Abstract: The purpose of this study is to investigate the pathogenicity and drug resistance of Clostridium perfringens (CP) derived from large-scale chicken farms, and to evaluate the preventive effect of avilamycin and tylosin phosphate premixture on Necrotizing enteritis (NE) caused by CP infection, which can guide the prevention and control of chicken NE. Feces were randomly collected in large-scale chicken farms with a history of NE caused by CP in Hebei and Shanxi Province. CPs were isolated and their toxin types were determined by multiplex PCR. Three strains of type A CPs were selected from three farms to infect 14-day-old SPF chicken at dose of 10⁹CFU for 5 days. Diarrhea and intestinal lesions were observed and the CPs' pathogenicity were evaluated. Minimum inhibitory concentration (MIC) of avilamycin, lincomycin and tylosin were measured using the microbroth dilution method. According to the MIC results, avilamycin premixture and tylosin premixture were selected to prevent chickens with no clinical symptoms from NE disease caused by CP by observing mental appetite, diarrhea symptoms and intestinal lesions, counting the CP detection rate and NE incidence, thus evaluate the prophylactic effect of the two drugs. Results were as follows:91 CP were isolated from 753 chicken stool samples, and all toxin types were type A. The results of the challenge test showed that both the incidence of NE lesions and intestinal lesion score were significantly higher in the A, B, C challenged group than D unchallenged group (P<0.05). Compared with the unchallenged group, the challenged group showed obvious diarrhoea symptoms and intestinal lesions, with significant differences (P<0.05), and all three CP strains could cause chicken NE. Results of drug sensitivity test:The MIC range of avilamycin, lincomycin and tylosin were 0.25-4, 0.125-128 and 0.25-32 μg·mL^-1, respectively. Results of drug efficacy test:Feeding avilamycin premixed for 21 consecutive days could decrease the NE symptoms or prevent the lesion deterioration, significantly reduce the incidence of NE and NE lesion score (P<0.05), and significantly reduce the CP detection rate (P<0.05). The effect of feeding tylosin premixture for 7 consecutive days was comparable to that of avilamycin premixture, but a few of the chickens showed NE symptoms after the drug withdrawal. At present, most of the CPs detected from large-scale poultry farms in Hebei, Shanxi Province are type A which can cause NE. Meanwhile, type A CP is still sensitive to avilamycin and tylosin. Their premix are highly effective in chicken NE prevention.

Key words: Clostridium perfringens, chicken necrotizing enteritis, toxin type, minimal inhibitory concentration, animal model, drug efficacy