Analysis of serological profiles in SPF chickens infected with classical or variant infectious bronchitis viruses

Infectious bronchitis virus (IBV) remains a major challenge in poultry farming. Both an ELISA assay and Haemagglutination Inhibition (HI) test are standard serological methods for detecting antibodies against IBV, to monitor vaccine efficacy or to evaluate potential exposure to virulent field strains. This study evaluates clinical and serological differences in SPF chicks following infection with IBV strains QX, M41 or 793B. Clinically, the onset of respiratory signs varied among strains. All three IBV strains (QX, M41, and 793B) caused respiratory signs, with the M41-inoculated birds exhibiting the highest number of respiratory signs. For the detection of IBV antibodies, significant differences were observed among ELISA kits. The QX antisera group exhibited the highest correlation between ELISA kits, whereas the 793B group showed the lowest. Homologous HI titres correlated strongly with ELISA kits A, B, and C in the QX antisera group, whereas other homologous HI titres showed no significant correlation with any ELISA kit. Heterologous HI tests showed high sensitivity, demonstrating some antigenic similarity among QX, M41, and 793B. However, reduced cross-reactivity indicates that this assay can identify the dominant infection strain in flocks. ELISA kit A was the most effective for QX antisera, and other kits showed variable sensitivity. The performance of the ELISA kit varied depending on the specific kit and the antisera group. Principal component analysis (PCA) identified two major components explaining 67.52% of the variance, capturing ELISA and HI test variability. Strong correlations were observed between ELISA kits A and B, as evidenced by Pearson correlation coefficients. Additionally, the M41 HI titre predicted the positivity of the ELISA kits B and C, indicating that false-negative ELISA results were observed when the M41 HI titres were low. Our findings emphasise the need to select the appropriate serological method for each antisera group to ensure accurate detection of different IBV strains.