Methods and materials
Study design and area
A school-based cross-sectional study was conducted at Sebatamit primary school from April to May, 2018. The school is located 7 km from Bahir Dar city which is the capital city of Amhara Regional State. The altitude of the area is 1820 m above sea level. The annual temperature of the area is 11–29 °C. The residents at the study area use Abbay and Andassa Rivers for recreational, irrigation, drinking and cooking purposes.
Sample size determination and sampling technique
Sample size was determined using 50% prevalence since there was no study conducted regarding this topic in the area, 95% CI, 5% margin of error with 10% non-response rate; hence using single proportion formula, 422 students were included in the study. The total number of children from grade 1–8 in the school was 1734. There were four sections in each grade. Then, proportional allocation formula was used to determine to the number of children sampled from each grade. Hence, the numbers of students sampled from grade 1–8, respectively were: 65, 62, 58, 55, 59, 43, 46 and 34 having a total of 422 children. Then, by using class rosters as a sampling frame, systematic sampling technique was used to select the sections from each grade and the school children from the selected sections.
Stool sample collection for parasitological examination
Each of the school children was informed to provide total of 2 g of stool sample. Then, the stool samples were preserved with 10% formalin and transported to Bahir Dar University Microbiology and Parasitology teaching Laboratory for Kato-Katz. Moreover, standardized questionnaires were used to collect data on socio-demographic characteristics of the study participants and associated risk factors of S. mansoni infection.
Parasitological stool examination by Kato Katz technique
For Kato-Katz technique, two Kato slides were prepared for every school children’s stool sample using fixed quantity of sieved 41.7 mg of stool on a punched template . They were then mounted on slides and covered with malachite green impregnated cellophane. The slides were observed within 1 h under the microscope at a magnification of 10× objective. The mean total number of eggs was expressed as eggs per gram (EPG) of stool.
Ten percent of the examined Kato-Katz slides were randomly selected and re-examined at the end by qualified laboratory technologist who were blind for the first result. Finally, all results were documented on log book by its unique ID number and copied to the questionnaire by principal investigators.
Data were analyzed using SPSS version 23. The associated risk factors for S. mansoni infection were first analyzed by bivariate logistic regression. Then, to control the possible confounding factors, variables with p value < 0.2 were adjusted by multivariate logistic by stepwise variable selection. Finally, variables with p value < 0.05 were considered as statistically significant.
Ethical clearance was obtained from Bahir Dar University College of Medicine and Health Sciences institutional review board (IRB) and approved with protocol number 070/18-04 on 04 April 2018 prior to start the study. Permission letter was obtained from Bahir Dar city education department and Sebatamit primary school director office. Informed written consent was obtained from the children’s parents and assent was obtained from the children. Those school children who were positive for S. mansoni were treated accordingly.
Sociodemographic characteristics of the study participants
Out of 422 school children, 223 (52.8%) and 199 (47.2%) were males and were females, respectively. The age of the school children ranged from 7 to 18 years with the mean age of 11.5 year. Two hundred sixty out of 422 (53.6%), 150/422 (35.5%) and 46/422 (10.9%) of the school children were from 11 to 14, 7 to 10 and 15 to 18 years age group, respectively. Most of the school children were from grades 1–4 accounted 240/422 (56.9%) followed by 182/422 (43.1%) from grades 5–8.
The prevalence of S. mansoni infection
One hundred five children were found to be positive to S. mansoni infection giving an overall prevalence of 24.9%. The males were found to be more infected than the females with 66 (15.6%) of the males being infected. The age group with the highest prevalence value for S. mansoni infection was between 11 and 14 years with 66 (15.6%) of them found to be infected while the least prevalence of 1.4% was among 6 children in the age group 15–18 years as shown in Table 1.
Intensity of S. mansoni infection
Out of 105 school children, 75 (71.4%), 25 (23.8%) and 5 (4.8%) had light, moderate and heavy infections, respectively according to WHO classification . Thirty (76.6%), 7 (17.9%) and 2 (5.2%) of female students had light, moderate and heavy infections, respectively where as 45 (68.2%), 18 (27.3%) and 3 (4.5%) of male students had light, moderate and heavy infections, respectively. The age group with the highest light infections was between 11 and 14 years with 48 (72.7%) while the least was between 15 and 18 years with 4 (66.8%). Both 7–10 and 11–14 years had similar moderate infections with 8 (24.2%) and 16 (24.2%), respectively while the highest heavy infections, 1 (16.6%) was found in 15–18 years.
