Travel to mainland Tanzania as risk factor for malaria and further transmission in Zanzibar

Abstract

Introduction: Malaria pre-elimination is reached in Zanzibar. Travel has earlier been identified as a risk factor for malaria in Zanzibar and import of malaria from Tanzania mainland has been proposed to fuel the residual transmission in Zanzibar. Objectives: To assess travel to mainland Tanzania as a risk factor and to describe characteristics of malaria patients in Zanzibar during 2016. Methods: This was a retrospective, descriptive and case-control study using quantitative data from a malaria surveillance system in Zanzibar. Malaria cases were clinical and confirmed by malaria rapid diagnostic test (mRDT) or microscopy. Questionnaire answers provided data for known risk factors for malaria such as recent travel history (within 30 days), not having slept under long lasting insecticide treated net (LLIN) (previous night) or not having done insecticide residual spraying (IRS) recently (within 8 months). Results: 48% of cases at health facilities had recent travel history outside Zanzibar. Recent travel was found to be a strong risk factor for malaria, unadjusted OR’s for different periods ranging 222-486 (CI 124-710, p<0.001). Tanzania mainland was reported as travel destination by 94% of all travel cases. LLIN was used by 64% and IRS done recently by 31% of all malaria cases, coverage varying by district. Conclusions and implications: A high proportion of malaria cases reporting recent travel suggests a large proportion of all malaria in Zanzibar is imported. Maintained uptake of interventions such as LLIN and IRS and continued surveillance and case follow-up are factors that affects the risk for onward transmission of imported malaria. Limiting imported malaria with suitable strategies could potentially help to accelerate further reduction and eliminate malaria in Zanzibar.