Determinants of Schistosoma mansoni transmission in hotspots at the late stage of elimination in Egypt
Authors: Ghazy Ramy Mohamed , Ellakany Walid Ismail , Badr Mai M. , Taktak Nehad E. M. , Elhadad Heba , Abdo Sarah M. , Hagag Ayat , Hussein Abdel Rahman , Tahoun Mohamed Mostafa 
Published: September 2022

Abstract
Background
In certain settings, the prevalence and severity of schistosoma infection do not lessen despite repeated rounds of preventative chemotherapy; these areas are known as hotspots. This study aimed to investigate the role of human practices, besides environmental and malacological factors, in the maintenance of the Schistosoma mansoni infection transmission chain in hotspot areas in Egypt.

Methods
This cross-sectional study was conducted between July and November 2019 in Kafr El-Sheikh Governorate, Egypt. A pre-designed structured interviewing questionnaire was used to collect humanitarian data. Stool samples were collected from children aged 6–15 years on three successive days and examined using the Kato-Katz technique. Simultaneously, water and snail samples were taken from watercourses surrounding houses. Snails were identified based on their shell morphology and structure and tested for cercaria shedding. Water samples were analyzed for their physicochemical and biological characteristics.

Results
A total of 2259 fecal samples (1113 in summer and 1146 in fall) were collected from 861 children. About 46.9% of the participants were males, and 31.8% were aged 6–10 years. The prevalence of S. mansoni infection was higher during the summer than during the fall (19.1% vs 7.2%, respectively, P < 0.01). The intensity of infection (light, moderate, and heavy) during summer versus fall was (93.55 vs 89.38%, 6.45 vs 8.85%, and 0.00% vs 1.77%), respectively (P < 0.05). A higher prevalence of human infection was observed among males than females [OR = 1.63, 95% confidence interval (CI):1.10–2.40, P = 0.015], children aged 11–15 years than among their counterparts aged 6–10 years (OR = 2.96, 95% CI: 1.72–5.06, P < 0.001), and mothers with a low level of education (OR = 3.33, 95% CI: 1.70–6.52, P < 0.001). The main identified risk factors were contacting the main body of water-canal for washing clothes (OR = 1.81, 95% CI: 1.12–2.49, P = 0.015), land irrigation (OR = 2.56, 95% CI: 1.32–4.96, P = 0.004), water collection (OR = 2.94, 95% CI: 1.82–4.73, P < 0.001), bathing (OR = 2.34, 95% CI: 1.21–4.31, P = 0.009), and garbage disposal (OR = 2.38, 95% CI:1.38–4.12, P < 0.001). The count of Biomphalaria alexandrina was distinct between seasons (P < 0.01) in consistent with statistically significant differences in water temperature, salinity, turbidity, the total concentration of coliforms, depth, velocity, and water level (P < 0.01). The presence of grasses and duckweeds was significantly associated with snail infection (P = 0.00 l). Significant effects of water depth, pH, temperature, and total dissolved solids on snail count were also observed (P < 0.05).

Conclusions
The persistence of the infection is due to adoption of risky behaviors and environmental factors that enhance snail survival and infection. Schistosomiasis elimination in hotspots requires an integrated control approach that combines preventive chemotherapy with other complementary measures.