Longitudinal surveillance of Anopheles mosquitoes across different settings in Tanga and Unguja: increased distribution of An. merus in coastal and inland areas of Tanzania

Background Effective malaria vector control in endemic areas requires an understanding the distribution and composition of Anopheles species, as shifts in malaria vector species and composition can influence the efficacy of control interventions and transmission patterns. The current study explored the temporal and spatial distribution of Anopheles species and their infection with Plasmodium in different transmission settings in Tanga region and Unguja, Zanzibar, United Republic of Tanzania. Methods From September 2021 to December 2023, monthly entomological surveys were conducted in 11 villages in Tanga and four Shehias in Unguja. Anopheles mosquitoes were sampled every month in each of 11 villages in Tanga and four Shehias in Unguja, 10 households were consented to participate in each village or Shehia. Mosquitoes were collected indoors and outdoors using CDC light traps, Furvela tent traps, Indoor and Outdoor prokopack. Species identification was performed using PCR, and Plasmodium infections were detected using TaqMan real-time PCR assay. Results A total of 3,766 and 905 Anopheles mosquitoes were collected in Tanga and Unguja respectively, with An. gambiae s.s . (43.8%) and An. merus (37.1%) being predominant. In Unguja, An. arabiensis (55.7%) and An. merus (41.9%) were most common. Seasonal variations were observed, with An. gambiae s.s . and An. funestus s.s. peaking in the short rainy season, An. arabiensis peaking in both dry and long rainy seasons, and An. merus did not show a clear seasonal pattern; instead, its abundance peaked during both the wet and dry seasons, suggesting relatively stable occurrence throughout the year. Plasmodium infection rates for An. gambiae s.s ., An. funestus s.s ., An. arabiensis , and An. merus were 3.0% in Tanga and 1.2% in Unguja but only found in An. arabiensis . In Tanga, An. gambiae s.s., An. merus , and An. funestus s.s. were more abundant in upland and lowland areas than in the highlands, with urbanization limiting An. merus occurrence. In Unguja, An. arabiensis and An. merus were less common in semi-urban areas but showed a sharp increase during the wet season. Conclusions The study indicates a shift in An. gambiae s.l. sibling species composition has taken place in the study areas compared to previous reports. In the past, An. merus was not considered an important vector in Tanzania. However, in this study An. merus was observed as the second most abundant species across coastal and inland areas of Tanga and Unguja during both wet and dry season. Combined with its observed infection with P. falciparum , the findings suggest An. merus may contribute to perennial transmission of malaria in the region. This presents a new challenge to malaria vector surveillance and control including the need for a year-round multi-strategic approach.