Influence of Exposure History on the Immunology and Development of Resistance to Human Schistosomiasis Mansoni

Schistosomiasis is a parasitic blood fluke infection of 200 million people worldwide. We have shown that humans can acquire immunity to reinfection after repeated exposures and cures with the drug praziquantel. The increase in resistance to reinfection was associated with an increase in schistosome-specific IgE. The ability to develop resistance and the rate at which resistance was acquired varied greatly in two cohorts of men within close geographic proximity and with similar occupational exposures to schistosomes. These differences are likely attributable to differences in history of exposure to Schistosoma mansoni infection and immunologic status at baseline, with those acquiring immunity faster having lifelong S. mansoni exposure and immunologic evidence of chronic S. mansoni infection. As many conflicting results have been reported in the literature regarding immunologic parameters associated with the development of resistance to schistosome infection, exposure history and prior immune status should be considered in the design of future immuno-epidemiologic studies

Cluster randomized trial comparing school-based mass drug administration schedules in areas of western Kenya with moderate initial prevalence of <i>Schistosoma mansoni</i> infections

<div><p>Background</p><p>Mass drug administration (MDA) using praziquantel is the WHO-recommended approach for control of schistosomiasis. However, few studies have compared the impact of different schedules of MDA on the resultant infection levels. We wished to evaluate whether annual MDA was more effective than less frequent treatments for reducing community-level prevalence and intensity of <i>Schistosoma mansoni</i> infections.</p><p>Methods</p><p>We performed a cluster randomized trial (ISRCTN 14849830) of 3 different MDA frequencies over a 5 year period in 75 villages with moderate (10%-24%) initial prevalence of <i>S</i>. <i>mansoni</i> in school children in western Kenya. Praziquantel was distributed by school teachers to students either annually, the first 2 years, or every other year over a 4 year period. Prevalence and intensity of infection were measured by stool examination in 9–12 year old students using the Kato-Katz method at baseline, each treatment year, and for the final evaluation at year 5. <i>S</i>. <i>mansoni</i> prevalence and intensity were also measured in first year students at baseline and year 5.</p><p>Results</p><p>Twenty-five schools were randomly assigned to each arm. <i>S</i>. <i>mansoni</i> prevalence and infection intensity in 9–12 year old students significantly decreased within each arm from baseline to year 5 but there were no differences between arms. There were no differences in infection levels in first year students either within or between arms.</p><p>Conclusions</p><p>Strategies employing 2 or 4 rounds of MDA had a similar impact in schools with moderate initial prevalence, suggesting that schistosomiasis control can be sustained by school-based MDA, even if provided only every other year.</p></div

Prevalence and intensity of <i>S</i>. mansoni infection for first year students in each arm at baseline (Year 1) and final evaluation (Year 5).

<p>Prevalence and intensity of <i>S</i>. mansoni infection for first year students in each arm at baseline (Year 1) and final evaluation (Year 5).</p

Location of schools included in study by arm, Western Kenya (n = 25 per arm).

<p>Map was created with ArcGIS (version 10.3, ESRI, Inc., Redlands, CA) using layers from ESRI data and Maps 10.1.</p

Comparisons of change in 9–12 year old prevalence and intensity of <i>S</i>. <i>mansoni</i> infection between year 1 and year 5 for each arm, 25 villages per arm.

<p>Adjustment of PRs and AMRs are for age, sex, and village sample size.</p

Overall mean prevalence and mean prevalence by infection intensity category by arm and by year.

<p>Total bar height represents mean <i>S</i>. <i>mansoni</i> infection prevalence in each year. Light grey represents the prevalence of individuals with low intensity infections (1–99 epg); darker grey represents prevalence of moderate intensity infections (100–399 epg); and black represents prevalence of individuals with high intensity infections (≥ 400 epg).</p

Prevalence and intensity of infection in 9–12 year olds in each arm at baseline (Year 1) and final evaluation (Year 5).

<p>Prevalence and intensity of infection in 9–12 year olds in each arm at baseline (Year 1) and final evaluation (Year 5).</p

Comparisons of 9–12 year old prevalence and intensity of <i>S</i>. <i>mansoni</i> infection between arms at year 5.

<p>Adjustment of PRs and AMRs are for age, sex, and village sample size.</p

Prevalence and intensity of <i>S</i>. mansoni infection for first year students in each arm at baseline (Year 1) and final evaluation (Year 5).

<p>Prevalence and intensity of <i>S</i>. mansoni infection for first year students in each arm at baseline (Year 1) and final evaluation (Year 5).</p