Efficacy of a bivalent vHVT-IBD vaccine against early challenge with a reassorted IBDV strain
(Northwestern European A3B1)
Willem Dekkers1, Sjaak de Wit1,2, Irene Jorna1, Yee Cheng Lee3, Stephane Lemiere3
1 Royal GD, Deventer, the Netherlands;
2 Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands;
3 Boehringer Ingelheim Animal Health B.V.
Since 2016, a new reassorted infectious bursal disease virus (IBDV) strain has become prevalent in Northwestern Europe. This virus combines segment A of vvIBDV (DV86-like) and segment B of a classical attenuated strain (D78-like), resulting in genotype A3B1 according to Islam et al. (2021). This study aimed to test the efficacy of VAXXITEK® HVT+IBD (VAXX), a bivalent vHVT-IBD vaccine, in protecting against A3B1 at 18 days post-vaccination.
Materials and methods
At day of hatch (day 0) 35 SPF-broilers were randomly divided into three groups (Table 1). The birds were housed in three different HEPA-filtered isolators. Vaccination was performed at day 0 by the subcutaneous route, using a commercial batch of vaccine and diluent at a dose of 103.91 PFU/0.4 mL per bird. Challenge was performed at day 18 by the ocular nasal route with 102.30 EID50/0.1 mL per bird. The study ended at day 28, with determination of the bursa to body weight ratios (BBWR) and bursa lesion scores (BLS). BBWR were calculated as the weight of the bursa in grams divided by the body weight before bleeding in grams and multiplied by 1,000. BLS were scored according to Table 2 after haematoxylin and eosin staining. BLS were divided in two categories (high BLS defined as BLS 4 and 5; low BLS defined as BLS 1, 2 or 3) and analyzed by a Chi-square test. BBWR were tested for normal distribution and analyzed by a linear regression model.
Results
The average BLS in Mock-A3B1 was 4.9, which was significantly higher than in VAXXA3B1 (1.6) (p<0.001) and Mock-Mock (0) (p<0.001). The BLS in VAXX-A3B1 did not differ significantly from Mock-Mock (see Figure 1). Additionally, the average BBWR in VAXX-A3B1 (2.1) was not significantly different (p=0.348) from Mock-Mock (2.4). The average BBWR in Mock-A3B1 (0.6) was significantly lower than VAXX-A3B1 (p<0.001) and Mock-Mock (p<0.001) (see Figure 2).
Note. N=35 SPF-broilers. VAXXITEK® HVT+IBD (VAXX) or mock vaccinated at day 0, A3B1 or mock challenged at day 18. Bursa lesion scores (BLS) and bursa to body weight ratios (BBWR) were assessed 10 days after challenge (day 28). Each data point represents an individual bird. Horizontal bars represent group medians.
w.dekkers@gdanimalhealth.com www.gdanimalhealth.com
Note. N=35 one-day-old SPF-broilers.
Mock-Mock: mock vaccinated and mock challenged. VAXX-A3B1: VAXX vaccinated and A3B1 challenged. Mock-A3B1: mock vaccinated and A3B1 challenged.
Table 2. Bursa lesion scoring system Score Follicles affected Morphology
0 0% No lesions, normal bursa
1 1–25% Lymphodepletion with influx of heterophils
2 26–50% Lymphodepletion with necrosis and heterophils
3 51–75% Lymphodepletion with necrosis and heterophils
4 76–99% Nearly complete lymphodepletion with hyperplasia, cysts, necrosis and heterophils
5 100% Nearly complete lymphodepletion, complete loss of follicular structure, epithelial folding; fibrosis
Note. Bursa lesion scoring system adapted from Muskett et al. (1979).
Conclusions
This study shows that VAXXITEK® HVT+IBD, a bivalent vHVT-IBD vaccine, provides effective protection against the reassorted Northwestern European A3B1 IBDV strain at day 18 post-vaccination. The significant reduction in BLS and maintenance of a high BBWR in the vaccinated group showed that the vaccine reduces the immunosuppressive effects of the virus and prevents severe damage to the bursa.
References
Muskett 1979 PMID: 224563; Islam 2021 PMID: 33410703.
