ackground:With the increasing number of free-range domestic chickens, helminth parasites have potentially become more of a threat to commercial flocks in recent years, and routine poultry deworming is needed to improve the efficiency of chicken production. The present study deals with a field trial to study the efficacy of two generally used anthelmintics, fenbendazole and levamisole, against gastrointestinal nematodes of domestic chickens in north-ern Iran.Methods:Of 45 domestic chicken flocks involved in the study, 20 flocks were selected to participate in fecal egg count reduction testing based on flock size from April 2017 to September 2018. The infected chickens were randomly divided into three equal groups of 30 each. Ninety chickens in the infected groups received one of the following treatments (d 0 of treatment): Group 1: 5 mg kg−1 body weight (BW ) fenbendazole for 3 consecutive days; Group 2: 16 mg kg−1 BW levamisole; Group 3 control: placebo, water + DMSO (dimethylsulfoxide). The efficacy of the treat-ments was evaluated by comparing fecal egg counts in the treated and control groups.Results:Examination of three flocks of chickens from the control group showed that 95.0% of the animals were infected with gastrointestinal nematodes with an average geometric value of 361 eggs per gram of feces. Fenbenda-zole at a dose of 5 mg kg−1 BW for 3 days showed an efficacy of 83.7% (P≥ 0.05), and levamisole at a dose of 16 mg kg−1 BW showed 71.8% efficacy (P≥ 0.05) with geometric mean eggs in a gram of feces of 100 and 199.6, respec-tively. In general, fenbendazole and levamisole treatment led to significantly lower activity. The result of this study revealed that fenbendazole was a better and more effective dewormer than levamisole on the three Iranian domestic chicken flocks, but the difference was not significant. Capillaria spp. were the most generally resistant nematodes fol-lowed by Trichostrongylus spp. and Amidostomum anseris.Conclusion:Our results indicated that fenbendazole and levamisole effectively reduced the number of nemathodes in three Iranian domestic chicken flocks. Given the results of our study, resistance can be expected in the parasitic

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