Comparative analysis of two methods used to show interspecific associations in naturally acquired parasite nematode communities from the abomasum of ewes.

The aim of our study was to compare a multivariate and a univariate method to evaluate the associations between nematode species in natural infections. We used a data set based on the 3-year study of the community of abomasal nematodes in ewes of the middle-Atlas region of Morocco. Frequency data (percentage of each species in the individual host community) were processed by principal component analysis. Euclidean distances between species (D2) were calculated from projections of principal component analysis on three axes and compared to a bivariate estimate (1 - correlation coefficient R). Low distances were recorded between morphs of the same species (Teladorsagia circumcincta and T. trifurcata, for example), which indicates a positive association. Only two species, Trichostrongylus axei and T. circumcincta, were negatively associated having D2 values much over 1. The distances D2, i.e., multivariate measure of association intensity were similar in most cases to 1- R estimates, indicating probably the absence of strong multispecies association. The D2 fitted a Motomura geometric progression model indicating that associations were arranged in a continuum from low to high values, but they did not depart much from what could be expected from random associations for most species. The D2 interest is mostly to relate associations with environmental variables, and comparing with 1 - R, to assess the degree of interaction in the community.