Malaria control efforts in Zanzibar are facing new challenges as Anopheles arabiensis, the primary malaria vector, continues to thrive, especially during long rainy seasons. A recent study has uncovered that this mosquito species exhibits strong outdoor and early biting habits, making it difficult to control using conventional methods like insecticide-treated nets (ITNs) and indoor residual spraying (IRS).

Rise in malaria vector density
The study, published on Malaria Journal, was conducted by scientists from the Zanzibar Malaria Elimination Program (ZAMEP) under the Ministry of Health of Zanzibar, Population Services International (PSI), Ifakara Health Institute, and the U.S. President's Malaria Initiative.

The research found a significant increase in mosquito density during the rainy season between April and June 2023 compared to the same period in 2022. This surge coincided with a rise in malaria cases in Unguja, raising serious public health concerns.

Climate change and its impact on malaria vectors
According to researchers, the increase in mosquito populations may be linked to the El Niño Southern Oscillation, a global climate phenomenon. They noted, “While the specific contributors to this increase are not fully characterized, the 2023 weather patterns were associated with El Niño events, which are considered to be associated with broader climate change.” 

“Monitoring climatic events—particularly rainfall—and how they affect disease-vector transmission dynamics and vector control intervention efficacy should be considered to allow timely adjustment of intervention strategies,” emphasized the scientists.

Outdoor transmission poses a growing threat
The study highlights the growing risk of malaria transmission for individuals engaged in outdoor nighttime activities, such as night watchmen, police officers, and those dining outdoors. These individuals are often exposed to mosquitoes before they retire to bed under ITNs. 

The research underscores the need for targeted interventions for these high-risk groups, noting that while ITNs remain effective, additional measures are necessary for individuals whose outdoor activities prevent them from using these interventions.

“Evidence from this study indicates that people engaging in nighttime outdoor behaviours or activities were at higher risk for malaria… and, while the widespread use of ITNs has proven highly effective and must be sustained, interventions specifically targeting populations at high risk for malaria due to occupational or behavioural factors, where ITN use is not practical are necessary to interrupt malaria transmission.” 

Persistence of other malaria vectors
While An. arabiensis dominates in most parts of Zanzibar, another major malaria vector, Anopheles gambiae s.s., remains prevalent in Stone Town. The study suggests this may be due to the delayed implementation of IRS and persistently low ITN use in the area, a pattern also observed in other urban centers like Dar es Salaam. Meanwhile, Anopheles funestus s.s. was rarely detected, but An. parensis, a secondary vector, was found across both Pemba and Unguja islands. 

The need for new malaria control strategies
These findings highlight the limitations of current malaria interventions, which primarily target indoor-biting mosquitoes with studies showing that even with universal ITN and IRS coverage, outdoor exposure could still lead to over 10 million additional malaria cases in Africa each year.

To address this challenge, the authors recommend introducing complementary strategies that target outdoor-biting mosquitoes. These include developing mosquito control measures for people engaged in outdoor nighttime activities, assessing the feasibility of outdoor-focused interventions in Zanzibar, and promoting community awareness on personal protection methods, such as using repellents and wearing protective clothing.

Strengthening data collection and surveillance
The scientists also stress the importance of monitoring climatic events, particularly rainfall, and how they affect disease-vector transmission dynamics and vector control intervention efficacy. Continuous tracking of these factors would enable timely adjustments to intervention strategies, ensuring better malaria control. 

Integrating human behavior into surveillance
The researchers also recommend that the ZAMEP, which leads routine malaria vector surveillance, integrate human behavior surveys into routine entomological monitoring. 

This integration would improve data accuracy and provide a clearer picture of malaria transmission risks. As the authors note, “Human behaviours, including time spent outdoors in the evening, time spent sleeping indoors and under ITNs, and time waking up in the morning, integrated with human biting rate over time and space, are critical for determining human exposure and impact of interventions.”

Addressing outdoor malaria transmission
While ITNs and IRS have significantly reduced malaria transmission in Zanzibar, the persistence of outdoor-biting mosquitoes poses a major challenge. Without targeted interventions, Zanzibar’s malaria elimination goals could face setbacks. This study strengthens the evidence for outdoor malaria transmission risk and calls for complementary interventions to accelerate malaria elimination efforts.

Ifakara scientists play key role in the study
Ifakara scientists Nicodem Govella, Victoria Githu, and Victor Mero were instrumental in study, leading the data analysis process, with Govella also serving as the corresponding author. They collaborated with researchers from the Zanzibar Malaria Elimination Program (ZAMEP), the PMI Dhibiti Malaria Project, Population Services International, and the U.S. President’s Malaria Initiative in Tanzania.

Read the publication here.