Abstract:

The aim was to investigate the efficacy of plant essential oil (Commercial name: Shu-ning 500) on chicks artificially infected with avian infectious bronchitis virus (IBV). A total of 260 1-day-old non-immunized healthy chicks were divided into 13 groups, including blank group, model control groups (A, B, C), preventive groups (A, B, C), treatment groups (A, B, C), and positive drug groups (A, B, C), with 20 chicks in each group. Except the blank group, all the chicks in group A were mono-infected with IBV strain GX-QZ20181028 (LX4 type) at 8-day-old, and all the chicks in group B were mono-infected with IBV strain GX-NN20200723 (Taiwan type) at 8-day-old. All group C chicks were co-infected with IBV strains GX-QZ20181028 (LX4) and GX-NN20200723 (Taiwan type) at 8-day-old. The preventive groups were given the drug(essential oil, 1 mL·L^-1 water)in drinking water from 3-day-old (5 days before infection). Both the treatment groups and the positive drug group (Shuanghuanglian Oral Liquid, 0.7 mL·L^-1 water) were administered when more than half of the chicks showed clinical symptoms, and the drug was administered continuously for 5 days. Clinical symptoms and efficacy were evaluated at 8, 9, 13, 14, 15, 16, 17, 18, 19, and 20 days old, while pathological histopathological changes, immune organ indexes, and viral loads in trachea and kidneys were observed at 19 and 22 days old. IBV-specific antibodies, as well as cytokines IFN-γ, IL-4, and IL-6 in sera were detected at 8, 9, 15, 17, 19, and 22 days old. The results showed that the protection rates of prevention group A, B and C were 75.00%, 85.00% and 70.00%, respectively. The effective rates of treatment group A, B and C were 92.31%, 78.57% and 77.78%, respectively. In the model control groups, cilia of a large number of epithelial cells in the tracheal mucosa decreased or disappeared, and degeneration and necrosis occurred, and lymphocytes were infiltrated in the renal mesenchyme, and no obvious lesions were observed in other groups. The viral loads of trachea and kidney in the prevention, treatment and positive drug groups were significantly lower than those in the model control groups (P < 0.05). The thymus and bursa indices in the model control groups were significantly lower than those in the remaining groups (P < 0.05). At 17 days old (the 5th day after administration), the antibody levels of prevention, treatment and positive drug groups increased, and at 22 days old (the 5th day after withdrawal), the antibody levels were significantly higher than those in the model control groups (P < 0.05). The levels of IL-4 and IFN-γ in the model control groups were significantly lower than those in other groups from pre-administration to the 5th day after withdrawal (P < 0.05) at 13 to 22 day-old (5 days before treatment to 5 days after withdrawal), and the contents of IL-6 were significantly higher than those in other groups (P < 0.05). In conclusion, under the condition of the current study, the plant essential oil has good preventive and therapeutic effects on chicks both mono-infected and co-infected with IBV, and the preventive and curative effects are mainly exerted through inhibiting viral replication, promoting immune organ development and antibody production, and regulating cytokines. The present study provides a new idea for the prevention and treatment of avian infectious bronchitis (IB), and also provides a reference for the effective treatment and prevention of coronavirus diseases in other animals.

Key words: infectious bronchitis virus, essential oil, prevention, treatment