Onchocerciasis elimination in sub-Saharan Africa requires alternative strategies
Authors: Alfred K Njamnshi, Wepnyu Y Njamnshi, Joseph N Siewe Fodjo
Journal: The Lancet Global Health (2024)

As we close in on the 2030 deadline set by WHO to achieve the elimination of transmission of onchocerciasis, it is increasingly necessary to have an overview of progress thus far. Through a comprehensive systematic review and meta-analysis published in The Lancet Global Health, Nyamai Mutono and colleagues1Mutono N Basáñez M-G James A et al.Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysis.Lancet Glob Health. 2024; (published online March 11.)https://doi.org/10.1016/S2214-109X(24)00043-3Google Scholar have provided an updated overview of onchocerciasis transmission in sub-Saharan Africa and identified potential determinants for achieving elimination of transmission in endemic foci. Their analysis suggests that factors positively associated with elimination of transmission include biannual ivermectin treatment, 10 or more years of uninterrupted ivermectin treatment with optimal coverage (≥80%), and reporting 15–19 years of ivermectin mass drug administration. Expectedly, high baseline endemicity levels for onchocerciasis were associated with a reduced likelihood of achieving elimination of transmission.1Mutono N Basáñez M-G James A et al.Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysis.Lancet Glob Health. 2024; (published online March 11.)https://doi.org/10.1016/S2214-109X(24)00043-3Google Scholar All these findings are consistent with existing onchocerciasis models.2Basáñez MG Walker M Turner HC Coffeng LE de Vlas SJ Stolk WA Chapter five—river blindness: mathematical models for control and elimination.in: Basáñez MG Anderson RM Advances in parasitology. Academic Press, London2016: 247-341Google Scholar Surprisingly, Mutono and colleagues1Mutono N Basáñez M-G James A et al.Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysis.Lancet Glob Health. 2024; (published online March 11.)https://doi.org/10.1016/S2214-109X(24)00043-3Google Scholar found that a history of vector control (without vector elimination) was significantly associated with an increased risk of experiencing ongoing transmission. This could possibly be due to the fact that interventions against the blackfly vectors are often implemented in settings of extreme nuisance caused by high biting rates, which would translate into higher pre-control onchocerciasis endemicity. It is worth noting that the absence of published data on elimination of transmission from eight sub-Saharan African countries currently receiving ivermectin (Angola, Benin, Burundi, Congo, Gabon, Guinea-Bissau, Sierra Leone, and South Sudan) in this systematic review and meta-analysis1Mutono N Basáñez M-G James A et al.Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysis.Lancet Glob Health. 2024; (published online March 11.)https://doi.org/10.1016/S2214-109X(24)00043-3Google Scholar limits the generalisability of the findings, thus highlighting the need for more studies. Thus far, ivermectin mass drug administration has been the mainstay strategy for onchocerciasis control. Following successful elimination of transmission with mass drug administration in only some villages in two sub-Saharan African countries (Mali and Senegal),3Diawara L Traoré MO Badji A et al.Feasibility of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: first evidence from studies in Mali and Senegal.PLoS Negl Trop Dis. 2009; 3: e497Crossref PubMed Scopus (273) Google Scholar WHO's strategy shifted from control to elimination. However, the contraindication of ivermectin in loiasis-endemic areas and the unsatisfactory mass drug administration coverage make it difficult to achieve elimination everywhere with this sole strategy. For instance, in the Ntui focus of Cameroon, despite decades of ivermectin mass drug administration, onchocerciasis prevalence has been evolving in a sinusoidal manner: 26·4% in 2009, 10·7% in 2016, and 14·6% in 2021.4Siewe Fodjo JN Ngarka L Njamnshi WY Enyong PA Zoung-Kanyi Bissek A-C Njamnshi AK Onchocerciasis in the Ntui Health District of Cameroon: epidemiological, entomological and parasitological findings in relation to elimination prospects.Parasit Vectors. 