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

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.

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.

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 majority 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 district, 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.

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.