A Framework for Decision-Making for Mass Distribution of Mectizan(R) in Areas Endemic for Loa loa

Abstract

BACKGROUND: The occurrence of Loa loa encephalopathy following mass treatment of onchocerciasis with Mectizan(R) has adversely affected onchocerciasis control efforts in central Africa. Persons with very high densities of L. loa microfilaremia are at increased risk of encephalopathy, but little is known about the geographic distribution of these persons within central Africa. RAPLOA, a new technique that correlates the proportion of community members reporting a history of eyeworm with the prevalence of high-intensity L. loa microfilaremia in that community, may be useful for rapid assessment of areas at potential risk of treatment-related L. loa encephalopathy. Validation of RAPLOA is ongoing. The operational and risk-reduction advantages of RAPLOA over the current technique of village-by-village rapid epidemiologic assessment for onchocerciasis (REA) are unknown. METHODS: We developed a decision model to compare four strategies for minimizing sequelae of L. loa encephalopathy following mass treatment with Mectizan(R) in areas co-endemic for onchocerciasis and loiasis: REA; RAPLOA with threshold eyeworm prevalences of 40% and 20% (RAPLOA-40 and RAPLOA-20, respectively); and combined REA/RAPLOA-40. RESULTS: In the model, all four strategies significantly reduced risk of death and neurologic complications from L. loa encephalopathy, but RAPLOA-20 and REA resulted in half as many such cases as did RAPLOA-40 or combined REA/RAPLOA-40. CONCLUSION: RAPLOA is likely to be useful programmatically in reducing risk of L. loa encephalopathy following mass treatment with Mectizan(R). It also may be cost-saving. Before full-scale implementation, additional data are needed on geographic clustering of high-density L. loa microfilaremia and on RAPLOA's reliability and cost.

Figure 1

Prevalence of persons with L. loa microfilaremia ≥ 30,000 per ml as a function of the prevalence of a restricted definition of eyeworm, from Figure 10 of the multi-center report on RAPLOA [10]. See RAPLOA in the List of Abbreviations for details on the restricted definition of eyeworm.

Figure 2

Conceptual framework for current and alternative approaches to minimizing risks and maximizing benefits of mass treatment with Mectizan^® in areas suspected of being co-endemic for onchocerciasis and loiasis. Implementation of training, surveillance, and supportive care, indicated in shaded boxes, are assumed to reduce the risk of death associated with SAEs. + indicates the condition indicated -indicates the absence of the condition.

Figure 3

Decision model of four strategies to reduce risk of L. loa encephalopathy following treatment with Mectizan^® in areas that are 1) thought to be endemic for loiasis on the basis of remote sensing (the environmental risk model map); and 2) determined to be hyper- or meso-endemic for onchocerciasis by REMO. Abbreviations: oncho = onchocerciasis; Rx = treatment. Additional details on the model are available from the authors upon request. [+] indicates truncation of a branch to simplify the figure. For the remainder of this branch, see the structure of similarly labelled branches elsewhere in the figure.

Figure 4

Estimated rates of lifetime blindness (per 1000 population) and of death and chronic disability from L. loa encephalopathy, under conditions of 1) no mass treatment with Mectizan^®; 2) mass treatment with no risk reduction strategy; and 3) mass treatment using REA, RAPLOA-40, RAPLOA-20, or combined REA/RAPLOA-40. Data are shown for two levels of loiasis co-endemicity, both with highly endemic onchocerciasis. Similar patterns are observed in areas of low onchocerciasis endemicity (data not shown).