Improving public health control of schistosomiasis with a modified WHO strategy: a model-based comparison study

Abstract

Background: Schistosomiasis is endemic in many low-income and middle-income countries. To reduce infection-associated morbidity, WHO has published guidelines for control of schistosomiasis based on targeted mass drug administration (MDA) and, in 2017, on supplemental snail control. We compared the current WHO guideline-based strategies from 2012 to an alternative, adaptive decision making framework for control in heterogeneous environments, to estimate their predicted relative effectiveness and time to achievement of defined public health goals.

Methods: In this model-based comparison study, we adapted an established transmission model for Schistosoma infection that couples local human and snail populations and includes aspects of snail ecology and parasite biology. We calibrated the model using data from high-risk, moderate-risk, and lower-risk rural villages in Kenya, and then simulated control via MDA. We compared 2012 WHO guidelines with a modified adaptive strategy that tested a lower-prevalence threshold for MDA and shorter intervals between implementation, evaluation, and modification. We also explored the addition of snail control to this modified strategy. The primary outcomes were the proportion of simulations that achieved the WHO targets in children aged 5-14 years of less than 5% (2020 morbidity control goal) and less than 1% (2025 elimination as a public health problem goal) heavy infection and the mean duration of treatment required to achieve these goals.

Findings: In high-risk communities (80% baseline prevalence), current WHO strategies for MDA were not predicted to achieve morbidity control (<5% prevalence of heavy infections) in 80% of simulations over a 10-year period, whereas the modified adaptive strategy was predicted to achieve this goal in over 50% of simulations within 5 years. In low-risk and moderate-risk communities, current WHO guidelines from 2012 were predicted to achieve morbidity control in most simulations (96% in low-risk and 41% for moderate-risk), although the proposed adaptive strategy reached this goal in a shorter period (mean reduction of 5 years). The model predicted that the addition of snail control to the proposed adaptive strategy would achieve morbidity control in all high-risk communities, and 54% of communities could reach the goal for elimination as a public health problem (<1% heavy infection) within 7 years.

Interpretation: The modified adaptive decision making framework is predicted to be more effective than the current WHO guidelines in reaching 2025 public health goals, especially for high-prevalence regions. Modifications in current guidelines could reduce the time and resources needed for countries who are currently working on achieving public health goals against schistosomiasis.

Funding: University of Georgia Research Foundation, The Bill & Melinda Gates Foundation, and the Medical Scientist Training Program at Stanford University School of Medicine.

Figure 1

Standard WHO strategy for drug-treatment-based schistosomiasis control Treatment is targeted to the entire population of children aged 5–14 years, but to limit adverse reactions, children who are acutely ill are excluded. Coverage levels are estimated after each cycle of treatment.

Figure 2

Stratified worm burden modelling approach MDA=mass drug administration. h=human. E=egg. m=placeholder signifying integral sequence.

Figure 3

Modified adaptive strategy for schistosomiasis morbidity control The added modifications are: (1) to decrease the time interval between initiation and evaluation of treatment strategy outcomes to 2–3 years due to projected stagnation of further treatment impact, and (2) to lower the prevalence threshold for switching from annual to biannual treatment strategies. Although we chose to use 7% and 25% as the cutoff thresholds during the changeover year, these values are not rigid and could be adjusted as needed for local conditions.

Figure 4

Comparison of Schistosoma haematobium prevalence histories under standard WHO control strategy to those under modified adaptive strategy for high-prevalence communities Total prevalence levels and heavy infection prevalence (>50 eggs per 10 mL urine) levels in children aged 5–14 years are shown. The grey vertical dotted line shows strategy re-evaluation and possible switch depending on guideline recommendations, which occurred at year 5 for the standard WHO control strategy and year 3 for the modified adaptive strategy. Red and blue lines show two separate iterations of the model with the same initial values. Discrepancy between the two iterations shows the effect of modeling uncertainty (eg, environmental variation) for the same community. The grey horizontal lines indicate 50% and 10% prevalence.

Figure 5

Effectiveness in achieving morbidity control and elimination goals for Schistosoma haematobium under standard WHO control strategy and modified adaptive strategy for high, moderate, and low-prevalence communities Total prevalence and heavy infection prevalence (>50 eggs per 10 mL urine) in children aged 5–14 years are shown. Standard treatment coverage is 75% among children aged 5–14 years; increased treatment coverage is 85% in children aged 5–14 years and 40% in those aged 15 years and older. 2020 goals of morbidity reduction (<5% prevalence of heavy infection among children aged 5–14 years) are displayed by the blue horizontal lines and elimination as a public health problem (<1% prevalence of heavy infection among children aged 5–14 years) goals are displayed by the red horizontal lines, in the right-hand panels. Box-whisker charts show the range of possible outcomes (due to input uncertainty) for low, moderate, and high-prevalence villages. Re-evaluation for the standard guidelines is shown at year 5 for high-prevalence villages and year 6 in low and intermediate-prevalence villages. For modified WHO guidelines, re-evaluation occurs at year 3 for high-prevalence and year 4 for low and moderate-prevalence villages. Quartiles are different between WHO standard and modified strategies for year 1 due to modelled uncertainty. The grey horizontal lines indicate 50% and 10% prevalence.

Figure 6

Three-stage control strategy with addition of snail control to MDA in the third step Effects of a three-stage control strategy are shown in which the modified adaptive strategy is used for the first two steps. Snail control is then added 2 years after the second year of intensified MDA. Note the addition of snail control leads to a slower rebound of infection prevalence. The upper graphs show the history of prevalence values for children aged 5–14 years given treatment implementation strategy, and the lower box-whisker charts display the range of possible results. The WHO goal of morbidity reduction (<5% prevalence of heavy infection among children aged 5–14 years) is displayed by the blue horizontal lines and elimination as a public health problem (<1% prevalence of heavy infection among children aged 5–14 years) goals are displayed by the red horizontal lines, in the right-hand panels. The grey vertical dotted lines show the points where there was re-evaluation and strategy switching in the modified adaptive strategy. The grey horizontal lines indicate 50% and 10% prevalence. MDA=mass drug administration.