A second trial was performed in the present investigation to analyze the effect of the IVM on several biochemical and enzyme serum parameters indicators. According to the results achieved in the experiment 1, the sera from three groups of naturally infected foals (untreated, 0.2 mg/kg BW of oral IVM, and 1 mg/kg BW of topical IVM) were analyzed. Only CK and LDH exceeded normal ranges throughout the study, and a positive significant correlation between the cyathostomin FECs and LDH activity was also demonstrated. The increment in the serum concentrations of alkaline phosphatase and LDH has been related to the injury on the intestinal mucosa [20]. After the L3 cyathostomins exsheath in the gut lumen and enter the mucosa, two possibilities may occur: (1) larvae moult to L4, return to the lumen, and develop to adult stages, or (2) L3 arrest their development until appropriate stimuli [21]. The penetrationeemergence cycle of the larval stages causes mucosal damage, which worsens when the adult worms feed [22], thus explaining the elevated levels of LDH observed in the infected untreated foals during the trial.