Spatio-temporal dynamics of malaria in Zanzibar, 2015–2020
Authors: Bisanzio Donal , Lalji Shabbir , Abbas Faiza B. , Ali Mohamed H. , Hassan Wahida S. , Mkali Humphrey R. , Al-mafazy Abdul-wahid , Joseph Joseph J. , Nyinondi Ssanyu S. , Kitojo Chonge , Serbantez Naomi , Reaves Erik , Eckert Erin , Ngondi Jeremiah , Reithinger Richard 
Published: April 2022

Background

Despite the continued high coverage of malaria interventions, malaria elimination in Zanzibar remains elusive, with the annual number of cases increasing gradually over the last 3 years. The aims of the analyses presented here were to (i) assess the spatio-temporal dynamics of malaria in Zanzibar between 2015 and 2020, and (ii) identify malaria hotspots that would allow Zanzibar to develop an epidemiological stratification for more effective and granular intervention targeting, thereby allowing for more effective programmatic resource allocations.

Methods

Data routinely collected by Zanzibar’s Malaria Case Notification (MCN) system were analyzed. The system collects detailed socio-demographic and epidemiological data from all malaria cases who are passively detected and reported at the islands’ 313 public and private health facilities (defined as primary index cases), as well as through case follow-up and reactive case detection (defined as secondary cases). Using these data, spatio-temporal analyses were performed to identify the spatial heterogeneity of case reporting at shehia (ward) level during transmission seasons and its correlation with 2015–2020 rainfall.

Results

From January 1, 2015, to April 30, 2020, 22,686 index cases were notified by health facilities and reported through the MCN system. Number of cases reported showed declining trends from 2015 to 2017, followed by an increase from 2018 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. Change point analysis identified 10 distinct periods of malaria transmission across the study period, and the seasonality of reported index cases was significantly correlated to the amount of precipitation that occurred during the previous rainy season. Out of Zanzibar’s 388 shehias, 79 (20.3%) were identified as malaria hotspots in any given year between 2015 and 2020; these hotspots reported 52% of all index cases during the study period. Of the 79 shehias identified as hotspots, 12 (3% of all shehias) were hotspots in more than four years, i.e., considered temporally stable, reporting 14.5% of all index cases.

Conclusions

Our findings confirm that the scale-up of malaria interventions has greatly reduced malaria transmission in Zanzibar since 2006, with mean annual shehia incidence being 3.8 cases per 1,000 over the 2015–2020 study period. Spatio-temporal analyses identified hotspots, some of which were stable across multiple years. From a programmatic perspective, malaria efforts should progress from an approach that is based on universal coverage of interventions to an approach that is more tailored and nuanced, with resources prioritized and allocated to a select number of hotspot shehias.