Overview of main vector control strategies for onchocerciasis

Vector control for Onchocerca volvulus primarily involves aerial and ground larviciding with temephos, which targets Simulium larvae in fast‑flowing rivers. Aerial larviciding, developed under the OCP, allows treatment of large and inaccessible breeding sites, while ground larviciding is used in smaller river systems, with both requiring repeated applications due to reinfestation. The Slash and Clear method which is the removal of trailing vegetation along riverbanks is a low‑cost, community‑led approach effective in small, accessible rivers but less feasible in large or heavily vegetated systems. Esperanza Window Traps (EWTs), odour‑baited and insecticide‑treated, provide a targeted option for reducing adult blackfly biting, especially when breeding sites cannot be reached. Other supplementary measures include environmental management and integrated strategies that combine vector control with ivermectin MDA to reduce both vector density and human parasite reservoirs.

Beyond pharmacological interventions, vector control has demonstrated substantial potential in reducing onchocerciasis transmission. In the elimination era, renewed attention has focused on vector control, which historically achieved interruption of transmission in confined settings such as Bioko Island, yet remains underutilized across mainland Africa due to operational, financial, and environmental constraints [8,10]. Since 2016, the Expanded Special Project for the Elimination of Neglected Tropical Diseases (ESPEN) has supported countries to integrate such strategies into post‑APOC elimination efforts [16]. However, despite decades of MDA and localized vector control activities, persistent transmission in several foci highlights gaps in our understanding of the determinants of successful implementation. Building on earlier experiences such as the OCP’s demonstration that vector control alone can achieve long‑term interruption [17], there is a clear need to clarify which determinants influence program performance in diverse African settings.

To address this gap, this systematic review aims to synthesize existing evidence to identify and categorize the key barriers and facilitators affecting vector control outcomes for onchocerciasis in sub‑Saharan Africa. Therefore, the central guiding question of this review is: What factors are associated with the success or failure of vector control for onchocerciasis in Sub-Saharan Africa as reported in the existing literature?

Ethics statement

Ethics waiver was obtained from the University of the Witwatersrand Health Research Ethics Committee, with the number (REF: W-CBP-230324–01) as this study is a systematic review.

Study design

This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines [18]. The protocol was registered with PROSPERO on 28 November 2022 (CRD42022376792).

Eligibility criteria

We included peer‑reviewed experimental and quasi‑experimental studies evaluating the implementation of vector control interventions for onchocerciasis in sub‑Saharan Africa between 1 January 2000 and 31 March 2025. Eligible studies assessed environmental, operational, biological, or chemical factors influencing the effectiveness of interventions targeting Simulium spp. Observational studies, case reports, conference abstracts, commentaries, reviews, and non‑English publications were excluded. We focused on experimental designs because they offer stronger attribution between interventions and measurable outcomes (e.g., changes in biting rates or infection indices). Excluding observational studies narrows contextual breadth but was necessary to ensure methodological comparability across included interventions. The chosen date range was selected to capture contemporary vector control strategies and emerging operational practices relevant to the 21st‑century elimination agenda. To maintain methodological rigor, we excluded studies unrelated to onchocerciasis control, as well as non‑English publications, case reports, conference abstracts, commentaries, observational studies, and reviews. The exclusion of non‑English literature reflects the authors’ language constraints; while this may limit completeness, it ensures accurate appraisal and interpretation of the included studies. (Full eligibility criteria and PICOST table, see Table 1 below.

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Table 1. Eligibility criteria and PICOST.

https://doi.org/10.1371/journal.pntd.0014402.t001

Search strategy

A comprehensive search was conducted in PubMed, Scopus, Web of Science, and the Cochrane Library. Search terms incorporated MeSH and Emtree headings and multiple synonyms for onchocerciasis, vector control, and Simulium spp. Search strategies for each database are provided in Tables A – D in S1 Appendix. The initial search covered February 1–March 1, 2023, with an updated search on March 6, 2025 to capture recent publications.

Study screening and selection

Records were imported into Rayyan (rayyan.ai, a web-based systematic review management platform that facilitates collaborative screening and deduplication of references) for screening and duplicate removal. Two reviewers independently screened titles and abstracts against the eligibility criteria, followed by full‑text review of potentially relevant studies. Disagreements were resolved through discussion, and unresolved cases were adjudicated through a consensus involving a third reviewer. Screening procedures adhered to PRISMA standards [18].