2022; 15: 444Crossref PubMed Scopus (1) Google Scholar Therefore, alternative onchocerciasis elimination strategies are strongly encouraged in such settings.5Boussinesq M Fobi G Kuesel AC Alternative treatment strategies to accelerate the elimination of onchocerciasis.Int Health. 2018; 10: i40-i48Crossref PubMed Scopus (70) Google Scholar Vector control constitutes an important alternative strategy to accelerate onchocerciasis elimination. The potential benefit of this strategy has been suggested by modelling studies2Basáñez MG Walker M Turner HC Coffeng LE de Vlas SJ Stolk WA Chapter five—river blindness: mathematical models for control and elimination.in: Basáñez MG Anderson RM Advances in parasitology. Academic Press, London2016: 247-341Google Scholar, 6Smith ME Bilal S Lakwo TL et al.Accelerating river blindness elimination by supplementing MDA with a vegetation "slash and clear" vector control strategy: a data-driven modeling analysis.Sci Rep. 2019; 915274Crossref Scopus (10) Google Scholar and confirmed during recent prospective observations in the Maridi onchocerciasis focus (South Sudan).7Hadermann A Jada SR Amaral L-J Colebunders R Bol YY Siewe Fodjo JN Rapid diagnostic testing for onchocerciasis in Maridi (South Sudan) before and after improving elimination strategies: a repeated cross-sectional survey.Open Res Eur. 2023; 3: 206Crossref Google Scholar In this onchocerciasis focus, the slash and clear vector control method contributed to lowering Ov16 seropositivity among 3-year-old children.7Hadermann A Jada SR Amaral L-J Colebunders R Bol YY Siewe Fodjo JN Rapid diagnostic testing for onchocerciasis in Maridi (South Sudan) before and after improving elimination strategies: a repeated cross-sectional survey.Open Res Eur. 2023; 3: 206Crossref Google Scholar Furthermore, natural and human-driven ecological events such as floods, deforestation, and dam construction can also affect the blackfly population. In the case of dams, the effect is varied: water reservoirs upstream of the dam tend to reduce blackfly abundance,8Zarroug IMA Hashim K ElMubark WA et al.The first confirmed elimination of an onchocerciasis focus in Africa: Abu Hamed, Sudan.Am J Trop Med Hyg. 2016; 95: 1037-1040Crossref PubMed Scopus (50) Google Scholar whereas fast-flowing downstream river segments can become conducive for vector breeding.9Lakwo TL Raimon S Tionga M et al.The Role of the Maridi Dam in causing an onchocerciasis-associated epilepsy epidemic in Maridi, South Sudan: an epidemiological, sociological, and entomological study.Pathogens. 2020; 9: 315Crossref PubMed Scopus (24) Google Scholar More generally, it has been suggested that climate change will result in increased onchocerciasis prevalence.10Figueiró R Docile T Climate change and onchocerciasis: insights from an analysis of public datasets.Peer Rev. 2023; 5: 1-11Crossref Google Scholar All these factors must be considered when planning for future human projects and for designing interventions that would sustainably combat onchocerciasis. Although it is unlikely that elimination of Onchocerca volvulus transmission will be achieved in all foci by 2030, the work of Mutono and colleagues provides important evidence regarding the factors that must urgently be acted upon to accelerate the process. In line with the authors' recommendations, ivermectin mass drug administration coverage should be sustainably improved and mass drug administration should be supplemented with environmentally-friendly vector control strategies, such as slash and clear, which would further enhance the public health impact of the intervention. Ultimately, onchocerciasis elimination interventions should ideally comprise both antiparasitic and antivectorial components, and the format and frequency must be tailored to the realities of each endemic focus. We declare no competing interests. Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysisMass drug administration duration, frequency, and coverage; baseline endemicity; and vector elimination or disappearance are important determinants of elimination of onchocerciasis transmission in sub-Saharan Africa. Our findings underscore the importance of improving and sustaining high therapeutic coverage and increasing treatment frequency if countries are to achieve elimination of onchocerciasis transmission. Full-Text PDF Open Access