Spatial niche competition among helminth parasites in horse's large intestine.

The spatial distribution of large intestinal helminth parasites of 50 horses was studied. Both adult and mucosal larval stages were examined and counted within the ventral colon, dorsal colon and caecum. Thirty-three species of adult parasites were identified and their distribution in the three large intestinal regions reported, together with the localization of cyathostome encysted larvae and of empty mucosal cysts. In order to highlight interspecific interactions, both Spearman pairwise correlation (among all parasites) and multivariable negative binomial regression (among the most abundant parasites) were performed. Interactions among parasite species are very important from the ecological and parasitological point of view as they can modulate the abundance and the spatial distribution of parasites, as well as the composition of the infracommunity. The main results were: the selection of preferred sites of horse's large intestinal helminths has been confirmed; all statistically significant pairwise correlation coefficients (359 out of 666) were positive; the build up of multivariable regression models brought to light both negative and positive interactions; the most interesting competitive interactions have been observed between the most abundant and relatively large strongyle species: Strongylus edentatus and Strongylus vulgaris in the caecum and ventral colon. When S. edentatus is in the caecum, the favourite site of S. vulgaris, the latter decreases especially in the caecum. On the other hand, when S. edentatus is in the ventral colon, its favourite site, there is no negative relationship with S. vulgaris in the ventral colon and the positive correlation observed with bivariate analysis is maintained. The common belief that the horse large intestinal parasite community is isolationist, due to the absence of negative pairwise interaction, is therefore contradicted by the present study.