Intestinal schistosomiasis is an ancient endemic disease in Egypt targeted for elimination. Epidemiological studies are indicated for elucidating the dynamics of transmission in endemic areas because each site has different biological, ecological, social, and economic characteristics that may affect this process. Transmission of schistosomiasis is the result not only of the interplay between humans, snails, and parasites but also of complex interactions between all these factors [31]. The WHO Expert Committee on Epidemiology and Control of schistosomiasis in 1978 asserted that “comprehensive understanding of environmental, demographic, social, human behavioral and economic factors in schistosomiasis is essential for designing control programs that are successful on the long run [32]. The current schistosomiasis elimination strategy is mainly based on preventive chemotherapy with periodic administration of the anti-schistosomal drug; PZQ to school-aged children and other high-risk groups [33]. PZQ reduces morbidity and might have an impact on transmission, but rarely eliminates the infection [34, 35].
Various factors are responsible for the persistent transmission of schistosomiasis in KES Governorate in the three mentioned villages (Village I, II, and III).This study aimed at identifying different ecological factors responsible for the sustained transmission of diseases like seasonal variation and its reflection on the environmental factors affecting the prevalence of human and snail infection. Besides, sociodemographic and behavioral risk factors are playing an important role.
Seasonal variation and schistosomiasis prevalence in snails and humans:
In this study, the overall prevalence of S. mansoni infection was (13.1%); 7.2% (31/432) during summer and 19.2% (82/429) during the fall. The studied villages were considered of low and moderate endemicity during summer and fall, respectively [25]. The current prevalence of schistosomiasis is relatively lower than that was previously reported two years earlier in Arab El-Mahder village (30%), KES. The prevalence of infection was nearly the same during summer and fall in the Village II (26.3 and 27.1%, respectively), while it increased in Village I from 0.7% during summer to 18.2% during fall. This low prevalence during summer can be explained by chemotherapy campaigns conducted four months before the survey or difference in the sampling technique between the two seasons. It is important to note that the number of collected B. alexandrina snails per spot during summer was higher than during fall with an equal ratio of infected watercourses in each season, however, the prevalence of human infection was higher during fall. This contradiction between human and snail infection is explained by human activity; Residents of Village I preferred to swim in other villages’ watercourses where their farms are located. Also, the watercourses of Village II were severely polluted, and of a high-water level which resulted in a small snail number despite the high prevalence of human infection. The snail prevalence is not the only predictor of human infection, furthermore, the point prevalence of S. mansoni infection should not be used to assess the yearly prevalence of infection.
Intensity of infection
Indeed, the effectiveness of MDA programs for S. mansoni is mainly monitored by measuring changes in the prevalence of infection, drug treatment coverage, and also based on the prevalence of heavy infection (≥400 epg) [36]. Regarding the intensity of infection in this study, the majority of children 98.2% (111/113) were of either light or moderate infection, and the prevalence of infection was either between 10% - 50% in other villages. This finding is due to the intensive MDA campaign adopted by the MOHP.
Socio-demographic factors
Age: In endemic areas, the infection is usually acquired during childhood [37]. The prevalence and intensity of infection rise with age and peaks at approximately 15 to 20 years. In older adults, the prevalence of infection does not change significantly, but the intensity (parasite burden) decreases dramatically [38]. In this study, the mean age of infected children was significantly higher than the uninfected. Additionally, the age category of 11-15 years harbored infection more than the age category of 6-10 years. Indeed, children aged 11–15 years can become more vulnerable for schistosomiasis during recreational activities, i.e., swimming and playing in the water, or while fetching water for household use, or agriculture activities On the same line, the finding that different age groups had different susceptibility to infection was reported in different studies. [39, 40]. On the other hand, no difference in the prevalence of S. mansoni infection was observed among the three investigated age groups in a study conducted in Côte d’Ivoire [41].
Sex: There are no global figures on the distribution of schistosomiasis by sex. From the scattered surveys available, it seems that men and women are infected in equal numbers, but that women are generally more intensely affected by the disease than men [42]. Nonetheless, in the current study, the prevalence of infection among girls was lower than boys, with no difference in intensity of infection as they are more often to come in contact with water than females, this is explained by sex-specific water-contact activities. Actually, during swimming (the predominant method of water contact by boys), boys exposed their whole bodies in the water, while girls usually only exposed their legs and hands into the water, mainly concerning carrying out domestic activities (e.g. fetching water, washing clothes and dishes). In line with this finding, another study conducted in Senegal stated that higher prevalence was reported among males [43]. Interestingly, similar prevalence rates for boys and girls were reported in the Côte d’Ivoire study, however, it may be due to a considerably higher number of boys than girls (727 vs. 460) [41].
