This investigation was developed at the sabanas of the deparment of Casanare, Colombia, between june/2006 and april/2007, with the objective to classify the parasite population that affect gastrointestinal and pulmonary system. The equine faecal samples were taken of four towns of Casanare: Aguazul, Paz de Ariporo, Maní and El Yopal. Ten fresh samples were taken from each town, directly on pasture and completely radomized. The faecal samples were analyzed by MacMaster`s technique that showed gastrointestinal nematodes eggs of type Strongylido spp., Strongyloide spp., Triodonthoforus spp., Habronema spp., and Oxyuris spp.. By Coprocultives the parasite genere found were: Small strongylus (86.2%), Strongylus vulgaris (6.2%), Strongylus edentatus (3.6%), Strongylus equinus (2.8%), Strongyliodes westeri (1.1%) and Trichostrongylus axei (0.20%), and by Baermann`s technique it was not found Dyctiocaulus spp.

This study was conducted in the grasslands of the Department of Casanare, seeking to determine the degree of anthelmintic susceptibility or resistance against macrocyclic lactones in small populations of Strongylus in this region of the country. Samples were taken from four municipalities in the department: Aguazul, Paz de Ariporo, Maní and El Yopal, between June, 2006 and April, 2007, where ten fresh fecal samples were collected in each municipality, directly from the field and in a completely random way. The samples were processed using the MacMaster coprological technique, thus determining the highest count of fecal epg by municipality. L3 larvae were extracted from each of the six samples with the highest count of fecal epg through the coprological and Baermann-Wetzel tests with which the Larvae Development Analysis (LDA) test was run, detecting Small Strongylus highly susceptible to the action of macrocyclic lactones (ivermectin).