The Zanzibar Malaria Elimination Programme has primarily relied on insecticide-treated nets (ITNs) as the main vector control method, supplemented by reactive focal indoor residual spraying (IRS). Despite these efforts, local malaria transmission continues, with underlying reasons remaining unclear. Identifying gaps in existing interventions and opportunities for complementary strategies is crucial for optimizing vector control to achieve malaria elimination.
Entomological characterization of malaria transmission dynamics was conducted to assess vector behavior and human exposure risks. Monthly adult malaria vector collections were performed for two consecutive nights at ten sentinel sites from October 2022 to September 2023. Hourly indoor and outdoor human landing catch (HLC) methods were used to capture mosquitoes from 18:00 to 06:00.
Findings revealed that Anopheles arabiensis was the predominant malaria vector across most sites, except in the urban district of Unguja, where Anopheles gambiae sensu stricto was more common. Malaria parasite-infected An. arabiensis bites occurred indoors between 22:00 and 02:00 and outdoors from 18:00 to 21:00, posing a risk of early evening transmission before typical sleeping hours.
Sustained low human Plasmodium falciparum malaria prevalence has been maintained over the past decade through ITNs and IRS. However, local transmission persists due to evolving mosquito behaviors. ITNs and IRS primarily target indoor-biting and resting mosquitoes, yet changes in vector composition and behavior reduce intervention effectiveness. An. arabiensis, which has become more dominant, exhibits flexible feeding and resting behaviors, including outdoor biting and feeding on non-human hosts. These behaviors allow the mosquitoes to evade indoor-targeted interventions and survive to an age capable of transmitting malaria.
Epidemiological data from 2022 to 2023 indicated a sharp rise in malaria cases, with 19,007 cases reported from July 2022 through June 2023 compared to 4,603 cases in the previous year. Changes in vector species composition have been observed across sub-Saharan Africa since 2005, with An. gambiae sensu stricto declining and An. arabiensis increasing in prevalence. Unlike An. gambiae sensu stricto, which prefers human blood and bites indoors late at night, An. arabiensis exhibits early evening biting and outdoor activity, reducing its exposure to ITNs and IRS.
Current malaria control efforts in Zanzibar do not fully address outdoor malaria transmission. The effectiveness of ITNs and IRS is compromised by outdoor and early evening biting patterns, increasing the likelihood of residual transmission. Understanding the relationship between human and mosquito behaviors is essential for designing effective malaria vector control strategies in elimination areas.
Mosquito sampling was conducted using the HLC method, where volunteers sat on chairs, exposing their legs and collecting mosquitoes attempting to bite them with a mouth aspirator. Informed consent was obtained from volunteers, who received malaria prophylaxis and deworming medication as part of mass drug administration programs. Collections were carried out hourly from 18:00 to 06:00 at randomly selected sentinel houses in each district, with mosquitoes placed in labeled paper cups and transported to the laboratory for analysis.
Morphological identification of mosquitoes was performed using a dissecting microscope, with further molecular species confirmation through polymerase chain reaction (PCR). The head and thorax of each specimen were tested for sporozoite infection using enzyme-linked immunosorbent assay (ELISA).
Data analysis involved descriptive statistics and generalized linear mixed models (GLMM) with Poisson distribution to assess differences in indoor versus outdoor biting densities. Data collected from 2021/2022 and 2022/2023 were compared, with mosquito density stratified for April, May, and June to evaluate seasonal malaria surges. The year of data collection was treated as a fixed effect, with mosquito catches as the response variable. Statistical significance was considered at p-values below 0.05.
These findings highlight the importance of integrating outdoor mosquito control strategies with existing interventions. Addressing early evening and outdoor biting behaviors is critical for closing gaps in malaria elimination efforts. Complementary interventions, such as spatial repellents, insecticide-treated clothing, and improved environmental management, may help mitigate outdoor malaria transmission and support Zanzibar’s goal of malaria elimination.
