This study aimed to determine the AR of gastrointestinal nematodes (GIN) to commercial drugs in sheep flocks naturally infected, grazing in irrigated (IA) and dry (DA) areas of the semiarid region in northeastern Brazil. Fecal egg count reduction tests (FECRT) were performed at 10 farms. From each flock, 36 adult sheep were selected and divided into five groups (G1 (0.08% ivermectin), G2 (10% albendazole), G3 (5% levamisole), G4 (1% moxidectin), G5 (10% closantel) and one control group, G6). All the commercial drugs were found to reduce the number of eggs per gram of feces (EPG). Resistance to ivermectin (37.1%), albendazole (52.1%), and levamisole (52.0%) was detected at all the farms, but nematodes proved to be susceptible to moxidectin (87.9%) and closantel (83.9%). The overall average efficacy of the commercial drugs was significantly higher (P < 0.05) in DA (49.2%), where moxidectin (90.4%) showed high effectiveness. The presence of the parasite Haemonchus contortus predominated at all the farms. The variables irrigated area (P = 0.002), intensive breeding (P = 0.018), uncovered enclosures (P = 0.05), cultivated (P = 0.043) and native/cultivated (P = 0.007) pastures, and rotational grazing (P = 0.013) were significantly associated with GIN infection; irrigated area (P = 0.009), semi-intensive breeding (P = 0.05), rotational grazing (P = 0.045), cultivated (P = 0.021) and native/cultivated (P = 0.04) pastures, and estimated weighing of animals (P = 0.002) were significantly associated with AR. Therefore, improved management practices and strategic deworming must be implemented to prevent the development of AR.
Keywords: Effectiveness; Gastrointestinal parasites; Multidrug resistance; Small ruminants.