Effect of Soybean Isoflavones on Growth Performance, Immune Function, and Viral Protein 5 mRNA Expression in Broiler Chickens Challenged with Infectious Bursal Disease Virus

SIMPLE SUMMARY: Infectious bursal disease virus (IBDV) is characterized by inflammation and subsequent atrophy of the bursa of Fabricius and immune suppression. However, nutritional strategies are able to ameliorate the negative effects of viral infections. Therefore, the aim of the present study was to determine the effect of different levels of soybean isoflavones (SI) on broiler chickens challenged with IBDV. Based on the findings, supplemental 10~20 mg/kg SI may have a positive effect on broiler chickens infected with IBDV, probably because SI decrease the severity of bursa lesions and viral protein 5 mRNA expression, and have strong antioxidant activity. ABSTRACT: A total of 200 one-day-old male broilers were assigned to five groups, and each group consisted of four replicates with 10 birds per replicate. Chicks were fed the basal diet with 0 (non-infected control), 0 (infected control), 10, 20, and 40 mg/kg soybean isoflavones (SI) for 42 days. At 21 days of age, chickens were inoculated with an infectious bursal dose (causing 50% morbidity) of the infectious bursal disease virus (IBDV) BC 6/85 strain by the eye-drop and nasal route (except for the non-infected group). Average daily gain (ADG) and average daily feed intake (ADFI) decreased (p < 0.05) in broilers infected with infectious bursal disease virus (IBDV) from 22 to 42 days. However, infected broilers fed 10 and 20 mg SI/kg had the maximum (p <0.05) ADG and ADFI from 1 to 42 days. Body weight (BW) increased (p < 0.05) in infected broilers fed the 10 and 20 mg SI /kg diet. The bursa weight at 7 days post-infection (dpi) was increased (p < 0.05) by the supplemental 10 mg SI/kg diet. Infected broilers showed the highest (p < 0.05) bursa lesions, with an average score of 4.0 ± 0.0, while the severity of bursa lesions was decreased (p < 0.05) at 3 dpi and 7 dpi by the supplemental 20 mg SI/kg diet. Supplemental SI at 20 mg/kg decreased (p < 0.05) the viral protein 5 (VP5) mRNA expression at 3 dpi and 7 dpi. The level of interferon gamma (IFNγ) was elevated (p < 0.05) in the infected group at 3 dpi and 7 dpi as compared with the control group, while its level was decreased (p < 0.05) by supplemental 10 mg/kg SI at 3 dpi. The level of nuclear factor κB in the bursal tissue showed the lowest value (p < 0.05) with supplemental 10 and 20 mg SI/kg diet at 7 dpi. Supplemental 10, 20, 40 mg/kg SI improved (p < 0.05) the serum total antioxidant activity (T-AOC) in infected broilers at 3 dpi. In addition, the serum level of malondialdehyde (MDA) decreased (p < 0.05) in the group fed 20 mg/kg SI at 7 dpi. In conclusion, supplemental 10~20 mg/kg SI may have a positive effect on broiler chickens infected with IBDV, probably because SI decrease the severity of bursa lesions and viral protein 5 mRNA expression, and have strong antioxidant activity.