Data extraction and synthesis

Data were extracted independently by one reviewer into a standardized Microsoft Excel form and cross‑checked by a second reviewer. Extracted fields included study location, design, intervention type, entomological outcomes, implementation characteristics, and reported facilitators or barriers. We conducted a narrative synthesis following established guidelines [19,20]. This involved four main steps, first, a preliminary synthesis of findings from individual studies into broad descriptive categories. Second, we grouped and coded findings thematically to identify patterns across studies. Third, we explored relationships within and between studies, such as common contextual factors, vector ecology, or intervention outcomes. Finally, we assessed the robustness of the synthesis by considering the methodological quality and consistency of findings across studies. This approach allowed us to thematically organize and interpret the diverse findings from the selected studies for our systematic review.

Quality appraisal

We assessed the methodological quality of all included studies using the Joanna Briggs Institute (JBI) Critical Appraisal Checklist for Quasi‑Experimental Studies [21,22]. Each study was independently appraised by two reviewers who scored all checklist items and documented justifications for each judgment. Any discrepancies in scoring or interpretation between the two reviewers were first discussed in a consensus meeting. If agreement could not be reached, a third senior reviewer acted as an adjudicator to resolve the disagreement and reach a final decision. This three‑step disagreement‑resolution process ensured consistency, minimized bias, and strengthened methodological rigor.

Evidence synthesis

Seventeen quasi-experimental studies were included in the review. Six of these studies were conducted in Uganda, two studies in each of Guinea, South Sudan, Cameroon, and Ghana, and only one study from each of Equatorial Guinea, Tanzania, and Burkina Faso. The studies included in the synthesis cover various aspects of onchocerciasis control, including vector control strategies, larviciding interventions, and the evaluation of different approaches.

Some of the studies included a preparatory phase before the actual study and data collection began. In terms of vector control methods, some of the studies explored the effectiveness of larviciding in reducing blackfly biting rates and transmission, while others explored slashing down trailing vegetations along fast flowing rivers. Furthermore, in this review, a number of studies focused on specific aspects related to vector ecology and behaviour, while another one study evaluated the effectiveness of the Esperanza Window Trap in reducing biting rates of Simulium damnosum sensu lato in Northern Uganda. In practice, the programmatic goal for much of Africa is not absolute vector elimination but reducing blackfly biting rates below the threshold necessary to sustain transmission, particularly when combined with regular ivermectin mass drug administration. Several interventions reviewed ranging from ground larviciding to community-led “Slash and Clear” approaches achieved substantial reductions in biting rates, thereby contributing to transmission suppression even in high-transmission settings.

Overall, the studies highlight the various factors which contribute to the success or failure of blackfly vector control programs. They include programmatic factors, vector-related factors (including ecology and biology), environmental factors, and human related factors such as technical (entomology) capacity or community involvement and commitment.

Programmatic factors refer to different elements essential for the successful and effective implementation of vector control programs. Four studies discussed programme related factors, one of these concerns is the duration of effect after a specific type of intervention such as ‘slash and clear’ [23–25]. If this is too short it can lead to a re-colonization of blackflies after treatment of breeding sites [1,26]. The effectiveness of Slash and Clear was facilitated by the presence of accessible trailing vegetation at breeding sites, which could be easily removed. However, the studies also highlighted that intervention feasibility is constrained by accessibility; inaccessible breeding sites remain potential sources of reinfestation. Seasonal fluctuations in river flow and vegetation growth further influence the optimal timing and sustainability of the intervention [23–25].

Biological and ecological factors drawn from characteristics of blackflies vectors and their interactions with the environment were identified by Four studies as important determinants of successful implementation of vector control programs [26–29]. For instance, blackfly vector susceptibility and resistance profiles to larvicides was mentioned in two studies as chief determinants to the success of larviciding control programs [26,28]. Additionally, understanding the blackfly habitat preference, the biting activity, and genetic mechanisms underlying insecticide resistance in blackflies is essential for developing effective strategies to combat resistance and tackle onchocerciasis. Other biological factors like natural predation and competition may have some consequences in the relative abundance of blackfly vectors [29].

