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 stages and mucosal larval stages were examined and counted in the ventral colon, dorsal colon and caecum. Thirty-three species of adult parasites were identified and their distribution in the three large intestinal tracts reported, together with the localization of 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) was 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 following: the selection of preferred sites in horse large intestinal helminths has been confirmed; all statistical 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 S. vulgaris in the caecum and ventral colon. When S. edentatus is in the caecum, the favourite site of S. vulgaris, this latter one decreases especially in the caecum. On the other hand, when S. edentatus is in the ventral colon, his favourite site, there is no negative relationship with S. vulgaris in the ventral colon and the positive correlation observed with bivariate analysis is mantained. The common belief that the large intestinal horse parasite community is isolationist, due to the absence of negative pairwise interaction, is threfore contradicted by the present study.