Chemical Control of Snail Vectors as an Integrated Part of a Strategy for the Elimination of Schistosomiasis-A Review of the State of Knowledge and Future Needs

Abstract

WHO promotes the implementation of a comprehensive strategy to control and eliminate schistosomiasis through preventive chemotherapy, snail control, clean water supply, improved sanitation, behaviour change interventions, and environmental management. The transmission of schistosomiasis involves infected definitive hosts (humans or animals) excreting eggs that hatch (miracidia), which infect freshwater snail vectors (also referred to as intermediate snail hosts) living in marshlands, ponds, lakes, rivers, or irrigation canals. Infective larvae (cercariae) develop within the snail, which, when released, may infect humans and/or animals in contact with the water. Snail control aims to interrupt the transmission cycle of the disease by removing the vector snails and, by so doing, indirectly improves the impact of the preventive chemotherapy by reducing reinfection. Snail control was, for many years, the only strategy for the prevention of schistosomiasis before preventive chemotherapy became the primary intervention. Snails can be controlled through various methods: environmental control, biological control, and chemical control. The chemical control of snails has proven to be the most effective method to interrupt the transmission of schistosomiasis. The current review aims to describe the vector snails of human schistosomiasis, present the chemicals and strategies for the control of snails, the challenges with the implementation, and the future needs. Snail control can play a key role in reducing schistosomiasis transmission and, thus, complements other interventions for disease control. There is a need to develop new molluscicide products or new formulations and methods of applications for existing molluscicides that would target snail vectors more specifically.

Keywords: behavioural change; chemical snail control; community; environmental management; health education; molluscicides; schistosomiasis; snails; water supply and sanitation.

Figure 1

Neotropical (1–5) and African (6–15) Biomphalaria species, (1) B. glabrata; (2) B. tenagophila; (3) B. straminea; (4) B. havanensis; (5) B. helophila; (6) B. alexandrina; (7) B. angulosa; (8) B. camerunensis; (9) B. pfeifferi; (10) B. salinarum; (11) B. sudanica; (12) B. choanomphala; (13) B. rhodesiensis (14) B. smithi; and (15) B. stanleyi (prepared by HM).

Figure 2

Representative species of the Bulinus species groups, Oncomelania, Robertsiella, and Neotricula species. The B. africanus group: (1) B. abyssinicus; (2) B. africanus; (3) B. nasutus; (4) B. ugandae; (5) B. jousseaumei; (6) B. obtusus; (7) B. obtusispira; (8) B. umbilicatus; (9) B. globosus; (10) B. productus. B. truncatus/tropicus complex: (11) B. angolensis; (12) B. liratus; (13) B. natalensis; (14) B. tropicus; (15) B. nyassanus; (16) B. succinoides; (17) B. transversalis; (18) B. trigonus; (19) B. truncatus. The B. forskalii group: (20) B. bavayi; (21) B. beccarii; (22) B. canescens; (23) B. cernicus; (24) B. crystallinus; (25) B. forskalii; (26) B. scalaris; (27) B. senegalensis. The B. reticulatus group: (28) B. reticulatus; (29) B. wrighti. Asian species: Oncomelania hupensis. (30) smooth form and (31) ribbed form; (32) O. quadrasi; (33) Neotricula aperta; (34) Robertsiella kaporensis (prepared by HM).

Figure 3

(a) Preparation for molluscicide application in Zanzibar; (b) Molluscicide application in a temporary pond in Zanzibar; (c) Preparation of niclosamide for application in Cameroon; (d) Molluscicide application in China using a drone ((a–c): AGD; (d): courtesy NIPD, China).