Assessment of Soil Transmitted Helminth Infection (STHI) in School Children, Risk Factors, Interactions and Environmental Control in El Salvador.

Background: Soil transmitted helminth infections (STHI) are important Neglected Tropical Diseases (NTD). The three main STHI are infections with Ascaris lumbricoides, Trichuris trichiura and hookworms. STHI have a significant effect on the growth and development of children. A national survey for STHI in El Salvador by Pan American Health Organization and Ministry of Health in 2012 in school children aged 8years to 10 years. The survey collected data on age, gender, behavioral habits, and source of drinking water, type of toilet facility used and ecological zone of residence. A) We did an analyses of the data with an aim to determine the prevalence of STHI in El Salvador, assess the risk factors and risk interactions. B) We also aimed to determine the efficacy of urea as a potential additive for inactivation of Ascaris suum in solar toilets. Methods: A) Data from 1310 subjects was analysed for determination of prevalence of STHI in El Salvador. Risk factor assessment was done by chi-square test, unadjusted logistic regression and fully adjusted logistic regression. Risk factor interactions was tested on multiplicative and additive scale. B) Urea was tested for efficacy in inactivation of Ascaris suum ova in 20 solar toilets. Under conditions of controlled pH and moisture, concentration of gas ammonia, peak temperature were measured along with duration of treatment with urea to determine viability of Ascaris suum samples placed in the solar toilets. Results: I) The prevalence of Ascaris lumbricoides in 8-10 year old school children is 2.75%, Trichuris trichiura is 4.1% and hookworm is 1.83%. A) For Ascaris lumbricoides infection: Significant risk in individuals from volcanic chains and central depression compared to those from the mountains. Spring or well water when used as source of drinking water was associated with higher risk of infection when compared with piped water. Higher infection was also associated with open air defecation compared to use of septic tank or flush toilet. Use of sandals or no footwear was associated with a higher risk of infection when compared to use of closed footwear at all times. B) For Trichuris infection: Coastal plains were associated with a higher risk of infection compared to the mountains while rural status was protective against infection. Spring or well water when used as source of drinking water was associated with higher risk of infection when compared with piped water. Use of sandals or no footwear was associated with a higher risk of infection when compared to use of closed footwear at all times. C) For hookworm infection: Risk of infection was higher in individuals from urban regions. Spring or well water when used as source of drinking water was associated with higher risk of infection when compared with piped water. Use of sandals or no footwear was associated with a higher risk of infection when compared to use of closed footwear at all times. Poor handwashing was shown to be protective against infection with hookworm. Significant risk factor interactions were identified for infection with each of the three soil transmitted helminths. II) Urea as an additive at 1%w/w to feces tested in solar toilets showed an inactivation rate of nearly half the Ascaris suum ova samples. Fifty percent or higher inactivation rates were associated with ammonia gas concentrations of 109.5 ppm or higher and duration of treatment of 72 hours or higher. Conclusions: Prevalence of STHI in 8-10 year old school children for 2012 in El Salvador is low. Significant risk factors for STHI in El Salvador are eco-epidemiologic zone, source of drinking water, type of sanitation, use of shoes behavior and urban status of place of residence. Use of urea for inactivation of soil transmitted helminth ova in feces is a possible intervention for environmental control of STHI