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. 2022 Jun 7;21(1):172.
doi: 10.1186/s12936-022-04200-y.

Outdoor biting and pyrethroid resistance as potential drivers of persistent malaria transmission in Zanzibar

Revocatus M Musiba  1 , Brian B Tarimo  2 , April Monroe  3 , Dickson Msaky  2 , Halfan Ngowo  2 , Kimberly Mihayo  2 , Alex Limwagu  2 , Godlove T Chilla  2 , Gasper K Shubis  2 , Ahmada Ibrahim  2 , George Greer  4 , Juma H Mcha  5 , Khamis A Haji  5 , Faiza B Abbas  5 , Abdullah Ali  5 , Fredros O Okumu  2 , Samson S Kiware  2   6
Affiliations

Outdoor biting and pyrethroid resistance as potential drivers of persistent malaria transmission in Zanzibar

Revocatus M Musiba et al. Malar J. .

Abstract

Background: Low-level of malaria transmission persist in Zanzibar despite high coverage of core vector control interventions. This study was carried out in hot-spot sites to better understand entomological factors that may contribute to residual malaria transmission in Zanzibar.

Methods: A total of 135 households were randomly selected from six sites and consented to participate with 20-25 households per site. Mosquito vector surveillance was carried out indoors and outdoors from 6:00 pm-7:00 am using miniaturized double net trap (DN-Mini™). Additional collections were done indoors using mouth aspirators to retrieve resting mosquitoes from wall and ceiling surfaces, and outdoors using resting bucket and pit traps. All collected mosquitoes were morphologically and genetically (PCR) analysed in the laboratory. All collected anopheline and blood-fed mosquitoes were analysed for sporozoite infection and blood meal host preferences by Circumsporozoite Protein ELISA and blood meal ELISA, respectively. The differences between indoor and outdoor mosquito biting rates were analysed using generalized linear mixed models. Levels of resistance to commonly used insecticides were quantified by WHO susceptibility tests.

Results: Out of 704 malaria vectors collected across 135 households, PCR analysis shows that 98.60% were Anopheles arabiensis, 0.6% Anopheles merus and 0.6% Anopheles gambiae sensu stricto. Sporozoite ELISA analysis indicates that all mosquitoes were negative for the malaria parasite. The results show that more An. arabiensis were collected outdoor (~ 85%) compared to indoor (~ 15%). Furthermore, large numbers of An. arabiensis were caught in outdoor resting sites, where the pit trap (67.2%) collected more mosquitoes compared to the outdoor DN-Mini trap (32.8%). Nearly two-thirds (60.7%) of blood-fed mosquitoes had obtained blood meals from non-human hosts. Mosquitoes displayed non-uniform susceptibility status and resistance intensity among the tested insecticides across the study sites to all WHO recommended insecticides across the study sites.

Conclusion: This study suggests that in contexts such as Zanzibar, testing of novel techniques to complement indoor protection and targeting outdoor biting and/or resting mosquitoes, may be warranted to complement existing interventions and contribute to malaria elimination efforts. The study highlights the need to implement novel interventions and/or adaptations of strategies that can target outdoors biting mosquitoes.

Keywords: Entomology; Insecticide resistance; Malaria transmission; Novel tools; Pyrethroids.

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Conflict of interest statement

The authors have declared that they have no competing interests.

Figures

Fig. 1
Fig. 1
The figure presents the six study sites in Unguja and a snapshot of their characteristics
Fig. 2
Fig. 2
Miniaturized Double Net Trap (DN-Mini). A Provides a clear picture of DN-Mini trap and B show a volunteer collecting mosquito while seating down but not freely exposed to mosquitoes
Fig. 3
Fig. 3
Mean number of An. gambiae s.l. collected indoor and outdoor per night in Mbaleni, Donge Mchangani and Miwani
Fig. 4
Fig. 4
Hourly mean number of An. gambiae s.l. caught by DN-Mini in Mbaleni (A), Donge (B), and Miwani (C)
Fig. 5
Fig. 5
Showing blood meal host preferences displayed by An. gambiae s.l. caught by MD-Min both outdoor and indoor dwellings
See this image and copyright information in PMC

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