Introduction

Schistosomiasis is one of the most prevalent neglected tropical diseases (NTDs) worldwide, posing a significant public health burden in many low and middle-income countries, mainly in sub-Saharan Africa [1,2]. The disease is caused by blood flukes of the genus Schistosoma, with Schistosoma mansoni being the predominant species responsible for intestinal schistosomiasis in Africa [3]. Transmission occurs through skin contact with freshwater contaminated by cercariae released from infected Biomphalaria snail intermediate hosts, making the disease closely associated to environmental, behavioral, and socio-economic factors [4,5].

Globally, around 230 million people are estimated to be infected with schistosomes and 700 million people living in areas at risk of infection. More than 90% of the global schistosomiasis burden is in Sub-Saharan Africa countries [2,6]. Ethiopia is one of the countries where S. mansoni infection remains a major public health problem, particularly in areas where people use rivers, streams, irrigation schemes, and dams which facilitates snail breeding and human water contact [7,8]. As reported in different National and regional studies, there are varying prevalence levels, reflecting ecological diversity and differences in local transmission dynamics [9]. SAC and young adults are often affected more, than others. This is because of increased exposure during swimming, bathing, washing clothes, and agricultural activities [10,11]. However, evidence suggests that populations other than SAC, particularly individuals engaged in river-related activities such as irrigation, fishing, and washing, are also at increased risk of schistosomiasis infection [1].

Chronic S. mansoni infection may result in significant morbidity, such as abdominal pain, diarrhea, hepatosplenomegaly, periportal fibrosis, anemia, malnutrition, growth retardation, and impaired cognitive development [4,12]. All these complications can affect educational achievement, work productivity, and community development, indicating the importance of effective control measures against S mansoni infection [13,14].

Mass drug administration with praziquantel targeting primarily to reduces morbidity by killing adult worms and lowering egg output in human hosts is the cornerstone of schistosomiasis control. It is assumed that reducing egg excretion may also partially reduce environmental contamination and transmission, however, the effect is usually incomplete in some highly endemic areas for S. mansoni infection particularly, where adults and other at-risk groups are often untreated [1,15]. In some areas reinfection occurs most of the time, and transmission persist, especially, where human-water contact persists and snail intermediate hosts remain abundant [16]. Consequently, the community level epidemiological profile of S. mansoni infection including prevalence, and population groups at risk may remain unchanged or change only slowly over time even in areas under MDA programs.

Recently, the World Health Organization (WHO) NTD Roadmap 2021–2030 has shifted the global strategic focus for schistosomiasis from morbidity control toward the interruption of transmission and eventual elimination as a public health problem in endemic countries [17]. Achieving these targets requires robust community-level epidemiological evidence to identify residual transmission foci, understand infection patterns among different population groups, and evaluate whether current control strategies are sufficient to interrupt transmission. In settings where preventive chemotherapy is primarily targeted to SAC, monitoring infection among the wider community is particularly important for identifying potential reservoirs that may sustain ongoing transmission [1,17].

Despite a number of earlier studies conducted in Ethiopia, regular and current community level epidemiological data are crucial, particularly in regions where MDA programs are still in place. These kinds of data are essential for tracking the effectiveness of programs, locating hotspots for residual transmission, assessing the results of interventions, and directing complementary strategies such as expanding MDA to other at-risk groups, snail control and improvements in water, sanitation, and hygiene (WASH) [18,19]. The findings of the present study therefore provide important evidence to support adaptive, evidence-based decision-making and sustain progress toward the control and eventual elimination of schistosomiasis.

In Southwest Ethiopia, particularly in Mizan-Aman and the Bench-Sheko Zone, local community level epidemiological data on S. mansoni infection with regard to MDA program remains limited. The presence of freshwater bodies, water contact activities, and environmental factors in the area point to the possibility of continuous transmission, which supports the necessity for an updated epidemiological assessment after MDA program in the community. Thus, the purpose of this study was to determine the community-level prevalence and determinants of S. mansoni infection in a setting where MDA targets only SAC in river-crossing Kebeles of mizan Aman, Bench-Sheko Zone, Southwest Ethiopia. It is anticipated that the results would be valuable baseline information for local public health authorities and contribute to improved, evidence-based control strategies.

