The booster immunization using commercial vaccines effectively protect chickens against novel variants of infectious bursal disease virus (genotype A2dB1)

Abstract

The novel variant IBDV (nVarIBDV, genotype A2dB1), characterized by bursal atrophy of fabricius and decreased lymphocytes, has been emerging on a large scale in Asia (including China) since late 2018. nVarIBDV is a new threat to the poultry industry, yet the currently licensed commercial vaccines, including the live viral vector vaccine, IBDV immune complex vaccine or VP2 subunit vaccine, are ineffective against nVarIBDV infection. In this study, specific-pathogen-free (SPF) chickens and broilers divided into 3 groups were vaccinated with the live viral vector vaccine, the VP2 subunit vaccine or the IBDV immune complex vaccine at 1 day-old, respectively. The SPF chickens received a secondary vaccination with the live B87 strain vaccine at 11-day-old. The bursa/body weight ratio, histopathology lesion of the bursa, and the differentiation between infected and vaccinated animals (DIVA) by qRT-PCR confirmed that the live viral vector vaccine or immune complex vaccine plus live B87 strain booster could provide at least 80% protection against the FJ2019-01 strain of nVarIBDV in SPF chickens. The broilers also received a secondary vaccination using a live W2512 G-61 strain vaccine at 14-day-old, and analyses showed that the VP2 subunit vaccine or immune complex vaccine plus the live W2512 G-61 strain booster also provided more than 80% protection against the FJ2019-01 strain of nVarIBDV. Unfortunately, the live viral vector vaccine plus live W2512 G-61 strain booster provided poor to moderate protection against FJ2019-01 in broilers. These findings suggest that combining commercial vaccines with rational booster immunization can effectively protect chickens against an nVarIBDV challenge.

Keywords: booster immunization; efficacy; infectious bursal disease virus; novel variant IBDV.

Figure 1

Efficacy of commercial vaccines and booster immunization against the nVarIBDV FJ2019-01 strain in SPF chickens. (A, B) ELISA detection of IBDV antibodies at 11 or 21-day-old (cutoff titer = 1002), respectively. (C) Bursa: body weight index of chickens challenged with the FJ2019-01 strain after 10 d. (D) Histopathology lesions in the bursa at 10 d after chickens were challenged with the IBDV FJ2019-01 strain. (E) The bursal viral load in FJ2019-01-challenged chickens. Samples were quantified using the standard curve, and the average viral genome copy number was calculated. The results were shown as the viral genome copy number of the log₁₀ (viral copies). G1: negative control; G2: challenged control; G3: vaccinated with live viral vector vaccine at 1-day-old and live B87 strain vaccine at 11-day-old; G4: vaccinated with VP2 subunit vaccine at 1-day-old and live B87 strain vaccine at 11-day-old; G5: vaccinated with IBDV immune complex vaccine at 1-day-old and live B87 strain vaccine at 11-day-old.

Figure 2

Pathological sections of bursa in different groups of SPF chickens challenged with the nVarIBDV FJ2019-01 strain. Three typical images were selected from each group.

Figure 3

Efficacy of commercial vaccine and booster immunization against the nVarIBDV FJ2019-01 strain in broilers. (A, B) ELISA detection of IBDV antibodies at 14- and 24-day-old (cutoff titer = 1002). G0 was maternal IBD antibody titers in broilers at 1-day-old. (C) Bursa: body weight index of chickens challenged with the FJ2019-01 strain after 10 d. (D) The bursal viral load in FJ2019-01-challenged chickens. Samples were quantified using the standard curve, and the average viral genome copy number was calculated. The results were shown as the viral genome copy number of the log₁₀ (viral copies). G1: negative control; G2: challenged control; G3: vaccinated with live viral vector vaccine at 1-day-old and live W2512 G-61 strain vaccine at 14-day-old; G4: vaccinated with VP2 subunit vaccine at 1-day-old and live W2512 G-61 strain vaccine at 14-day-old; G5: vaccinated with IBDV immune complex vaccine at 1 day-old and live W2512 G-61 strain vaccine at 14-day-old.

Figure 4

Pathological sections of bursa in different groups of broilers challenged with the nVarIBDV FJ2019-01 strain. Three typical images were selected from each group.