a common scenario seen in almost all schistosomiasis is studies in the country and in the current study area.
S. mansoni was Ten times more likely to happen in participants with frequent habit of swimming in Lake Tana than those who had no habit of swimming. Frequent swimming had accounted for infection in a study carried out in Tigray, Ethiopia, (31), and Western Kenya (10). This is nding was in contrast to a report from Blantyre, Malawi where frequency of water contact was not signi cantly associated with Schistosoma infection (36). This variation might be due to time of water contact which is associated with shedding of infective S. mansoni cercaria.
The odds of getting S. mansoni infection among individuals Fishing and Farming was 7.9 times higher than those who do not. This is nding correlates with a report from western Kenya, where shing in Lake Victoria was associated with a higher risk of infection (10).
With regard to proximity of water, the present study has demonstrated higher likely infection odds of S. mansoni infection among individuals living 1 km away from the shoreline; 1.7 times higher than those who living 1 km away from the shoreline at Lake Tana. This nding is supported by a study carried out in western Kenya where the school nearest the lake less than 1 km had a prevalence of 80% and the mean prevalence decreased to approximately 20% at a distance of 4 km. In addition beyond the 4 km line, the prevalence decreased further to 2.8% for children living beyond 9 km from the shoreline (10). Similar nding in Kwale, Kenya, showed a signi cant low infection risk among participants who were residing far away from a river (11).
In Blantyre, Malawi, the odds of getting S. mansoni infection among individuals living less than 1 km was 5.3 times higher than 1 km living (36). This nding is not surprising as children who live closer to the lake are more likely to have contact with water on a regular basis (10).

Ethical Considerations
Ethical approval was obtained from the school of Biomedical and laboratory science (SBLS) ethical committee and a permission letter was obtained from Central Gondar Zonal Health O ce, western Dembia District Health Bureau and selected study site Kebeles leaders. When the inhabitants of nearby villages in proxy to water sources are identi ed, the purpose, objectives and bene ts of the study was explained. Consent and assent for participation was obtained from the children's parents/guardians.
Study participant who was found positive for S. mansoni and other intestinal parasites infection was linked with the health center and treated with standard dose of anti-schstosomal and anti-helminthic treatment.

Consent for publication
Not applicable. This study does not contain any individual or personal data.

Availability of data and materials
Data is available upon request.

Funding
This research work was nanced by University of Gondar, Ethiopia. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Authors' contributions AA, MA, LW, AG, AD and DA, were involved in study conception, data collection and analysis, drafting the manuscript. AA, MA, AG and DA were critically reviewed the manuscript for intellectual content. All authors have read, edited and approved the manuscript.      Spatial distribution S. mansoni infection and snail survey sites the shoreline of Lake Tana in Gorgora town, Western Dembia, Northwest Ethiopia 2020. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Cercariae that emerged from Biomphalaria pfeifferi in Lake Tana, in Gorgora town, Western Dembia, Northwest Ethiopia 2020.