Understanding the Epidemiology of Malaria in Zanzibar through Molecular and Serological Analysis of Samples collected during Reactive Case Detection.

Background: The Zanzibar archipelago has been a pre-elimination region for malaria thanks to rigorous control interventions, but recent surges in malaria cases have been observed. The contribution of non-falciparum species to the current malaria situation is unknown.

Methods: This study investigates the seroepidemiology of falciparum and non-falciparum malaria species in Zanzibar. Leveraging plasma extracted from dried blood spots (DBS) collected during reactive case detection (RCD) activities across Unguja island from May 2022 to May 2023, we measured immunoglobulin G (IgG) responses to MSP1-19kD antigens using a multiplex bead-based immunoassay. Additionally, active infections were detected using species-specific real-time PCR.

Results: Out of 1,618 participants surveyed in 35 RCDs, 35.3% had exposure to any malaria species, with being the most common (29.8%). Seroprevalences for non-falciparum species were lower: (5.8%), (5.9%), and (5.9%). Active infections were detected in 6.0% of participants, predominantly (4.6%). Travel to mainland Tanzania was a dominant risk factor for seropositivity for all four malaria species. Other factors associated with seropositivity (high-risk occupations, female status) were not associated with seropositivity for non-falciparum species. The geographic distribution of non-falciparum exposure differed compared to falciparum, with relatively higher seroprevalences in rural districts, especially Kazkazini A in northern Unguja.

Discussion: This study suggests a significant contribution of non-falciparum species to the local epidemiology in Zanzibar. Current control and elimination efforts, focused on , may not adequately address exposure to non-falciparum species.