Behaviours in trematode cercariae that enhance parasite transmission: patterns and processes

Abstract

Cercariae, like miracidia, are non-parasitic larval stages implicated in the life cycle of all trematodes for the host-to-host parasite transmission. Almost all cercariae are free-living in the external environment. With a few exceptions (cercariae of Halipegus occidualis (Halipegidae) can live several months, Shostak & Esch, 1990a), cercariae have a short active life during which they do not feed, living on accumulated reserves. Most cercariae encyst as metacercariae in second intermediate hosts which are prey of the definitive host; in certain species, the interruption of the active life is achieved by an encystment in the external environment (or a simple immobile waiting strategy in a few species). In some two-host life cycles, the cercariae develop into adults after penetration (this is the case for various species causing human schistosomiasis). Some cercariae do not leave the mollusc which must then be ingested by the definitive host.

New scope on the relationship between rotifers and Biomphalaria alexandrina snails

“…Accordingly, the poor cercarial emergence together with the lazy cercarial swimming observed with cercariae shed from SG II-B snails, could account for the decline in the infectivity rates and the existence of non penetrating cercariae seen in this subgroup, as compared to SG II-A. Learning to distinguish normal from abnormal cercarial behavior is very important, since abnormal cercarial behavior likely pinpoints problem areas that might adversely affect infectivity [36,37] . The most influential anti-schistosome effect of the rotifer infestation is known to be the restriction of cercarial motility, which was subsequently accused of the decreased penetration of skin by rotifer contaminated-cercariae [16,21] .…”

“…This represents the normal S. mansoni cercarial behaviors; typically known to swim to the surface of water, rest momentarily by lying quiescent before resuming their swimming. By resting for a while, cercariae are therefore adapted to store their energy in order to maximize their chances to encounter their specific definitive host species [36] . Cercarial swimming behavior was studied in many species and the enormous diversity of movement patterns and responses to environmental and host cues was noticed [36,37] .…”

“…By resting for a while, cercariae are therefore adapted to store their energy in order to maximize their chances to encounter their specific definitive host species [36] . Cercarial swimming behavior was studied in many species and the enormous diversity of movement patterns and responses to environmental and host cues was noticed [36,37] . The normal swimming attitude of cercariae is belonging to SG II-A accounts for the shown successful infectivity of all mice in the present study, with the absence of any remaining non-penetrating cercariae at the end of the infectivity experiment in this subgroup.…”

“…Cercariae of schistosomes possess tails with longitudinal striated muscles extending beneath the tegument and used by cercariae to swim in their journey searching for the definitive host. Glycogen stores inside cercarial tail provide a non-renewable energy source, which is depleted during active swimming [36] . Cercarial swimming behaviors are known to be influenced by several stimuli; including intensity of light, chemical signals produced from the host and biotic pressures (for example competition with other organisms for common nutritional resources within the host) [38] .…”

“…In this work, cercarial attachment coincided with their failure to accomplish any animal infectivity, coupled with the presence of non-penetrating cercariae (cercariae failed to penetrate the mouse skin and cause the infection). Cercarial activity is known to relate to infectivity [36] . Conversely, cercarial attachment was shown to negatively correlate to infectivity to the definitive host, since altered cercarial behavior was shown to limit or completely inhibit their skin penetration capability [30] .…”