From high to low malaria transmission in Zanzibar—challenges and opportunities to achieve elimination

Abstract

BackgroundSubstantial global progress in the control of malaria in recent years has led to increased commitment to its potential elimination. Whether this is possible in high transmission areas of sub-Saharan Africa remains unclear. Zanzibar represents a unique case study of such attempt, where modern tools and strategies for malaria treatment and vector control have been deployed since 2003.MethodsWe have studied temporal trends of comprehensive malariometric indices in two districts with over 100,000 inhabitants each. The analyses included triangulation of data from annual community-based cross-sectional surveys, health management information systems, vital registry and entomological sentinel surveys.ResultsThe interventions, with sustained high-community uptake, were temporally associated with a major malaria decline, most pronounced between 2004 and 2007 and followed by a sustained state of low transmission. In 2015, the Plasmodium falciparum community prevalence of 0.43% (95% CI 0.23–0.73) by microscopy or rapid diagnostic test represented 96% reduction compared with that in 2003. The P. falciparum and P. malariae prevalence by PCR was 1.8% (95% CI 1.3–2.3), and the annual P. falciparum incidence was estimated to 8 infections including 2.8 clinical episodes per 1000 inhabitants. The total parasite load decreased over 1000-fold (99.9%) between 2003 and 2015. The incidence of symptomatic malaria at health facilities decreased by 94% with a trend towards relatively higher incidence in age groups > 5 years, a more pronounced seasonality and with reported travel history to/from Tanzania mainland as a higher risk factor. All-cause mortality among children < 5 years decreased by 72% between 2002 and 2007 mainly following the introduction of artemisinin-based combination therapies whereas the main reduction in malaria incidence followed upon the vector control interventions from 2006. Human biting rates decreased by 98% with a major shift towards outdoor biting by Anopheles arabiensis.ConclusionsZanzibar provides new evidence of the feasibility of reaching uniquely significant and sustainable malaria reduction (pre-elimination) in a previously high endemic region in sub-Saharan Africa. The data highlight constraints of optimistic prognostic modelling studies. New challenges, mainly with outdoor transmission, a large asymptomatic parasite reservoir and imported infections, require novel tools and reoriented strategies to prevent a rebound effect and achieve elimination.Electronic supplementary materialThe online version of this article (10.1186/s12916-018-1243-z) contains supplementary material, which is available to authorized users.

Falciparum malaria from coastal Tanzania and Zanzibar remains highly connected despite effective control efforts on the archipelago

“…Multiple studies have been conducted on travel patterns of Zanzibarian residents as it relates to importation of malaria [49][50][51], one of which estimated that there are 1.6 incoming infections per 1000 inhabitants per year. This is also in accordance with the estimate of about 1.5 imported new infections out of a total of 8 per 1000 inhabitants in a recent epidemiological study [2]. None of these studies have leveraged parasite population genetics to understand importation patterns.…”

“…Zanzibar has been the target of intensive malaria control interventions for nearly two decades following the early implementation of ACT therapies in 2003 [2]. Despite sustained vector control practices and broad access to rapid testing and effective treatment, malaria has not been eliminated from the archipelago [2]. Here WGS of P. falciparum isolates from Zanzibar and nearby sites on the mainland was used to investigate ancestry, population structure and transmission in local parasite populations.…”

“…through a combination of robust vector control and access to efficacious anti-malarial treatment, the archipelago of Zanzibar has been deemed a pre-elimination setting, having only low and mainly seasonal transmission [2]. Despite significant efforts, however, elimination has been difficult to achieve in Zanzibar.…”

Patterns of human exposure to malaria vectors in Zanzibar and implications for malaria elimination efforts

“…In Zanzibar, a higher percentage of males was recorded to be away throughout the night compared to females, and qualitative research ndings suggest males are more likely to engage in nighttime occupations, to travel, and to stay outdoors later socializing at night, all of which may impact exposure to malaria vectors. Likewise, travel to and from mainland Tanzania has been found to be a risk factor for malaria infection in Zanzibar [1].…”

“…Despite historically high transmission, the islands experienced a dramatic decline in malaria cases and deaths following the introduction of artemisinin-based combination therapy (ACT) and effective vector control interventions, namely insecticide-treated nets (ITNs) and indoor residual spraying (IRS) [1]. Since 2008, low level transmission has been maintained with malaria prevalence below 1% [1]. Field evidence suggests remaining cases are geographically focused and coincident with areas with high vector abundance [2].…”