Safety and efficacy of an infectious bronchitis virus used for chicken embryo vaccination

Abstract

Commercial vaccines for in ovo vaccination have not yet been developed for infectious bronchitis virus (IBV), the major coronavirus in the poultry industry. Recombinant IBVs based on the Beaudette strain expressing the Beaudette spike protein (Beau-R) or that from the virulent M41 strain (BeauR-M41(S)) were assessed for their potential as prototype vaccines for application to 18-day-old embryos. Pathogenicity was assessed by observing the effect on hatchability, and/or the production of nasal discharge and/or the effects on ciliary activity in the trachea at various time points post hatch. In contrast to commercial IBV vaccines given in ovo, the Beau-R and BeauR-M41(S) strains did not reduce hatchability or cause nasal discharge, and caused minimal damage to the ciliated epithelium of the trachea. The presence of the spike protein from a virulent virus did not increase the pathogenicity of the virus according to the criteria used. Assessment of the BeauR-M41(S) strain for efficacy showed that it protected up to 90% of chicks against challenge with virulent IB virus (M41) in a dose dependent manner. Further egg passage of the BeauR-M41(S) strain (BeauR-M41(S) EP10) did not increase its pathogenicity though it did improve its efficacy, based on serology and protection against a virulent challenge. BeauR-M41(S) EP10 was more efficacious than BeauR-M41(S) protecting more birds against virulent challenge and providing a better serological antibody response. BeauR-M41(S) EP10 induced a serological response similar to that of a commercial vaccine given at day-old though the commercial vaccine provided slightly higher efficacy. These results are promising for the development of embryo safe efficacious IBV vaccines for in ovo application.

Fig. 1

The effect of IBV in ovo vaccination on hatchability. Groups of 33 (Experiment 1, panel A), 100 (Experiment 2, panel B), 60 (Experiment 3, panel C) or 50 (Experiment 4, panel D and Experiment 5, panel E) fertile SPF eggs were inoculated with either a commercial vaccine (CV1, Experiment 1: 10⁴, 10² or 10¹ EID₅₀ or CV2, Experiment 4: 10⁴ or 10¹ EID₅₀, panel D), M41-CK (Experiment 5: 10⁴ EID₅₀ panel E), a Beaudette-derived virus, or a placebo. The Beaudette-derived viruses were Beau-R (Experiment 1: 10⁶ EID₅₀ panel A, Experiment 2: 10⁴ EID₅₀ panel B, Experiment 4: 10⁴ EID₅₀ panel D), BeauR-M41(S) (Experiment 1: 10⁶ EID₅₀ panel A, Experiment 2: 10⁴ EID₅₀ panel B, Experiment 3: 10⁴ EID₅₀ panel C, Experiment 4: 10⁴ EID₅₀ panel D) or BeauR-M41(S) EP10 (Experiment 3: 10⁴ EID₅₀ panel C). The percentage of hatched birds at 21.5 days is shown.

Fig. 2

Analysis of post hatch clinical signs and ciliary activity. The hatched chickens following in ovo vaccination were assessed for nasal discharge 6 days post hatch (Experiment 1, panel A: Beau-R and BeauR-M41(S) 10⁶ EID₅₀, graded doses of a commercial vaccine as shown). The hatched chickens were euthanized and assessed for tracheal ciliary activity on day 6 post hatch (Experiment 1, panel B: Beau-R and BeauR-M41(S) 10⁶ EID₅₀, graded doses of a commercial vaccine as shown) or on days 2, 5 and 8 (Experiment 2, panel C: Beau-R, BeauR-M41(S) and BeauR-M41(S) EP10 10⁴ EID₅₀. A commercial vaccine CV1 was given at day-old 10⁴ EID₅₀ and tracheal ciliary activity assessed on days 5 and 8 (Experiment 2, panel C).

Fig. 3

Analysis of the serological antibody responses of hatched chickens following in ovo vaccination. The hatched chickens were bled after 4 weeks (Experiment 1, panel A and Experiment 3, panel C) or 6 weeks (Experiment 2, panel B) and sera analysed by ELISA for IBV-specific antibodies. Panel A: Beau-R and BeauR-M41(S) 10⁶ EID₅₀, graded doses of a commercial vaccine as shown; Panel B: Beau-R, BeauR-M41(S), BeauR-M41(S) EP10 10⁴ EID₅₀in ovo or CV1 commercial vaccine at 10⁴ EID₅₀ given at day-old; and Panel C: BeauR-M41(S) or BeauR-M41(S) EP10 10⁴ EID₅₀in ovo.

Fig. 4

Analysis of the efficacy of the candidate vaccines. Groups of 10 birds were challenged 4 weeks (Experiment 1, panel A and Experiment 3, panel C) or 6 weeks (Experiment 2, panel B) after hatch with virulent M41 via the oculonasal route. Birds were euthanized 5 and 7 days after challenge, the tracheas removed and assessed for ciliary activity over 10 areas. The scores are presented as percentage ciliary activity. Panel A: represents Beau-R and BeauR-M41(S) vaccinated in ovo at a dose of 10⁶ EID₅₀ and birds pooled from the 3 groups vaccinated with the commercial vaccine at graded doses. Panel B: represents Beau-R, BeauR-M41(S), BeauR-M41(S) EP10 10⁴ EID₅₀ inoculated in ovo or CV1 commercial vaccine at 10⁴ EID₅₀ given at day-old. Panel C: BeauR-M41(S) or BeauR-M41(S) EP10 10⁴ EID₅₀ inoculated in ovo.