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

## TL;DR

This study tracks Anopheles mosquitoes in Tanzania, finding that An. merus is now a significant malaria vector, requiring updated control strategies.

## Contribution

The study reveals the increased distribution and potential role of An. merus as a malaria vector in Tanzania, necessitating revised surveillance and control approaches.

## Key findings

- An. merus was the second most abundant species in Tanga and Unguja, showing stable presence across seasons.
- Plasmodium infection was detected in An. arabiensis, but not in other species like An. merus.
- An. merus occurrence was limited in urban areas of Tanga and increased in semi-urban areas of Unguja during wet seasons.

## Abstract

Effective malaria vector control in endemic areas requires 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.

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.

A total of 4771 Anopheles mosquitoes collected (3,766 and 905 in Tanga and Unguja respectively), PCR amplification failed in 100 samples. Among successfully identified specimens, An. gambiae s.s. (43.8%) and An. merus (37.1%) were 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 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.

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.

The online version contains supplementary material available at 10.1186/s12936-026-05811-5.