Impact of annual ivermectin distribution on the intensity (a) and prevalence (b) of microfilarial infection. Red, blue and green lines correspond, respectively, to a baseline endemicity of 80%, 60% and 40% microfilarial prevalence. Microfilarial intensity is quantified as the mean microfilarial load per mg of skin in those aged ≥ 20 years. The dashed horizontal lines illustrate the upper and lower bounds (5% and 1% prevalence) of the current operational thresholds for cessation of treatment, namely an observed microfilarial prevalence below 5% in all surveyed villages and 1% in 90% of the surveyed villages. Assumptions are as in legend of Figure 3. The inset in Figure 4 (a) zooms in microfilarial infection intensity (in the ≥ 20 yr of age) for the last four years of the simulated intervention programme.