Can chemotherapy alone eliminate the transmission of soil transmitted helminths?

Abstract

Background: Amongst the world's poorest populations, availability of anthelmintic treatments for the control of soil transmitted helminths (STH) by mass or targeted chemotherapy has increased dramatically in recent years. However, the design of community based treatment programmes to achieve the greatest impact on transmission is still open to debate. Questions include: who should be treated, how often should they be treated, how long should treatment be continued for?

Methods: Simulation and analysis of a dynamic transmission model and novel data analyses suggest refinements of the World Health Organization guidelines for the community based treatment of STH.

Results: This analysis shows that treatment levels and frequency must be much higher, and the breadth of coverage across age classes broader than is typically the current practice, if transmission is to be interrupted by mass chemotherapy alone.

Conclusions: When planning interventions to reduce transmission, rather than purely to reduce morbidity, current school-based interventions are unlikely to be enough to achieve the desired results.

Figure 1

Coverage of preventive STH chemotherapy in preschool-age children (Pre-SAC, blue) and school-age children (SAC, red), (a) internationally and (b) WHO African Region, calculated by WHO as the proportion of the total population of pre-SAC and SAC living in all the endemic areas in a country which require preventive chemotherapy for STH by year, 2003–2011[1].

Figure 2

Age-intensity profiles for mean intensity of infection (top row) and prevalence (%, second row) for the three major soil transmitted helminths; Ascaris [[27]] (left column), Trichuris [[28]] (middle column) & hookworm [[29]] (right column). Colours indicate age-group.

Figure 3

The critical treatment surfaces for Ascaris (row 1: a, b, c, and 3: g, h, i) and hookworm (row 2: d, e, f). Reproductive number R₀ is 2, 3 and 5 in columns 1 (a, d, g), 2 (b, e, h) and 3 (c, f, i), respectively. In rows 1 (a, b, c) and 2 (d, e, f), treatment is yearly while in row 3 (g, h, i), treatment is every 6 months. Model parameters as in Additional file 1: Table S2. The proportion treated effectively (proportion treated x drug efficacy) in each age grouping (pre-SAC, SAC & Adults) must lie above the ‘critical’ surface in the 3D plots (at the plateau of 1.0 all must be treated effectively) for eradication to occur. The colours indicate coverage in SAC.

Figure 4

Numerical solutions of the model for different transmission settings (R₀ of 2 (a, d), 3 (b, e) and 5 (c, f)) and yearly (a, b, c) or twice-yearly (d, e, f) treatment with 80% treatment in Pre-SAC and SAC. The mean worm burden in Infants (black), Pre-SAC (blue), SAC (red) and Adults (orange) is shown reducing over time. Other parameters as in Additional file 1: Table S2.