Projected Number of People With Onchocerciasis-Loiasis Coinfection in Africa, 1995 to 2025

Abstract

Background: Onchocerciasis elimination through mass drug administration (MDA) is hampered by coendemicity of Loa loa, as people with high L. loa microfilariae (mf) density can develop serious adverse events (SAEs) after ivermectin treatment. We assessed the geographical overlap of onchocerciasis and loiasis prevalence and estimated the number of coinfected individuals at risk of post-ivermectin SAEs in West and Central Africa from 1995 to 2025.

Methods: Focusing on regions with suspected loiasis transmission in 14 countries, we overlaid precontrol maps of loiasis and onchocerciasis prevalence to calculate precontrol prevalence of coinfection by 5 km2 × 5 km2 pixel, distinguishing different categories of L. loa mf intensity. Using statistical and mathematical models, we predicted prevalence of both infections and coinfection for 2015 and 2025, accounting for the impact of MDA with ivermectin.

Results: The number of people infected with onchocerciasis was predicted to decline from almost 19 million in 1995 to 4 million in 2025. Of these, 137 000 people were estimated to also have L. loa hypermicrofilaremia (≥20 000 L. loa mf/mL) in 1995, declining to 31 000 in 2025. In 2025, 92.8% of coinfected cases with loiasis hypermicrofilaremia are predicted to live in hypoendemic areas currently not targeted for MDA.

Conclusions: Loiasis coinfection is a major concern for onchocerciasis elimination in Africa. We predict that under current strategies, at least 31 000 coinfected people still require treatment for onchocerciasis in 2025 while being at risk of SAEs, justifying continued efforts in research and development for safer drugs and control strategies.

Keywords: Loa loa; ONCHOSIM; mass drug administration; onchocerciasis; serious adverse events.

Figure 1.

Map of the estimated precontrol overlap between the prevalence of palpable onchocercal nodules and the prevalence of a history of eye worm in African Programme for Onchocerciasis Control countries. Abbreviations: CAR, Central African Republic; DRC, Democratic Republic of Congo.

Figure 2.

Maps showing the estimated prevalence of L. loa hypermicrofilaremia (≥20 000 mf/mL) in L. loa-mapped areas for 3 time points: 1995, precontrol (A); 2015 (B); 2025 (C). Abbreviations: CAR, Central African Republic; DRC, Democratic Republic of Congo; L. loa, Loa loa; mf, microfilariae.

Figure 3.

Maps showing the estimated prevalence of loiasis–onchocerciasis coinfections with L. loa hypermicrofilaremia (≥20 000 mf/mL) in L. loa-mapped areas coendemic for onchocerciasis for 3 time points: 1995, precontrol (A); 2015 (B); 2025 (C). Abbreviations: CAR, Central African Republic; DRC, Democratic Republic of Congo; IVM, ivermectin; L. loa, Loa loa; mf, microfilariae; O.v., Onchocerca volvulus.

Figure 4.

Sensitivity of the estimated number of onchocerciasis–loiasis coinfected cases by year for the following assumptions regarding the impact of mass drug administration on loiasis: no effect of IVM: IVM has no effect at all on the intensity and prevalence of Loa loa hypermicrofilaremia; effect of first round of IVM: IVM causes a change in the L. loa mf count frequency distribution among the population that received a first IVM treatment; the resulting L. loa mf count frequency distribution is sustained after subsequent treatment without any further changes; and exponential effect of IVM: IVM reduces the prevalence and intensity of L. loa mf after each repeated treatment with IVM, resulting in an exponential effect of repeated treatments on L. loa mf. Abbreviations: IVM, ivermectin; mf, microfilariae.