Materials and methods

Results

Socio-demographic characteristics of study participants

In the present study a total of 349 participants were enrolled in the study, yielding a response rate of 96%. Males accounted for 52.1%, and females for 47.9% of the study participants, with a median age of 20 years. Nearly all participants (97.4%) had lived in the study area for at least one year. Almost half were students, and Protestant Christianity was the predominant religion (Table 1).

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Table 1. Socio-demographic characteristics of study participants in Mizan Aman Town, Bench Sheko Zone, Southwest Ethiopia (n = 349).

https://doi.org/10.1371/journal.pntd.0014506.t001

Prevalence of S. mansoni and other intestinal parasites

Out of 349 screened study participants the overall prevalence of S. mansoni infection was 81 (23.2%); 95% CI: (18.9-28.0). Mono-infection with S. mansoni accounted for 10.9%, while 12.3% of participants had S. mansoni co-infection with other helminths. Overall, 50.7% of participants were infected with at least one intestinal parasite (Table 2).

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Table 2. Prevalence of S. mansoni, soil-transmitted helminths, and other intestinal parasite infections among study participants in Mizan Aman, Southwest Ethiopia (n = 349).

https://doi.org/10.1371/journal.pntd.0014506.t002

Infection Intensity of S. mansoni Infection Among Study Participants

Among the 349 study participants examined, 81 (23.2%) were infected with S. mansoni. Regarding infection intensity, 64 (18.3%) participants had light-intensity infection, 13 (3.7%) had moderate-intensity infection, and 4 (1.2%) had heavy-intensity infection. Light-intensity infection was the predominant category among the study participants (Table 3).

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Table 3. Prevalence of S. mansoni infection intensity among study participants in Mizan Aman, Southwest Ethiopia (n = 349).

https://doi.org/10.1371/journal.pntd.0014506.t003

Factors associated with S mansoni infection

In this study variables with p < 0.25 in bivariate analysis were included in the multivariable logistic regression model (Table 4a). In the final model, use of river water, bathing in rivers, swimming habits, and farmland proximity to rivers remained statistically associated with S. mansoni infection. Participants who used river water for domestic purposes were nearly six times more likely to be infected compared to those were not used. Individuals who reported bathing in rivers had approximately three times higher odds of infection. Similarly, participants with a habit of swimming in rivers were more than two and a half times more likely to be infected. Furthermore, those who had farmland located near rivers were 2.28 times more likely to be infected compared to those without such exposure. Other factors, including sex, age, and river crossing, did not show statistically significant associations with S. mansoni infection in the multivariable analysis (Table 4b).

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Table 4. (a) Bivariate logistic regression analysis of factors associated with S. mansoni infection in Mizan Aman, Southwest Ethiopia (n = 349). (b) Multivariable logistic regression analysis of factors associated with S. mansoni infection in Mizan Aman, Southwest Ethiopia (n = 349).

https://doi.org/10.1371/journal.pntd.0014506.t004

Discussion

Schistosomiasis is one of the NTD transmitted by a freshwater snail host, that remains a public health problem in many parts of sub-Saharan Africa countries, including Ethiopia [23,24]. Schistosomiasis control strategies in Ethiopia primarily rely on periodic MDA with praziquantel, which is mainly delivered through school-based platforms and targets SAC as the primary risk group for morbidity [24]. Although this approach has been effective in reducing infection intensity and morbidity among treated children, it does not fully interrupt transmission when other exposed population groups such as preschool children, out-of-school adolescents, and adults at risk remain untreated and continue to contaminate freshwater bodies with parasite eggs [1,25]. As a result, persistent community-level transmission has been reported in several endemic settings despite repeated rounds of school-based MDA [19,26,27]. In this context, the present community-based study was conducted to generate up-to-date epidemiological evidence on the prevalence and determinants of S. mansoni infection in Mizan-Aman Town.

This community-based study revealed that S. mansoni infection remains a moderate public health problem in Mizan-Aman Town community, with an overall prevalence of 23.2%. As per WHO classification, areas with a prevalence range between 10% and 50% are considered moderately endemic, showing sustained transmission in the community despite ongoing control efforts [25]. In Ethiopia, schistosomiasis control depends majorly on school-based MDA focusing only SAC, whereas adults and other community members are not routinely included, with the exception of small, focused campaigns [24]. Although this strategy can reduce morbidity among SAC, its limited coverage of other at-risk populations may reduce its effectiveness in interrupting transmission and achieving elimination at the community level, as suggested by the findings of the present study.

