The ‘at risk population’ of the surveyed areas in each APOC country was estimated by multiplying the surface of the surveyed area in the country with the country-specific average population density for CDTI projects. The latter was obtained for each APOC country by dividing the total population of the CDTI projects in the country in 2011 by the total surface area of these projects.
The nodule prevalence map was used to divide the surveyed area in each country into three endemicity classes with nodule prevalence of 0–4.5%, 5–19.9% and >20% respectively. The population in each class was estimated by multiplying the surface area with the average population density for CDTI projects in the country. For all surface calculations, the geographic coordinates were first projected using the ARCGIS (World) Cylindrical Equal Area projection.
In order to estimate the number of persons that would have been infected with O. volvulus by the year 2011 if there had been no onchocerciasis control, we used the recently published results of a study on the relationship between the prevalence of skin microfilaria in a village (all age groups combined) and the prevalence of palpable nodules in adult males in the same villages [42 (link)]. From this publication we used the main relationship for all study areas except one (Mbam), for which the pattern was different. This relationship was used to convert the 1 km resolution predicted nodule prevalence in adults, as generated during the geostatistical analysis, into the corresponding predicted prevalence of microfilaria for all ages combined. For each country, the predicted prevalence of microfilaria was then averaged over the total surveyed area and multiplied with the estimated at risk population of the surveyed areas in the country to obtain an estimate of the total number, T, infected with O. volvulus if there had been no onchocerciasis control. To obtain a confidence interval for this estimate, we sampled repeatedly from the joint predictive distribution of prevalence surface P(x), and from each sample calculated the corresponding estimate of T. Then, a 95% confidence interval for T is the range from the 2.5th to the 97.5th percentile of the empirical distribution of these estimates. For the APOC-wide total we used a similar procedure. Since nodule prevalence was modelled using three independent spatial processes with different means for the main area, Liberia and Bioko, we obtained a simulated sample for each from the joint predictive distribution of P(x), the estimated number of infected for the three areas separately and added these up. The 95% confidence intervals were then calculated from the resulting APOC-wide total distribution of T.
The nodule prevalence map was used to divide the surveyed area in each country into three endemicity classes with nodule prevalence of 0–4.5%, 5–19.9% and >20% respectively. The population in each class was estimated by multiplying the surface area with the average population density for CDTI projects in the country. For all surface calculations, the geographic coordinates were first projected using the ARCGIS (World) Cylindrical Equal Area projection.
In order to estimate the number of persons that would have been infected with O. volvulus by the year 2011 if there had been no onchocerciasis control, we used the recently published results of a study on the relationship between the prevalence of skin microfilaria in a village (all age groups combined) and the prevalence of palpable nodules in adult males in the same villages [42 (link)]. From this publication we used the main relationship for all study areas except one (Mbam), for which the pattern was different. This relationship was used to convert the 1 km resolution predicted nodule prevalence in adults, as generated during the geostatistical analysis, into the corresponding predicted prevalence of microfilaria for all ages combined. For each country, the predicted prevalence of microfilaria was then averaged over the total surveyed area and multiplied with the estimated at risk population of the surveyed areas in the country to obtain an estimate of the total number, T, infected with O. volvulus if there had been no onchocerciasis control. To obtain a confidence interval for this estimate, we sampled repeatedly from the joint predictive distribution of prevalence surface P(x), and from each sample calculated the corresponding estimate of T. Then, a 95% confidence interval for T is the range from the 2.5th to the 97.5th percentile of the empirical distribution of these estimates. For the APOC-wide total we used a similar procedure. Since nodule prevalence was modelled using three independent spatial processes with different means for the main area, Liberia and Bioko, we obtained a simulated sample for each from the joint predictive distribution of P(x), the estimated number of infected for the three areas separately and added these up. The 95% confidence intervals were then calculated from the resulting APOC-wide total distribution of T.
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