Mathematical modeling of control strategies for the elimination of soil-transmitted helminthiases in Thailand

Abstract

Soil-transmitted helminthiases, caused by soil-transmitted helminths (commonly known as intestinal worms), are considered to be neglected tropical diseases (NTDs). In Thailand, school-age children (SAC) who live in remote areas are at risk of STH. A school-based combined intervention involving test and treat (TnT) and mass drug administration (MDA) programs has been implemented as part of the national STH control program since 2002, with a target to eliminate STH in SAC by 2026. To help achieve this target, we developed an STH transmission dynamic model, calibrated it using STH infection prevalence data from Thailand, and used the model to simulate the effects of the current intervention targeting only SAC and expanded interventions that also cover preschool-age children (PSAC) and adults. We also investigated modified interventions, including biannual MDA and TnT. Our model predicted that neither the current nor a biannual TnT can achieve elimination, even with expanded target populations. However, all biannual MDA treatment scenarios showed a reduced prevalence of STH in SAC, of less than 5%, by 2026. Our model also predicted that biannual MDA targeting SAC and adults would be more effective than targeting SAC and PSAC. Our findings suggest that if community-wide biannual MDA treatment were to be included in the control program, this would be beneficial for eliminating STH in Thailand.

Fig 1. Current interventions in the helminth control project, conducted between 2019 and 2024.

Fig 2. Schematic representation of the STH transmission dynamic model with interventions.

The compartments represent both mean worm populations (M_p, M_c, M_a) and estimated infections (P, C, A) from PSAC, SAC and adults respectively. The solid lines represent the flows from compartment to compartment in the ordinary differential equations, while the dotted lines represent the estimation of prevalence given the mean worm for each population.

Fig 3. The overall prevalence of any helminth infections between 2002 and 2020 based on the school types.

(A) Border patrol police schools, (B) Schools under the Office of the Basic Education Commission, (C) Community learning centers of the Office of the Non-formal Education Commission, (D) Monastic schools under the National Office of Buddhism, (E) Private Islamic schools under the Office of the Private Education Commission, (F) Schools under the local government and child development centers, (G) Schools under the Bangkok Metropolitan Administration (BMA), and (H) Rajaprajanugroh schools.

Fig 4. Comparing the observed and model estimates of prevalences of STH in Thailand between 2002 and 2020.

The black dots indicate observed prevalence data, and the black line indicates model estimate prevalences. The red dotted line indicates the 95% credible intervals (CrI) of model estimates.

Fig 5. Model prediction of the prevalence of STH and mean worm number between 2024 and 2033 under the current intervention with its expansion to adults.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 6. Model prediction of the prevalence of STH and mean worm number between 2024 and 2033 under the current intervention with its expansion to PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 7. Model prediction of the prevalence of STH and mean worm number between 2024 and 2033 under the current intervention with its expansion to both adults and PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 8. The model’s predictions of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual MDA expanded to adults.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 9. The model’s prediction of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual MDA expanded to PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 10. The model’s prediction of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual MDA expanded to both adults and PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 11. The model’s prediction of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual TnT expanded to adults.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 12. The model’s prediction of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual TnT expanded to PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.

Fig 13. The model’s prediction of prevalence of STH and mean worm number between 2024 to 2033 under the current intervention compared with biannual TnT expanded to both adults and PSAC.

The prevalence of STH and mean number of worms in PSAC (A and B), SAC (C and D), and adults (E and F) are shown, with the mean number of worms in the environment shown in G.