Epidemiological and entomological analysis of imported malaria in Pemba Island, Zanzibar: a study from 2020-2022

Q: Why is imported malaria considered a major threat in Zanzibar and on Pemba Island?

Although Zanzibar reduced malaria prevalence since 2008, imported malaria continues from mainland Tanzania. On Pemba Island, 67.8% of 2021 cases were classified as imported.

Q: What data sources and analyses were used to study imported malaria?

The study used retrospective analysis of malaria surveillance data from 2020 to 2022 to examine temporal and spatial patterns and their origins. It also used routine entomological data from four sentinel sites and applied negative binomial and regression models to evaluate associations and odds of importation.

Q: What factors were associated with higher imported malaria cases?

Imported cases were higher among males and adults over 18 years and varied by destination regions. The odds were higher in 2021 and 2022 relative to 2020, and Human Biting Rate (HBR) was significantly associated with imported case numbers, with Mkoani identified as the most receptive area.

Sep 30, 2026

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Background Since 2008, Zanzibar reduced malaria prevalence to around 1%, yet imported malaria from mainland Tanzania remains a sustained threat, particularly on Pemba Island where 67.8% of 2021 cases were classified as imported. Methods Retrospective analysis of 2020–2022 surveillance data characterized epidemiological and spatial patterns, while routine entomological data from four sentinel sites assessed local receptivity. Results 2646 cases were reported, with 53.3% imported; importation varied by sex, age, destination regions, and travel seasonality, and HBR showed a strong association with imported case numbers. Conclusions Imported malaria is a persistent barrier to elimination, requiring strengthened cross-border surveillance and spatially targeted interventions integrating epidemiological and entomological evidence.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

Abstract

Background

Methods

Results

Conclusion

Ali et al. Malaria Journal (2026) 25:14 [https://doi.org/10.1186/s12936-025-05694-y](https://doi.org/10.1186/s12936-025-05694-y) Malaria Journal RESEARCH Open Access Epidemiological and entomological analysis of imported malaria in Pemba Island, Zanzibar: a study from 2020–2022 Ali O. Ali 1,2,3*, Ali M. Ali 1, Mwema Felix Mwema4 and Yeromin P. Mlacha 1,5 Abstract Background Since 2008, Zanzibar has significantly reduced the prevalence of malaria, keeping it at 1% . However, imported malaria particularly from mainland Tanzania continues to pose a major threat to sustaining these gains, especially on Pemba Island where 67. 8% of cases in 2021 were classified as imported. This study analysed malaria cases reported from 2020 to 2022, focusing on epidemiological and spatial trends of imported infections to inform more effective surveillance and response. Methods Retrospective analysis of malaria surveillance data from 2020 to 2022 was conducted to characterize the epidemiological parameters focusing on spatial and temporal patterns of imported malaria cases and their origin. Entomological data obtained from routine surveillance at four sentinel sites used to assess the receptivity of these areas to malaria transmission. Geographical location, season, sex and malaria intensity in the travel destination regions were evaluated in relation to the trend of malaria importation using a negative binomial regression model. The odds of importation were assessed using both univariate and multivariate regression models. Results Between 2020 and 2022, 2646 malaria cases were reported in Pemba, of which 1411 (53 . 3%) were classified as imported. Imported cases were significantly higher among males (57 . 3%, p < 0. 0001) and adults over 18 years (68%, p < 0. 0001) . Tanga (61%), Dar es Salaam (12%), and the Coastal Region (10%) accounted for the major ity of cases (98%) that came from mainland Tanzania. Only 2% of cases were traced to three neighbouring countries. Importation rates were significantly lower in Micheweni(IRR = 0 .48, p = 0 . 001) than in Wete dis trict, and higher in Mkoani (IRR = 2 .45, p < 0. 001) . Lower number of imported cases was observed during the rainy season (IRR = 0 . 83, p = 0 . 049) and among travellers returning from regions with low or moderate transmission. The odds of malaria importation were found to be significantly higher in 2021 (OR = 3 . 58, p < 0. 001) and 2022 (OR = 5 . 62, p < 0. 001) relative to 2020. No significant correlation was identified between case distribution and Anopheles biting density (r = 0 . 12, p = 0 . 5) . However, a significant association was found between Human Biting Rate (HBR) and imported cases, with each unit increase in HBR predicting an 84. 81-unit increase in the log-transformed number of imported cases (95% CI 13.29–156. 32, p = 0 . 021) . Mkoani district was identified as the most receptive area. Conclusion Imported cases has been confirmed as a persistent barrier to elimination in Pemba. These findings underscore the need for enhanced cross-border surveillance, spatially targeted interventions, and the integration of entomological and epidemiological data to inform malaria elimination strategies. Keywords Imported cases, Malaria, Receptivity, Pemba Island, Zanzibar *Correspondence: Ali O. Ali [aaomar@ihi.or.tz](aaomar@ihi.or.tz) Full list of author information is available at the end of the article © The Author(s) 2025. Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images

FAQs

Why is imported malaria considered a major threat in Zanzibar and on Pemba Island?

Although Zanzibar reduced malaria prevalence since 2008, imported malaria continues from mainland Tanzania. On Pemba Island, 67.8% of 2021 cases were classified as imported.

What data sources and analyses were used to study imported malaria?

The study used retrospective analysis of malaria surveillance data from 2020 to 2022 to examine temporal and spatial patterns and their origins. It also used routine entomological data from four sentinel sites and applied negative binomial and regression models to evaluate associations and odds of importation.

What factors were associated with higher imported malaria cases?

Imported cases were higher among males and adults over 18 years and varied by destination regions. The odds were higher in 2021 and 2022 relative to 2020, and Human Biting Rate (HBR) was significantly associated with imported case numbers, with Mkoani identified as the most receptive area.

Sep 30, 2026

Upload by Mafia Boss