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Interactions between parasite species may influence their distribution and abundance within communities. Experimental single-species infections of Diplostomum spp. in the gut of the definitive host, the Ringed-billed Gull (Larus delawarensis Ord, 1815), were compared with mixed-species infections to explore interactions among parasites. Three species of Diplostomum von Nordmann, 1832 (Digenea), designated as Diplostomum sp. 1, Diplostomum sp. 4, and Diplostomum baeri Dubois, 1937, were examined for intestinal distribution and fecundity in single and mixed infections. In single-species infections, most specimens of Diplostomum sp. 1 and D. baeri were recovered from the mid-region of the intestine, whereas Diplostomum sp. 4 were mainly present in the anterior region. Significant spatial displacement was observed only for D. baeri when Diplostomum sp. 1 was also present. Intensity was directly correlated to the number of occupied intestinal segments, and there was no significant difference in mean linear span for each species between single-species and mixed-species infections. Diplostomum sp. 4 had the highest mean number of eggs per worm in utero in single-species infections. In mixed-species infections, the fecundity of Diplostomum sp. 4 declined dramatically in the presence of Diplostomum sp. 1, whereas fecundity of Diplostomum sp. 1 increased in the presence of D. baeri. These results highlight interspecific interactions that may play a role in population dynamics of Diplostomum spp. and community structure. Key words: Diplostomum, Ring-billed Gull, Larus delawarensis, fecundity, intestinal distribution, mixed infections, linear span. Les interactions entre especes de parasites peuvent influencer leur repartition et leur abondance au sein de communautes. Des infections experimentales a une seule espece de Diplostomum spp. dans le tube digestif d'hotes definitifs, des goelands a bec cercle (Larus delawarensis Ord, 1815), ont ete comparees a des infections mixtes a plus d'une espece afin d'explorer les interactions de parasites. Trois especes de Diplostomum von Nordmann, 1832 (digeniens), designees Diplostomum sp. 1, Diplostomum sp.4et Diplostomum baeri Dubois, 1937, ont ete examinees pour determiner les perturbations intestinales qu'elles causent et leur fecondite dans des infections a une seule espece et des infections mixtes. Dans les infections a une seule espece, la plupart des specimens de Diplostomum sp.1etD. baeri ont ete recuperes de la region moyenne de l'intestin, alors que les Diplostomum sp. 4 etaient principalement presents dans la region anterieure. Un deplacement spatial significatif n'a ete observe que pour D. baeri quand Diplostomum sp. 1 etait egalement present. L'intensite etait directement correlee au nombre de segments intestinaux occupes, et il n'y avait pas de difference significative de l'etendue lineaire moyenne de chaque espece entre les infections a une seule espece et les infections mixtes. Diplostomum sp. 4 presentait le nombre moyen d'ceufs in utero par ver le plus eleve dans les infections a une seule espece. Dans les infections mixtes, Diplostomum sp. 4 montrait une baisse drastique de fecondite en presence de Diplostomum sp. 1, alors que la fecondite de Diplostomum sp. 1 augmentait dans les infections mixtes avec D. baeri. Ces resultats font ressortir des interactions entre especes qui pourraient jouer un role dans la dynamique des populations de Diplostomum spp. et la structure de leurs communautes. [Traduit par la Redaction] Mots-cles: Diplostomum, goeland a bec cercle, Larus delawarensis, fecondite, repartition intestinale, infections mixtes, etendue lineaire., Introduction A parasite's niche, its physical distribution and ecological role, is dependent on many different factors such as the presence of other species of parasites and potential interactions among them. [...]

Globally, amphibians face a variety of anthropogenic stresses that include exposure to contaminants such as agricultural pesticides. Pesticides may negatively affect amphibian immune systems, concomitantly increasing susceptibility to parasitism. We quantified nematodes and evaluated leukocyte profiles of Northern Leopard Frogs (Rana pipiens Schreber, 1782) collected from five wetlands in southwestern Quebec, Canada, that spanned a gradient of pesticide exposure. Three taxa of nematode parasites (Rhabdias ranae Walton, 1929, genus Oswaldocruzia Travassos, 1917, and genus Strongyloides Grassi, 1879) were sufficiently numerous for detailed evaluation. When all frogs were pooled, frog size was negatively correlated with nematode species richness, abundances of each of the three nematode species, and densities of three different leukocytes. When all frogs were pooled, there was strong evidence of both negative and positive associations between pairs of parasite species. However, none of the previous relationships was significant within wetlands. Our results reveal strong spatial organization of amphibian-parasite communities and illustrate the importance of controlling for sampling locale in evaluating host-parasite associations. Finally, although several response variables varied significantly among wetlands, causes of this variation did not appear to be related to variation in nematode parasitism or pesticide exposure. Key words: leucocytes, nematodes, Northern Leopard Frog, Oswaldocruzia sp., pesticides, Rana pipiens, Rhabdias ranae, Strongyloides sp. A l'echelle planetaire, les amphibiens sont confrontes a divers stress d'origine humaine incluant l'exposition a des contaminants tels que des pesticides agricoles. Ces derniers peuvent avoir une incidence negative sur les systemes immunitaires des amphibiens, les rendant du coup plus sensibles au parasitisme. Nous avons quantifie les nematodes et evalue les profils de leucocytes de grenouilles leopards (Rana pipiens Schreber, 1782) prelevees dans cinq milieux humides du sud-ouest du Quebec (Canada) etales le long d'un gradient d'exposition aux pesticides. Trois taxons de nematodes parasites (Rhabdias ranae Walton, 1929, le genre Oswaldocruzia Travassos, 1917 et le genre Strongyloides Grassi, 1879) etaient suffisamment nombreux pour permettre une evaluation detaillee. Quand toutes les grenouilles etaient regroupees, leur taille etait negativement correlee a la richesse specifique des nematodes, a l'abondance de chacune des trois especes de nematodes et aux densites de trois leucocytes distincts. Quand toutes les grenouilles etaient regroupees, il y avait de fortes indications d'associations tant negatives que positives entre paires d'especes de parasites. Cependant, aucune de ces relations n'etait significative a l'echelle du milieu humide individuel. Nos resultats revelent une forte organisation spatiale des communautes de parasites d'amphibiens et illustrent l'importance de tenir compte du lieu d'echantillonnage dans revaluation des associations hote-parasite. Enfin, si plusieurs variables de reaction differaient significativement d'un milieu humide a l'autre, les causes de ces differences ne semblaient pas reliees a des differences sur le plan du parasitisme par les nematodes ou de l'exposition aux pesticides. [Traduit par la Redaction] Mots-cles: leucocytes, nematodes, grenouille leopard, Oswaldocruzia sp., pesticides, Rana pipiens, Rhabdias ranae, Strongyloides sp., Introduction Many populations of amphibians are now relegated to wetlands in fragmented agricultural landscapes (Knutson et al. 2004). In these wetlands, amphibians are often exposed to runoff that contains contaminants [...]