Assessment of tracheal mucosal thicknesses is a preferable method for evaluation of the immunogenicity of Mycoplasma gallisepticum vaccines in poultry

Live-attenuated vaccines are commonly used to control chronic respiratory disease (CRD) caused by Mycoplasma gallisepticum in poultry. A previous review found that measures of tracheal lesions are better indicators of the validity of a vaccine efficacy test than air sac lesions. Here we extend those observations by comparing the raw tracheal mucosal thickness (TMT) and gross air sac lesion score (ALS) data from published vaccine efficacy studies to provide insights into standardising methods for evaluation of M. gallisepticum vaccine efficacy by identifying the most discriminative and reproducible parameter to demonstrate the efficacy of vaccines and the validity of immunogenicity tests. Our analyses revealed that a higher proportion of trials detected a ≥ 80 % effectiveness of challenge based on TMT, with a significantly lower number of biological replicates, than ALS. Furthermore, a significantly higher proportion of vaccinated-and-challenged groups had a proportion protected of ≥80 %, a reduction in lesions of ≥30 % and a mitigated fraction of ≥0.80, with a significantly lower number of biological replicates, compared to the positive-control group when analyses were based on TMT rather than ALSs. These findings indicate that TMT is a more discriminative and reproducible parameter to assess the efficacy of a vaccine and the validity of an efficacy test, and hence that this should be the primary outcome variable used to evaluate M. gallisepticum vaccine efficacy studies. Furthermore, the ability of TMT to discriminate these groups with fewer biological replicates enhances animal welfare by reducing the number of animals needed for efficacy studies.