Field and laboratory findings following the large-scale use of intermediate type infectious bursal disease vaccines in Denmark.

Following a period of clinical outbreaks of very virulent infectious bursal disease virus (vvIBDV) in Denmark, the histological bursal lesion score (HBLS) was used on a national scale to screen broiler flocks vaccinated with intermediate IBD vaccines for lesions indicative of IBDV challenge. High lesion scores were detected in a high percentage of healthy and well performing flocks despite the lack of other indications of the presence of vvIBDV. RT-PCR and subsequent sequencing showed the frequent presence of H253Q and H253N IBDV strains that were genetically close to the sequence of the intermediate vaccines with a relative risk ratio of 13.0 (P < 0.0001) in intermediate vaccine A or B vaccinated flocks compared to unvaccinated flocks. The relevance of these H253Q and H253N strains was tested under experimental conditions using a protocol derived from the European Pharmacopoeia for safety of live IBD vaccines. The results confirmed the higher pathogenicity for the bursa of these strains compared to intermediate vaccines as well as the negative effect on antibody response to a Newcastle disease (ND) vaccination performed at the peak of the bursa damage. The efficacy of the ND vaccination was still 100% showing that the H253N and H253Q IBDV strains would be considered as safe vaccine viruses. In conclusion, the use of the HBLS to screen commercial broiler flocks vaccinated with intermediate IBD vaccines for the presence of vvIBDV does not seem to be a reliable method due to the frequent occurrence of H253N and H253Q strains in those flocks. For screening of IBD vaccinated flocks for the presence of vvIBDV or other field strains, the RT-PCR with subsequent sequencing seems to be most suitable.

The required sample size in vaccination-challenge experiments with infectious bronchitis virus, a meta-analysis.

For statistical, animal welfare and financial reasons the choice of the number of chickens per group in experiments is important. This estimation, together with the number of tracheal organ cultures (TOCs) that need to be examined from each chicken in order to assess protection, should be based on the difference in level of protection that one would like to be able to detect (effect size), the expected variability of the results between and within the chickens, the desired confidence level and the power of the study. To obtain data that would facilitate this estimation, a meta-analysis was performed on the data from 18 infectious bronchitis virus (IBV) vaccination-challenge experiments performed at the Dutch Animal Health Service Deventer, the Netherlands (GD) in order to determine and quantify the source of variation in the mean level of protection of different groups. For the calculations, 137 groups of chickens were subdivided into 10 clusters based on age (young or adult), vaccination (none, homologous or heterologous), challenge (IBV or mock infected) and location of vaccination (isolator at GD or in the field). The results were used to estimate the required number of chickens per group for the different clusters using 2, 5 or 10 TOCs per chicken to be able to detect effect sizes of 6.25%, 12.5%, 25% and 50% between groups of chickens with 95% confidence (P<0.05) and 80% power. The number of chickens that was required for the mentioned effect sizes varied greatly from 2 to 650. This meta-analysis provided data that allow research workers to estimate the number of chickens that should be included in each group in order to obtain reliable results based on particular combinations of infectious bronchitis vaccination and challenge strains as defined by the presented clusters.

Efficacy and safety of an attenuated live QX-like infectious bronchitis virus strain as a vaccine for chickens.

The attenuation of infectious bronchitis (IB) QX-like virus strain L1148 is described. The virus was passaged multiple times in embryonated specific pathogen free (SPF) chicken eggs, and at different passage levels samples were tested for safety for the respiratory tract and kidneys in 1-day-old SPF chickens. There was a clear decrease in pathogenicity for the respiratory tract and kidneys when the virus had undergone a large number of passages. Passage level 80 was investigated for safety for the reproductive tract in 1-day-old and 7-day-old SPF chickens. In 1-day-old chickens, 12.5% of the vaccinated birds had macroscopic lesions. No lesions were observed if the chickens had been vaccinated at 7 days of age. Passage level 80 was investigated for its ability to spread from vaccinated to non-vaccinated chickens and for dissemination in the body. The virus was able to spread from vaccinated chickens to groups of non-vaccinated chickens, and in the vaccinated birds the virus was found frequently in oro-pharyngeal and cloacal swabs. A fragment of the hypervariable region of the S1 protein of passage level 80 was sequenced and revealed nucleotide changes resulting in two amino acid substitutions. Passage level 80 was given additional passages to levels 82 and 85. Both passage levels were tested for efficacy in SPF chickens and passage level 85 was tested for efficacy in commercial chickens with maternally derived antibodies (MDA) against a challenge with QX-like strain IB D388. In both SPF chickens and chickens with MDA, the vaccines based on strain IB L1148 were efficacious against challenge.