Moreover, the moderate prevalence of S. mansoni observed in the general community suggests that transmission may still be ongoing in the study area despite the implementation of MDA targeting SAC. This may be due to continued human-water contact, the presence of suitable snail intermediate hosts, and the exclusion of other at-risk groups, including preschool children, out-of-school adolescents, and adults, from routine MDA programs. These untreated populations may serve as reservoirs of infection and contribute to continued transmission within the community. Similar observations have been reported from the Abbey and Didessa Valleys of western Ethiopia and the Lake Chamo area in the southern Rift Valley, where persistent S. mansoni transmission was documented despite ongoing MDA interventions [19,28,29].

Furthermore, in the current study, the high prevalence of the overall intestinal parasitic infections (50.7%) and the substantial proportion of S. mansoni co-infections with soil-transmitted helminths reflect ongoing environmental contamination and overlapping transmission routes. Similar patterns have been reported from several endemic areas of Ethiopian and East African settings, where inadequate sanitation, unsafe water sources, and poor hygiene practices facilitate polyparasitism [20–33]. Moreover, Polyparasitic infections are known to exacerbate morbidity, including anemia, undernutrition, and reduced work productivity, particularly among untreated adult populations [34].

Although achieving interruption of S. mansoni transmission still requires substantial effort, the present study found that infection was predominantly of light intensity, affecting 18.3% of the study participants, whereas moderate and heavy-intensity infections were less common, accounting for 3.7% and 1.2% of participants, respectively. The predominance of light-intensity infections may indicate a reduced parasite burden in the community, possibly reflecting the impact of ongoing schistosomiasis control interventions, including MDA, health education, and increased community awareness. However, the proportion of heavy-intensity infections (1.2%) remains slightly above the WHO threshold of <1% heavy-intensity infection required for the elimination of schistosomiasis as a public health problem. Therefore, continued and strengthened control efforts are necessary to further reduce infection intensity and move toward achieving the elimination target. [17].

In this study, factors such as use of river water for domestic purposes, bathing and swimming in rivers, and having farmland located near rivers were significantly associated with S. mansoni infection. These findings are supported with the established transmission dynamics of intestinal schistosomiasis, which depends on repeated human contact with cercariae-infested freshwater [4]. Adults are particularly vulnerable due to occupational and domestic activities such as farming, washing, irrigation, and fishing, which involve prolonged water contact [35]. Because these high-risk adult groups are not systematically covered by school-based MDA, they may serve as persistent reservoirs of infection, sustaining transmission within the community [1].

Although praziquantel-based MDA is effective in reducing worm burden and egg output among treated individuals, its impact on transmission interruption is limited when treatment coverage does not include the entire at-risk population [1,17]. Reduced egg excretion following treatment can lower environmental contamination temporarily; however, untreated individuals particularly, out-of-school adolescents and adults continue to discharge parasite eggs into freshwater bodies, maintaining the life cycle of S. mansoni and facilitating reinfection of treated children [17,36]. Several field studies have shown that morbidity control can be achieved with school-based MDA, but transmission elimination generally requires community-wide treatment or adding other interventions [37,38].

The lack of significant association between S. mansoni infection and age group in the multivariable analysis of present study suggests widespread exposure across demographic groups. Similar findings have been reported from endemic areas where adults remain untreated and experience frequent water contact, resulting in infection patterns that extend beyond SAC [39,40]. This underscores the limitation of relying only on school-based prevalence data to assess transmission intensity and program impact [41].

These findings highlight the critical importance of frequent and up-to-date epidemiological surveillance, particularly in areas implementing school-based MDA. WHO and other experts have emphasized that routine community-level surveys are essential to monitor residual transmission, assess reinfection patterns, and evaluate the long-term effectiveness of control programs [1,17]. Without updated epidemiological data that include adults and other high-risk groups, control programs may underestimate ongoing transmission and overestimate progress toward elimination goals [19].

Acknowledgments

First, we would like to thank Jimma University, Institute of Health. Thanks to all data collectors and participants for their willingness during data collection.

Ethical approval and consent to participate: Ethical clearance was obtained from the Institutional Review Board of the Jimma University Institute of Health (Approval Number/ID: Rf/IHRPGn/187). Legal permission was obtained from the Town health office administration as well as kebele administration. Written informed consent was also obtained from the study participants. All methods were performed following the relevant guidelines and regulations.

Availability of data and materials: The data supporting our findings are incorporated within the manuscript and. Additional data supporting the study are also available from the corresponding author upon request.