Intramolluscan Inter-trematode Antagonism: a Review of Factors Influencing the Host-parasite System and its Possible Role in Biological Control

Abstract

Publisher Summary The study of intramolluscan single and double trematode infections has proved to be a useful approach in better understanding host–parasite relationship. Modification and disruption of snail infection with trematodes is reviewed and the parameters and characteristics of various infection combinations are described. Biologically, however, the inter-trematode reactions and the responses of the snail host offer remarkable opportunities to examine and isolate various aspects of this host–parasite relationship. The single species infection patterns differ widely between the pure sporocyst type, exemplified by S. mansoni, and the redial type, typified by P. segregatum. Characteristics of each are reviewed. Their interaction is then reviewed, based on original data and comparison with other studies. Redial predation (“direct antagonism”) is discussed in terms of relative activity, efficiency, and predatory response to triggering stimuli in the snail. Possible mechanisms of indirect antagonism are reviewed, with a fuller consideration of snail immunity as induced by these infections and measured experimentally. The parameters or characteristics of single-species infection are described and related to one another in a preliminary fashion as an “adaptation index” (AI), which includes such parameters as infection rate, period for 50% snail death, and period to cercarial shed. The trematode method is locally applicable, using indigenous strains supplied and sustained at a high level by continual reseeding with eggs of the controlling trematode (or processed faeces from the appropriate final hosts). Effective biocontrol as a self-sustaining life-cycle appears highly improbable, as is the expectation that this method can be adopted on a wide scale with the same controlling agent.