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On the efficacy of anti-parasite behaviour: a case study of tadpole susceptibility to cercariae of Echinostoma trivolvis

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Many animals respond behaviourally to the infective stages of parasites, but the efficacy of such responses in reducing risk of parasitism often is not established. It was found that tadpoles of Rana clamitans Latr., 1801 (green frogs) and R. sylvatica LeConte, 1825 (wood frogs) increased their activity when exposed to live infective stages (cercariae) of the trematode Echinostoma trivolvis Rudolphi, 1809. The susceptibility to parasitism for green frog tadpoles subjected to three different treatments was compared. Tadpoles were housed at 20°C and allowed to respond to cercariae, held at 6–8°C and showing reduced behavioural responses, or anesthetized and showing no responses. Low levels of parasitism were found for tadpoles that responded behaviourally to cercariae; such responses are expected to occur under normal field conditions in the absence of factors suppressing activity of tadpoles. We also demonstrate that infectivity of E. trivolvis cercariae to non-responding (anesthetized) wood frog tadpoles was higher at warm than at cool temperatures. Thus, lowered parasitism at warm temperatures in the first experiment likely resulted from host behavioural responses and not from low infectivity of cercariae. These results have implications for observing effects of environmental factors on susceptibility to parasitism where susceptibility is thought or known to be mediated by host behaviour.

De nombreux animaux réagissent par leur comportement aux stades infectieux des parasites, mais l’efficacité de telles réactions dans la réduction du risque de parasitisme n’est souvent pas établie. Exposés aux stades infectieux (cercaires) du trématode Echinostoma trivolvis Rudolphi, 1809, les têtards de Rana clamitans Latr., 1801 (grenouilles vertes) et R. sylvatica LeConte, 1825 (grenouilles des bois) augmentent leur activité. Nous avons comparé la susceptibilité de têtards de la grenouille verte au parasitisme dans trois conditions expérimentales. Les têtards ont été gardés à 20 ºC et libres de réagir aux cercaires, ou alors maintenus à 6–8 ºC et capables seulement d’une réaction comportementale réduite, ou enfin anesthésiés et incapables de réagir. Les têtards qui ont des réactions comportementales aux cercaires ont de faibles taux de parasitisme; de telles réactions se produisent probablement en nature en l’absence de facteurs qui inhibent l’activité des têtards. Nous démontrons aussi que l’infectivité des cercaires de E. trivolvis chez les têtards inactifs (anesthésiés) de la grenouille des bois est plus importante aux températures chaudes qu’aux fraîches. Le parasitisme réduit aux températures élevées dans la première expérience résulte donc vraisemblablement des comportements de réaction des hôtes et non d’une infectivité réduite des cercaires. Ces résultats ont des conséquences lorsqu’on observe les effets des facteurs environnementaux sur la susceptibilité au parasitisme, si l’on sait ou l’on soupçonne que la susceptibilité est reliée au comportement de l’hôte.

Document Type: Research Article

Publication date: November 1, 2006

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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