Education: In this research maternal illiteracy was strongly associated with a higher prevalence of infection, 74.7% of infected children had illiterate mothers. A study from Santo Antonio de Jesus, Bahia State, Brazil, indicated that an increase in the education level of the head of the household was strongly associated with a decrease in the prevalence and intensity of infection in the household [44]. Nonetheless, the paternal level of education did not significantly affect the prevalence of infection. A similar conclusion was reached by [41], based on a study conducted in Côte d’Ivoire parental level of education was significantly associated with acquiring infection, but the maternal odds ratio was more than three times the odds of paternal education. The higher risk due to maternal illiteracy may be explained by the longer time spent by mothers with their children and the great impact they had on their children.
Social class or status: Some researchers argue that it is more accurate to understand the social production of schistosomiasis concerning social class or status rather than to urbanization. They often share conditions such as lower levels of education and unemployment as well as lack of sanitation, housing, transport, and access to health facilities. Their point is not that lack of education or poor housing will automatically lead to infection, but that if infective snails are present in local waterways, it will most likely be members of the poorer households who are infected. In this study, although a higher prevalence was reported among low socioeconomic class (13.98%) versus medium socioeconomic class (10.0%) this difference was not significant. The high social class effect could not be assessed as nearly all the children were of the low or intermediate class. Over time, extensive literature has been developed on the impact of poverty on prevalence, incidence, and cost impact of schistosomiasis. It is considered a clear example of disease of a poverty [45].
In this study, the proximity of the houses to watercourses was not significantly associated with S. mansoni infection. On contrary, a detailed epidemiological study was carried out in São Lourenço da Mata, Brazil, it showed that leisure water contact, particularly swimming, was the only type of water contact significantly associated with schistosomiasis among people between 10 and 25 years of age and that a decrease in water contact was associated with better socioeconomic conditions [46]. This was explained above, children may prefer to swim away from their houses.
Human water-contact activities
Water contact is mandatory to acquire the infection, however, 40.7% of infected children reported no contact with watercourses. This issue needs to be further discussed as if they did not encounter the water stream what the supposed route of infection would be. Direct observations were made with an emphasis on the behavior of community members to understand how they might become infected with schistosomiasis. Two explanations for this issue; firstly, these children may not recognize the risk of some adopted behaviors like getting rid of garbage by dumping into the watercourses. This may expose their legs or body to the water stream. Secondly, the stigma associated with using watercourses or being infected with S.mansoni, which could be a possible cause that made the children deny contacting watercourses. Site of contact is supposed to be associated with acquiring infection, 19.5% of the infected children were contacting the center of the canal, while 0.7% were contacting the periphery.
Health facilities within the villages
The limited accessibility to diagnostic, chemotherapeutic, and preventive services significantly constrains the health-seeking behavior of people infected with schistosomiasis and other infectious diseases, especially in developing countries. Besides, a lack of information, the costs of travel and health-services fees, geographic distance, social factors, and frequently unavailable services are also major barriers people face in accessing health services [47].
The major finding of this study was the small proportion (12%) of the population that reported visiting the local health care unit within the village although it was accessible for the population living within its catchment area. This issue was investigated while providing health education sessions. The stakeholders and fathers of screened children stated that doctors are not available all times and there is a severe lack of resources within the unit. Others stated that drugs are given to relatives and acquaintances. These issues make the local residence seek medical advice out of the local health care unit.
Environmental factors
Our results cast a light on the significant difference in the number of snails collected including B. alexandrina in-between different months, this difference was also found in temperature, salinity, turbidity, TC, and EC measured each month. Another promising finding was that there was no significant difference between different months in pH, TDS, and vegetation percentage. Furthermore, the current research provided evidence that there was a statistically significant difference between water depth, velocity, and level measured within each month. An important finding was that type of vegetation had a significant impact on population density; duckweeds and grasses had a significant association with the presence of snails which may be due to their importance as a food source, additionally, snails may be attached to various parts of the plants to escape the direct effect of sunlight, feed, or get access to oxygen [48]. It remains unclear to which degree snail population is attributed to these studied factors, as these environmental changes measured across months were not always going in the same direction with a total number of collected snails, this finding should shed light on other environmental factors that can be incriminated with snail survival. On equal terms, [49] reported that no particular environmental parameter is a major determinant of host snail distribution, environmental parameters can explain about 41 to 43% of the change in snail density.