Environmental factors were also found to have significant impact on the success or failure of vector control programs in Africa. Five studies discussed how environmental factors influence vector control programmes [24,30–33]. For example, seasonal variation of rainfall was identified as one important determinant of seasonal feasibility of vector control interventions This is because when the rainfall is high, it leads to large rivers bodies and fast flowing rivers which makes it difficult to reach breeding sites [24,28,30]. The presence of dams can also change the pattern of disease transmission by altering the speed and distribution of river flow, as well as creating varying pockets of breeding sites thereby creating more public health problems [29]. Isolation of regions or breeding sites such as islands can be considered an important criterion for achieving higher success.

Human related factors such as technical and entomological capacity of those involved in rolling out various vector control programs in Africa are part of the most important determinants of the success of vector control programs for blackfly, as highlighted in four studies from our review. [3,24,34,35] Entomological knowledge involves understanding blackfly’s ecological preference, their distribution and biting activity, knowing this will aid in the technical planning and preparatory phase for vector control programs as these could have significant implications [3,24,34,35]. For example, Simulium damnosum larvae more likely present in habitats near shrub [35]. On the other hand, local commitment, willingness of community members and community ownership of programs have been identified by several studies to be factors associated with vector control [3,24]. In Cameroon, while the immediate vector density reductions were notable, the study authors noted that vegetation regrowth and potential recolonization from untreated sites may require periodic repetition of the intervention [25]. They emphasized that, long-term success is dependent on sustained community motivation, ongoing resources, and the integration of slash and clear process into regular vector control cycles.

Other factors like presence of vegetation and shrubs, deforestation and agricultural activities have led to the temporary disappearance of freshwater crabs which are carriers to the blackfly larvae [1,26,36]. Political instabilities and insecurity have also hampered the sustainability and success of vector control activities in some regions [27]. These challenges disrupt operations, limit funding and resources, weaken health systems, hinder surveillance, and reporting, impede community engagement, and create coordination issues. The Fig 2 below shows a conceptual diagram with a snapshot of identified factors from our review.

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Fig 2. Conceptual diagram.

https://doi.org/10.1371/journal.pntd.0014402.g002

Quality appraisal

The JBI quality appraisal tools for quasi-experimental studies was employed for evaluating the quality of the included studies and gaining insights into the limitations of the evidence [22]. The quality appraisal table can be found in Table 3 below. All 17 included studies employed a quasi-experimental design. Overall quality scores, assessed using the Joanna Briggs Institute (JBI) critical appraisal checklist for quasi-experimental studies, ranged from 4 to 9, indicating variation in methodological rigor across the evidence base. Six studies (Studies 1, 2, 5, 7, 14, 17) scored 8–9, meeting most JBI criteria such as clear cause–effect attribution, appropriate comparison groups, valid and reliable outcome measures, and minimal risk of bias. These studies demonstrated a clear definition of cause and effect, with no confusion regarding the temporal relationships.

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Table 3. Quality Appraisal.

https://doi.org/10.1371/journal.pntd.0014402.t003

They also involved pre- and post-measurements of the outcome in a reliable way and employed appropriate statistical analysis methods. Seven studies (Studies 3, 9, 10, 12, 13, 15, 16) scored 6–7, meeting several quality criteria but showing limitations in areas such as blinding, completeness of follow-up, or clarity in participant recruitment. These studies further strengthened their design by including control groups for comparison. Four studies (Studies 4, 6, 8, 11) scored 4–5, reflecting greater methodological limitations, including limited baseline comparability, absence of control groups, and incomplete reporting of outcomes. These studies also had issues with the definition and measurement of exposure and outcome variables, raising concerns about the validity and reliability of their findings. Although quality varied, lower-scoring studies contributed valuable contextual insights, particularly on operational feasibility and real-world implementation of vector control strategies in challenging field settings. The inclusion of multiple high-scoring studies strengthens confidence in the synthesis, while findings from lower-quality studies should be interpreted with caution. In general, since the majority of the included studies indicated a moderate to high level of quality, the implication is that the findings from this review are somewhat reliable.