Targeting Simulium Vectors to Accelerate Control of Onchocerciasis in the Mahenge Area, Tanzania: A Systematic Review

Abstract

Background: The Mahenge Mountains in south-eastern Tanzania remain a persistent onchocerciasis hotspot despite many years of community-directed ivermectin (CDTI) distribution. The disease is primarily transmitted by Simulium species, especially members of the Simulium damnosum complex. Aims: This review brings together current evidence on vector-targeted interventions, including entomological surveillance, larviciding, and environmental management in Mahenge and comparable Tanzanian foci. This review aims to assess whether integrating vector control with ongoing CDTI could help accelerate the interruption of transmission. Methods: A comprehensive search was conducted across PubMed, Scopus, Web of Science, African Journals Online, WHO repositories, Google Scholar, and relevant grey literature, with no date limits. Search terms combined Simulium or blackfly, onchocerciasis, Mahenge or Tanzania, and vector-control related keywords. The study included field studies, operational reports, modelling papers, and entomological surveys that reported on vector species composition, breeding-site mapping, vector-control activities, or outcomes related to transmission. Two reviewers independently screened articles and extracted data. Quantitative findings were pooled when appropriate; otherwise, results were synthesised narratively. Results: Historical studies confirm S. damnosum s.l. as the dominant vector in Mahenge. Recent reports highlight ongoing transmission despite long-term ivermectin use and recommend supplementing CDTI with vector control. Evidence from other Tanzanian foci and modelling work suggests that well-implemented larviciding grounded in comprehensive breeding-site mapping and applied consistently can substantially reduce biting rates and shorten the time needed to achieve elimination compared with CDTI alone. Programmatic documents describe both successful reductions in vector density and episodes of transmission rebound when larviciding was incomplete or discontinued. However, there is still a shortage of published field trials from Mahenge directly evaluating the impact of larviciding, with most available studies being descriptive, model-based, or operational. Conclusions: The available evidence supports integrating targeted Simulium control, particularly larviciding accompanied by strong entomological surveillance, with existing CDTI in high-transmission areas such as Mahenge. Key priorities include: (1) updated breeding-site mapping using tools such as drone imagery with ground-truthing, (2) larval susceptibility testing to guide larvicide choice, (3) pilot larviciding accompanied by rigorous environmental and entomological monitoring, and (4) long-term investment in local capacity. Well-designed operational trials in Mahenge, using both entomological and epidemiological endpoints, are urgently needed to guide effective elimination strategies.