The influence of different mollusc associations in the triple system: Bulinus wrighti (target mollusc: TM)—Schistosoma bovis (parasite)—Melanopsis praemorsa (non-target mollusc: NTM) is analysed. The results show that: (i) for a TM/NTM density ratio of 1/1, the NTM stimulates both growth of the TM and parasite cercarial production; (ii) for a TM/NTM density ratio of 1/5, the effects of the NTM on TM growth and cercarial production were not additive and were less marked than for the 1/1 ratio. Hypotheses on a chemical explanation for these density-dependent effects are discussed.

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