Behaviours in trematode cercariae that enhance parasite transmission: patterns and processes.
TL;DR: Cercariae, like miracidia, are non-parasitic larval stages implicated in the life cycle of all trematodes for the host-to-host parasite transmission.
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.
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TL;DR: The results suggest that the small increases in air and water temperature forecast by many climate models will not only influence the geographical distribution of some diseases, but may also promote the proliferation of their infective stages in many ecosystems.
Abstract: Global warming can affect the world's biota and the functioning of ecosystems in many indirect ways. Recent evidence indicates that climate change can alter the geographical distribution of parasitic diseases, with potentially drastic consequences for their hosts. It is also possible that warmer conditions could promote the transmission of parasites and raise their local abundance. Here I have compiled experimental data on the effect of temperature on the emergence of infective stages (cercariae) of trematode parasites from their snail intermediate hosts. Temperature-mediated changes in cercarial output varied widely among trematode species, from small reductions to 200-fold increases in response to a 10 degrees C rise in temperature, with a geometric mean suggesting an almost 8-fold increase. Overall, the observed temperature-mediated increases in cercarial output are much more substantial than those expected from basic physiological processes, for which 2- to 3-fold increases are normally seen. Some of the most extreme increases in cercarial output may be artefacts of the methods used in the original studies; however, exclusion of these extreme values has little impact on the preceding conclusion. Across both species values and phylogenetically independent contrasts, neither the magnitude of the initial cercarial output nor the shell size of the snail host correlated with the relative increase in cercarial production mediated by rising temperature. In contrast, the latitude from which the snail-trematode association originated correlated negatively with temperature-mediated increases in cercarial production: within the 20 degrees to 55 degrees latitude range, trematodes from lower latitudes showed more pronounced temperature-driven increases in cercarial output than those from higher latitudes. These results suggest that the small increases in air and water temperature forecast by many climate models will not only influence the geographical distribution of some diseases, but may also promote the proliferation of their infective stages in many ecosystems.
389 citations
Cites background from "Behaviours in trematode cercariae t..."
...Although in many species the rate of cercarial emergence from snails is not constant and often shows daily peaks at particular times (e.g., Combes et al. 1994), this had no effect on the estimate of Q10 values....
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TL;DR: The high complexity, specificity and diversity of host-recognition strategies suggest that host finding and host recognition are important determinants in the evolution of parasite life cycles.
161 citations
TL;DR: It is shown that life cycle truncation has evolved independently many times in the phylogeny of trematodes, and the study of shorter life cycles offers an opportunity to understand the forces shaping the evolution of life cycles in general.
150 citations
TL;DR: Physiological analyses of the behaviour of several cercarial species which actively find and invade their hosts have revealed very complex sequences of behaviour patterns and responses to very different stimuli from the environment and the host.
Abstract: Physiological analyses of the behaviour of several cercarial species which actively find and invade their hosts have revealed very complex sequences of behaviour patterns and responses to very different stimuli from the environment and the host. A result of these physiological studies is that the behaviour patterns of each of the species investigated are surprisingly individual. The behavioural patterns of host-finding of those species analysed in some detail reveal profound adaptations to maximize transmission success. This can be demonstrated for movement patterns during swimming, for responses to environmental conditions such as gravity, light and temperature, for responses to stimuli emanating from the host such as shadows, water turbulence and chemical compounds and especially for the responses after contact with the host. The behaviour patterns can be interpreted as adaptations to: (1) dispersal by leaving the habitat of the snail intermediate host and distribution within the area; (2) long survival by energy saving swimming behaviour, by avoiding responses to inappropriate stimuli, by selecting favourable microhabitats and probably by avoiding predation; (3) finding and invading particular host types by selecting microhabitats frequented by the hosts and responding to sequences of specific stimuli emanating from the hosts.
129 citations
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1,546 citations
TL;DR: In this paper, the importance of parasitic species as regulators of host population growth is examined in light of empirical evidence and the type of information required from field studies to facilitate critical assessment of theoretical predictions.
Abstract: SUMMARY (1) Three categories of biological processes are shown to have a destabilizing influence on the dynamical behaviour of model host-parasite associations: parasite induced reduction in host reproductive potential, parasite reproduction within a host which directly increases parasite population size and time delays in parasite reproduction and transmission. (2) The importance of parasitic species as regulators of host population growth is examined in light of empirical evidence. Data from two particular laboratory studies used to indicate the magnitude of this regulatory influence. Suggestions are made concerning the type of information required from field studies to facilitate critical assessment of theoretical predictions.
1,118 citations
TL;DR: In parasitic life cycles, transmission events, that is, the passage of a parasite from an "upstream" host to a "downstream"Host, often involve behavioral adaptations that result in an increase of the chances of the parasite's being actually transmitted.
Abstract: In parasitic life cycles, transmission events, that is, the passage of a parasite from an "upstream" host to a "downstream" host, often involve behavioral adaptations that result in an increase of the chances of the parasite's being actually transmitted These behaviors stem from evolutionary changes in the parasites' genomes: either the parasites select responses (their own responses or those they provoke in upstream hosts) to stimuli arising from downstream hosts or from the environment where they live, or they select stimuli (arising from themselves, or from alterations they provoke in upstream hosts) that are addressed to downstream hosts The various processes are differentiated on the basis of behavioral adaptations and illustrated by examples The difficulty of demonstrating that the traits are actually the result of a selective process favoring transmission and several unresolved questions are discussed
178 citations