An Integrated District Mapping Strategy for Loiasis to Enable Safe Mass Treatment for Onchocerciasis in Gabon

Abstract

The lack of a WHO-recommended strategy for onchocerciasis treatment with ivermectin in hypo-endemic areas co-endemic with loiasis is an impediment to global onchocerciasis elimination. New loiasis diagnostics (LoaScope; Loa antibody rapid test) and risk prediction tools may enable safe mass treatment decisions in co-endemic areas. In 2017–2018, an integrated mapping strategy for onchocerciasis, lymphatic filariasis (LF), and loiasis, aimed at enabling safe ivermectin treatment decisions, was piloted in Gabon. Three ivermectin-naïve departments suspected to be hypo-endemic were selected and up to 100 adults per village across 30 villages in each of the three departments underwent testing for indicators of onchocerciasis, LF, and loiasis. An additional 67 communities in five adjoining departments were tested for loiasis to extend the prevalence and intensity predictions and possibly expand the boundaries of areas deemed safe for ivermectin treatment. Integrated testing in the three departments revealed within-department heterogeneity for all the three diseases, highlighting the value of a mapping approach that relies on cluster-based sampling rather than sentinel sites. These results suggest that safe mass treatment of onchocerciasis may be possible at the subdepartment level, even in departments where loiasis is present. Beyond valuable epidemiologic data, the study generated insight into the performance of various diagnostics and the feasibility of an integrated mapping approach utilizing new diagnostic and modeling tools. Further research should explore how programs can combine these diagnostic and risk prediction tools into a feasible programmatic strategy to enable safe treatment decisions where loiasis and onchocerciasis are co-endemic.

Analysis of diagnostic test outcomes in a large loiasis cohort from an endemic region: Serological tests are often false negative in hyper-microfilaremic infections

“…Sensitivity and specificity for loiasis-detection was also high, when tested against samples positive for other filarial infections [ 23 , 24 ]. In one study, the RDT was employed in the field, but individual diagnostic features of the test were not described [ 25 ]. Although RDT results were not available for our entire cohort, the analysis presented here is the largest and most comprehensive evaluation of this RDT in an endemic setting, with more than 960 study participants tested.…”

Sociodemographics, Clinical Factors, and Biological Factors Associated with Loiasis in Endemic Onchocerciasis Areas in Southern Gabon

“…In 2014, onchocerciasis prevalence was 17.3% in Memba, 59.0% in Nzoundou-Issinga, and 33.9% in Mbelnaletembe according to a National Parasitic Disease Control Program survey. 18 , 19 Since then, no mass treatment has been undertaken in these communities. The individual treatment is done after medical consultation by a specialist.…”

Geostatistical modelling enables efficient safety assessment for mass drug administration with ivermectin in Loa loa endemic areas through a combined antibody and LoaScope testing strategy for elimination of onchocerciasis

“…However, the performance of the device can be improved by changing the cut-off. The data we analysed here used cutoff of 157 reader units, which has been shown to deliver specificity of >95% for other filarial infections [ 9 ].…”

“…The data used for this study are from Loa loa surveys conducted in Gabon between December 2017 and December 2018 [ 9 ]. Surveys were conducted in 146 villages over 8 departments ( Fig 1 ), covering a total of 7,761 individuals aged 10 years or more.…”

“…Empirical prevalences for these 146 villages have been reported in Ella et al . [ 9 ]. Data were obtained using both the Loascope and the Ab test, resulting in the following outcomes for each individual:…”