Analysis of risk factors associated with S. mansoni infection
Among the potential associated risk factors of S. mansoni infection, swimming habit (p = 0.001), irrigational activities (p = 0.005) and washing clothes in the river (p = 0.045) were statistically significant with S. mansoni infection in bivariate logistic regression analysis. However, factors like crossing the river on bare foot and open defecation were not statistically significant (p > 0.05). From sociodemographic factors, sex of the children (p = 0.018) and age of the children (p = 0.046) were statistically associated in bivariate logistic regression analysis. But academic level of the students was not statistically significant (p = 0.23) as shown in Table 2.
All bivariate results that had p-value < 0.2 were subjected to multivariate logistic regression model to control possible confounding factors as shown in Table 2. After adjustment to multivariate logistic regression model, sex of the school children (p = 0.659) was not statistically significant. The school children from 11 to 14 years were 3.2 more likely to be infected with S. mansoni than those who were from 15 to 18 years (AOR = 3.2, 95% CI 1.274–8.151, p = 0.013). The odds of being infected by S. mansoni among school children who swam was 2.8 more likely as compared to those who did not (AOR = 2.8, 95% CI 1.702–4.699, p = 0.001). The school children who participated in irrigational activities were 1.675 more likely to be infected with S. mansoni than those who did not (AOR = 1.675, 95% CI 1.050–2.671, p = 0.030). The school children who washed their clothes in the river were 1.711 more likely to be infected with S. mansoni than those who didn’t (AOR = 0.1.711, 95% CI 1.028–2.851, p = 0.039) as shown in Table 3.
In this study, the overall prevalence of S. mansoni infection was 24.9%. This prevalence was slightly in with other studies such as 24% in Jimma , 23.9% in Mekelle , 26.5% in Democratic Republic of Congo . However, the prevalence of this study found to be lower compared to other findings in different parts of Ethiopia; including 73.7% in Bushulo village of Southern Ethiopia , 31% along lake Hawassa of Southern Ethiopia , 58.6% in Wolaita Zone , 67.6% in Horo Guduru Wollega , 73.9% and 42.4% in Tigray [23, 24], 37% in Zarima town of Gondar , 89.9% in Sanja General elementary school of Gondar , 82.8% in Sanja and Ewukat Amba primary school Gondar of , 56.6% in Tach Armachiho district of Gonde , 60.5% in Kenya , 84.01% in Tanzania .
The variations in the prevalence in this study might be due to some factors such as: the presence of fast running rivers (Abay and Andassa) in the area leading to low availability of vector snails that are mainly found to prefer stagnant or slow-moving water bodies ; the sociodemographic and other factors (immune status); the distance of the study area to the source of water may be a major cause of variation in prevalence; sample size..
On the other hand, the prevalence of this study was higher than 119 (20.6%) in Gorgora town , 14.3% in Ethiopian Orthodox church students around Lake Tana  and 2.8%) in Bahir Dar Shimbit elementary school . The reason for these variations might be due to poor sanitation; in our study area greater than 50% of the study participants had no latrine in their home.
In our study, most of infections were light infections. This is in line with the study done in Jimma zone  and Mekelle city . However, the highest moderate infections were observed in Sanja town [8, 9] and in Gorgora town  whereas in Wolayita zone  the highest heavy infections were observed. The difference in infection intensity could have been due to the frequency of exposure to contaminated water, the burden of the adult worms in hosts and immune status of the study participants.
In this study regarding the associated factors to S. mansoni infection, sex as an associated factor to S. mansoni infection was not retained on the multivariate analysis though it was significant in bivariate analysis which was similar to the findings from different parts of Ethiopia [8, 9 and 22]. Contrasting findings on S. mansoni infection among males and females were identified. Higher prevalence of S. mansoni infection among males was reported in Jimma Zone  and Wolayita Zone , whereas the study conducted in Gorgora and Mekelle [12, 17] reported the opposite findings (higher prevalence in females than males). These differences might be due to the difference in the exposure status of both sexes to cercarial contaminated water source.
The school children who were from 11 to 14 years were 3.2 more likely to be infected with S. mansoni infection than those in 15–18 years. This finding is in line with the study done in Gorgora , Jimma zone , Mekelle  and Kenya . This could be due to higher rate of water contact activities for recreational purposes and for bathing by the school children within 11–14 years and the concomitant immunity to re-infection in school children aged 15–18 years. Swimming habit, participation in irrigational activities and washing clothes in the river were factors associated with S. mansoni infection.
The overall prevalence of S. mansoni infection in the present study was 24.9%. Majority of the infections were light infection with mean intensity of 106.6 EPG. Age; swimming habit, irrigational activities, washing clothes in the river were the factors associated with S. mansoni infection. However, deworming program and community awareness should be strengthened to control S. mansoni infection.