Required duration of mass ivermectin treatment for onchocerciasis elimination in Africa: a comparative modelling analysis

Abstract

Background: The World Health Organization (WHO) has set ambitious targets for the elimination of onchocerciasis by 2020-2025 through mass ivermectin treatment. Two different mathematical models have assessed the feasibility of reaching this goal for different settings and treatment scenarios, namely the individual-based microsimulation model ONCHOSIM and the population-based deterministic model EPIONCHO. In this study, we harmonize some crucial assumptions and compare model predictions on common outputs.

Methods: Using a range of initial endemicity levels and treatment scenarios, we compared the models with respect to the following outcomes: 1) model-predicted trends in microfilarial (mf) prevalence and mean mf intensity during 25 years of (annual or biannual) mass ivermectin treatment; 2) treatment duration needed to bring mf prevalence below a provisional operational threshold for treatment interruption (pOTTIS, i.e. 1.4 %), and 3) treatment duration needed to drive the parasite population to local elimination, even in the absence of further interventions. Local elimination was judged by stochastic fade-out in ONCHOSIM and by reaching transmission breakpoints in EPIONCHO.

Results: ONCHOSIM and EPIONCHO both predicted that in mesoendemic areas the pOTTIS can be reached with annual treatment, but that this strategy may be insufficient in very highly hyperendemic areas or would require prolonged continuation of treatment. For the lower endemicity levels explored, ONCHOSIM predicted that the time needed to reach the pOTTIS is longer than that needed to drive the parasite population to elimination, whereas for the higher endemicity levels the opposite was true. In EPIONCHO, the pOTTIS was reached consistently sooner than the breakpoint.

Conclusions: The operational thresholds proposed by APOC may have to be adjusted to adequately reflect differences in pre-control endemicities. Further comparative modelling work will be conducted to better understand the main causes of differences in model-predicted trends. This is a pre-requisite for guiding elimination programmes in Africa and refining operational criteria for stopping mass treatment.

Fig. 1

Relationship between the annual biting rate (bites per person per year) and microfilarial (mf) prevalence in the population aged 5 years and above in the two models

Fig. 2

Comparison of expected trends in microfilarial (mf) prevalence during mass treatment, as predicted by ONCHOSIM and EPIONCHO, for settings with different baseline endemicity (mf prevalence in the population aged ≥ 5 years) assuming a coverage of 65 %

Fig. 3

Comparison of expected trends in arithmetic mean mf intensity during mass treatment, as predicted by ONCHOSIM and EPIONCHO, for settings with different baseline endemicity (mf prevalence in the population aged 5 years and above) assuming a coverage of 65 %

Fig. 4

Duration of mass ivermectin treatment in years that is needed to bring mf prevalence below the pOTTIS (red lines and symbols) or to eventually reach local elimination (blue lines and symbols), for ONCHOSIM (left) and EPIONCHO (right) and for annual (top) and biannual treatment (bottom). Dashed lines in each graph connect estimates obtained for different endemicity levels under the assumption that 65 % of the total population is treated per round (coverage). The vertical bars indicate how the duration would change if the coverage was 50 % per round (triangles) or 80 % (circles). To be able to differentiate the prediction intervals obtained for the different endpoints, the results are displayed slightly to the left or right of the actual simulated baseline prevalence (+/− 0.6 %)