Spatiotemporal dynamics of malaria in Zanzibar, 2015–2020

Abstract

BackgroundDespite high coverage of malaria interventions, malaria elimination in Zanzibar remains elusive, with the annual number of cases increasing gradually over the last 3 years.ObjectiveThe aims of the study were to (1) assess the spatiotemporal dynamics of malaria in Zanzibar between 2015 and 2020 and (2) identify malaria hotspots that would allow Zanzibar to develop an epidemiological stratification for more effective and granular intervention targeting.MethodsIn this study, we analysed data routinely collected by Zanzibar’s Malaria Case Notification (MCN) system. The system collects sociodemographic and epidemiological data from all malaria cases. Cases are passively detected at health facilities (ie, primary index cases) and through case follow-up and reactive case detection (ie, secondary cases). Analyses were performed to identify the spatial heterogeneity of case reporting at shehia (ward) level during transmission seasons.ResultsFrom 1 January 2015 to 30 April 2020, the MCN system reported 22 686 index cases. Number of cases reported showed a declining trends from 2015 to 2016, followed by an increase from 2017 to 2020. More than 40% of cases had a travel history outside Zanzibar in the month prior to testing positive for malaria. The proportion of followed up index cases was approximately 70% for all years. Out of 387 shehias, 79 (20.4%) were identified as malaria hotspots in any given year; these hotspots reported 52% of all index cases during the study period. Of the 79 hotspot shehias, 12 were hotspots in more than 4 years, that is, considered temporally stable, reporting 14.5% of all index cases.ConclusionsOur findings confirm that the scale-up of malaria interventions has greatly reduced malaria transmission in Zanzibar since 2006. Analyses identified hotspots, some of which were stable across multiple years. Malaria efforts should progress from a universal intervention coverage approach to an approach that is more tailored to a select number of hotspot shehias.

Micro-heterogeneity of transmission shapes the submicroscopic malaria reservoir in coastal Tanzania

“…In a global meta-analysis of the submicroscopic reservoir incorporating 259 surveys, a small but significant increase in the submicroscopic ratio was detected during dry season sampling, with submicroscopic infections less common during the wet season than the dry season (Whittaker et al, 2021). We found a similar association between dry months (based on monthly total rainfall) and submicroscopic carriage, but this association was not significant until we incorporated a 9-week “lag” to account for the time it takes for rainfall to affect mosquito populations and malaria transmission (Bisanzio et al, 2023; Krefis et al, 2011) (Table 1). Finally, submicroscopic carriage appeared to increase over the years of the study, perhaps driven by decreased rainfall but we are unable to rigorously assess this.…”

A cross-sectional study of imported malaria infections in Zanzibar

“…While residual local transmission persists in Zanzibar, with Anopheles arabiensis being the most abundant vector [35,36], previous analyses of malaria data have consistently identi ed travel to mainland Tanzania as a risk factor for malaria infection among residents of Zanzibar [18, 21,24,29,37].…”

“…Malaria incidence in Zanzibar exhibits clear seasonality following the bimodal rainfall patterns, with two rainy seasons in March to June and October to November [16,29].…”

Upsurge in cases of travellers’ malaria ex Zanzibar indicates that malaria is on the rebound in the archipelago

“…These measures, initiated in 2003, proved successful and in 2018, Zanzibar developed a national malaria strategic plan with the goal of elimination of malaria by 2023 [ 1 ]. Despite the early promise of success there appears to be a distinct “rebound” in the malaria situation in Zanzibar and this has been attributed to changes in malaria epidemiology and also to malaria being imported to Zanzibar from highly endemic areas of mainland Tanzania visited by local residents [ 2 , 3 ]. Flaws and challenges in recent and past eradication programmes have also been highlighted [ 4 ].…”

“…This is important for Zanzibar as many social activities are early evening activities as are occupational activities related to tourism. Several studies [ [2] , [3] , [4] , [5] ] have highlighted factors that explain the rebound in malaria on Zanzibar and these include: a shift to mosquito outdoor biting activities, a shift in peak mosquito biting times to early evening, a rise in pyrethroid-resistant Anopheles mosquitoes-reducing the effectiveness of currently used treated nets. Another study [ 6 ] highlighted the role of local travel from the residents of Zanzibar to mainland Tanzania, where malaria risk is high, as one of the main factors for continued and increased malaria transmission in the archipelago.…”