Are We on Our Way to Achieving the 2020 Goals for Schistosomiasis Morbidity Control Using Current World Health Organization Guidelines?

Abstract

Background: Schistosomiasis remains an endemic parasitic disease affecting millions of people around the world. The World Health Organization (WHO) has set goals of controlling morbidity to be reached by 2020, along with elimination as a public health problem in certain regions by 2025. Mathematical models of parasite transmission and treatment impact have been developed to assist in controlling the morbidity caused by schistosomiasis. These models can inform and guide implementation policy for mass drug administration programs, and help design monitoring and evaluation activities.

Methods: We use these models to predict whether the guidelines set by the WHO are on track for achieving their 2020 goal for the control of morbidity, specifically for Schistosoma mansoni. We examine whether programmatic adaptations; namely increases in treatment coverage and/or expansion to adult inclusion in treatment, will improve the likelihood of reaching the WHO goals.

Results: We find that in low-prevalence settings, the goals are likely to be attainable under current WHO guidelines, but in moderate to high-prevalence settings, the goals are less likely to be achieved unless treatment coverage is increased and expanded to at least 85% for school-aged children and 40% for adults.

Conclusions: To improve the likelihood of reaching the WHO goals, programmatic adaptations are required, particularly for moderate- to high-prevalence settings. Furthermore, improvements in adherence to treatment, potential development of candidate vaccines, and enhanced snail control and WASH (water, sanitation, and hygiene) measures will all assist in achieving the goals.

Figure 1.

Recommended programmatic adaptations (highlighted in blue boxes) to the current World Health Organization guidelines (in black boxes; using 75% coverage of school-aged children [SAC]) showing the frequency of treatment to be carried out according to prevalence in SAC in the region, where low prevalence is <10%, moderate prevalence is between 10% and 50%, and high prevalence is ≥50% [6].

Figure 2.

Imperial College London (A and B) and Case Western Reserve University (CWRU) (C and D) model scenarios showing prevalence of infection (eggs per gram [epg] >0) (A and C), and prevalence of heavy-intensity infections (epg ≥400) (B and D), in school-aged children (SAC) for settings of moderate baseline prevalence. Preventive chemotherapy once every 2 years reaches the morbidity goal by year 6 and may reach the elimination as a public health problem goal by year 10 (reached in 20% of the CWRU simulations). A and B, Shaded areas represent the range of basic reproductive number (R₀) values (R₀ = 1.22–1.241). C and D, Shaded areas represent the 95% confidence interval (CI) of uncertainty with the range of index of transmission potential (ITP) values (ITP = 1–5.6). The corresponding projections for the incidence of infection in the population are shown in Supplementary Figure 12B.

Figure 3.

Imperial College London (A and B) and Case Western Reserve University (CWRU) (C and D) model scenarios showing prevalence of infection (eggs per gram [epg] >0) (A and C), and prevalence of heavy-intensity infections (epg ≥400) (B and D), in school-aged children (SAC) for settings of high baseline prevalence. Preventive chemotherapy (PCT) is carried out once a year for 6 years and then at the same frequency or twice a year for 4 years depending on year 6 prevalence, which may reach one, both, or no goals (elimination as a public health problem goal reached by year 10 in 36% of the CWRU simulations). A and B, Shaded areas represent the range of basic reproductive number (R₀) values (R₀ = 1.2421–5.0). C and D, Shaded areas represent the 95% confidence interval (CI) of uncertainty with the range of index of transmission potential (ITP) values (ITP = 1–5.6); jagged lines due to 19% of the simulations having prevalence ≥50% at year 6 and therefore subjected to twice-yearly PCT. The corresponding projections for the incidence of infection in the population are shown in Supplementary Figure 12C.