Towards interruption of schistosomiasis transmission in sub-Saharan Africa: developing an appropriate environmental surveillance framework to guide and to support ‘end game’ interventions

By Prof. Russell Stothard

(Other Authors: Suzanne Campbell, Mike Y Osei-Atweneboana, Timothy Durant, Michelle C Stanton, Nana-Kwado Biritwum, David Rollinson, Deudonné R Eloundou Ombede, Louis-Albert Tchuem-Tchuenté)

Introduction

In sub-Saharan Africa, schistosomiasis is a waterborne parasitic disease of medical and veterinary importance particularly in impoverished, rural communities with limited access to safe water and adequate sanitation [1]. As many other trematodes, schistosomes have an intricate lifecycle involving two free-living motile larval stages, a ciliated miracidium and a birfurcate cercaria. Each stage resides in freshwater, both are short-lived being lecithotropic (non-feeding), but are exquisitely adapted to facilitate parasite transmission, by per-cutaneous routes, from vertebrate to intermediate snail host and vice versa [2]. This evolutionary specialisation has led to striking differences in the morphology, physiology and behaviour of the miracidium and cercaria, respectively [3, 4]. Although each stage is just visible to the naked eye, under the microscope they are so radically different in form and function that piecing them together within a coherent lifecycle was a major scientific break-through just over one hundred years ago [5]. Elucidation of the lifecycle revealed vulnerabilities and identified suitable attack points to control this formidable foe.

In terms of contemporary control of schistosomiasis in sub-Saharan Africa, preventive chemotherapy (PC) campaigns implementing mass drug administration (MDA) of praziquantel (PZQ), a broad spectrum anthelminthic, are the foundation of several national control programmes [6]. Each year, millions of school-aged children receive treatment with donated PZQ [7, 8], the only available schistosomicidal drug [9, 10]. Looking to the future, treatment targets in the WHO 2020 Roadmap encourage further scale-up of PC campaigns but despite desirable features, drawbacks of MDA include the inactivity of PZQ against immature worms, the poor cure rates associated with singe treatments, the inability of treatment to guard against re-infection and the challenge of maintaining adequate treatment coverage in currently targeted groups [9, 11, 12, 13, 14, 15]. Extra efforts to maximise the impact of PC are well-discussed and ideally should be set within an integrated control strategy inclusive of: water, sanitation and hygiene (WASH) interventions, health education with behavioural change, environmental modification and snail control with focal mollusciciding [15, 16, 17, 18], as highlighted by the World Health Assembly (WHA) in resolution WHA65.21. Whilst there are challenges ahead [13, 19], there is optimism grounded on epidemiological evidence and theory that elimination of schistosomiasis transmission in certain settings is achievable [14, 20, 21, 22]. Progress towards elimination is outlined in the WHO 2020 Roadmap and as campaigns transition from morbidity- to transmission-related control, formal investigation of environmental transmission is required [6]. Surprisingly, there are no international or national guidelines to do so in sub-Sahara Africa. The WHA65.21 resolution called on the WHO to prepare guidance for Member States towards the elimination of transmission, to establish procedures for the confirmation of the interruption of transmission, and to support countries with post-elimination surveillance to prevent reintroduction of transmission. Collectively, these can be considered as interventions themselves from the ‘end game’ perspective.

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