Our results confirmed the presence of both S. mansoni and S. haematobium among adults in the Taabo HDSS in the south-central part of Côte d’Ivoire, with the former being the predominant species. In agreement with prior observations in the study area (Fürst et al., 2013; Coulibaly et al., 2018), the prevalence of S. haematobium was low, confirming a strong decrease in the prevalence of urogenital schistosomiasis, which heavily affected shool-aged children in the mid-1990s (N’Goran et al., 1997). The relatively high prevalence of S. mansoni infection in adults observed in our study suggests an increase in the level of endemicity of intestinal schistosomiasis in the Taabo HDSS. The increase of S. mansoni might be governed by social-ecological conditions. Yet, the diagnostic methods used for case identification is also worth mentioning. Indeed, in the present study, a composite measure for detection of S. mansoni infection was employed, consisting of the widely used Kato-Katz technique, coupled with a much more sensitive POC-CCA urine cassette test (Bärenbold et al., 2018).
We observed a higher prevalence of S. mansoni infection in younger adults (18-41 years) compared to their older counterparts (aged 41 years and above), which might be explained by difference in behaviour. Young adults are more mobile and more engaged in farming activities that involve regular contacts with infested water bodies, which put them at a higher risk of S. mansoni infection than the older age group. This is particularly concordant in the village of Tokohiri, where most of people regularly cross the Bandama River to have access to their farms or visit their relatives living in hamlets at the opposite side of the river. Thus, the highest prevalence observed in this village may be attributed to the frequent contacts of residents with unprotected open freshwater that may contain infected intermediate host snails.
In line with previous investigations (Muhumuza et al., 2009; Bajiro et al., 2016), the prevalence of S. mansoni infection was similar among males and females. However, some studies have reported a significant sex difference, due to social or cultural considerations that can prevent women from having access to water bodies (Sady et al., 2013; Dawaki et al., 2016). In the current study area of south-central Côte d’Ivoire, women equally participated in outdoor water-related activities. Similarly, participants with formal and non-formal education were equally exposed to S. mansoni infection, which corroborates findings from Tanzania (Mazigo et al., 2017).
Low socioeconomic status was associated with higher odds of S. mansoni mono- and co-infections, suggesting that adults belonging to the poorest group constitute an important target for control of schistosomiasis in this setting. Poor hygiene was identified as a risk factor for co-infections. Indeed, in the study area, adults exhibited risky behaviours. For example, almost half and 88.7% of the individuals surveyed reported having no toilet at home, and hence, they disposed waste in the environment, thus potentially contaminating the environment with human and animal faecal pathogens (Ambesh et al., 2016; Samuel, 2016, Ahmed et al., 2019). As shown by Hürlimann et al. (2018), including WASH as part of integrated control of schistosomiasis and other helminth and intestinal protozoa infections could be an effective approach to improve people’s health and wellbeing in the study area.
The tendency for lower odds of healthcare use among participants with co-infections is not surprising. In many areas of Africa, several infectious diseases coexist (Erismann et al., 2016; Babamale et al., 2018; Tuasha et al., 2019). Hence, it is conceivable that people who do not seek treatment could accumulate the infections. An effective control strategy is regular mass administration of anthelminthic drugs outside the health care facilities (as recommended by WHO) and health education for prompt treatment of infections when symptoms occur.
Individuals with co-infections were more likely to rate their health as poor compared to their uninfected counterparts. A possible explanation of this association is that, unlike people harbouring no or showing a single infection, people exposed to two or more infectious agents may develop more episodes of illness or acute diseases that affect permanently their wellbeing. This result is consistent with another study conducted in South Africa, where infectious diseases have been found to be associated with self-rated poor health compared with absence of infection (Made et al., 2020).
Abdominal pain is one of the common symptoms described for intestinal schistosomiasis (Barsoum et al., 2013; Shuja et al., 2018). Other common manifestations of intestinal schistosomiasis include diarrhoea, bloody stool, fever, nausea, vomiting, chills, weakness, headache, anorexia and general malaise (Da Silva et al., 2005; Verjee et al., 2019). In our study area, it is understandable that many of the symptoms used to determine the symptom score were found among S. mansoni-infected individuals. It is important to note that those symptoms are often nonspecific and may be caused by other helminth infections or malaria (Rai et al., 2012). Given this, an association between co-infections and higher pain and symptom scores was also expected. However, a lack of association was observed. Pain and symptom scores were probably not reliable measurements of S. mansoni infection status among adults. Multimorbidity particularly due to chronic diseases (such as cardiovascular diseases, arthritis and other neuromuscular diseases) may have had an influence on participants’ pain and symptom responses relatively to their health outcomes (Arokiasamy et al., 2015).
In line with previous studies from Uganda (Tukahebwa et al., 2013), there was no evidence of the effect of S. mansoni infection status on anaemia. Anaemia is known to be multifactorial. Among adults, poor nutrition, for example, may be considered as a potential contributor (Dobner et al., 2018; Chaparro et al., 2019), which was, however, not assessed in our study.
Strengths and limitations
Strengths of the present study were that a comprehensive approach was used in investigating the correlates of Schistosoma infection, including concurrent infections as well as factors related to clinical morbidity, in the same adult population. The well-characterized CoDuBu study allowed the investigation of these questions in a detailed manner. Two different methods for the diagnosis of S. mansoni (i.e. Kato-Katz thick smear and POC-CCA urine cassette test) were employed to limit bias of misclassifications. The Kato-Katz thick smear technique is the recommended method for S. mansoni diagnosis. However, it has a low sensitivity, especially among lightly infected individuals (Berhe et al., 2004). The POC-CCA has the potential to provide sensitive and accurate results (Coulibaly et al., 2011; Bärenbold et al., 2018; Okoyo et al., 2018), with the potential bottleneck of “trace” result interpretation (Coelho et al., 2016). Different definitions of infection were included, which strengthens the fidelity of our results.
The study is limited by its cross-sectional nature, which precludes causal inferences. Given the very low prevalence of Kato-Katz positive cases, associations with infection intensity could not be investigated. However, the findings remain valid and call for future longitudinal research on adults, incorporating a parallel investigation in children (preschool- and school-aged), for improved understanding of the disease dynamics across a broader age range. It is also acknowledged that the study area faces a high burden from several non-communicable diseases (NCDs), such as hypertension and diabetes (Eze et al., 2019a; Eze et al., 2019b). Hence, additional studies are warranted to investigate the concomitance of schistosomiasis and NCDs. This will help fill the data gap in areas of potential links between infectious diseases and NCDs and shed new light on their respective determinants.