Comparative pathogenicity of Ma5 vaccine and a field isolate of the avian infectious bronchitis virus in SPF chickens.

Avian infectious bronchitis (IBV) is a worldwide distributed disease that causes significant economic losses in the poultry industry. The IBV virus, which belongs to the Coronaviridae family, can affect multiple bird systems, including the respiratory, gastrointestinal, reproductive, and renal tracts. It has also been reported to alter the composition of gut microbiota. Despite widespread vaccination programs, IBV remains a major health challenge. In Argentina, circulating genotypes include GI-1, GI-11, and GI-16; the latter comprises the A13 isolate, which was associated with a high-mortality outbreak in 2013. The Ma5 vaccine strain, which is widely used worldwide, belongs to the GI-1 genotype. It is used to prevent infections caused by Massachusetts phenotype variants. However, it remains unclear whether such vaccines may be more pathogenic than local field isolates. This study aimed to compare the pathogenicity of the A13 (GI-16) isolate with that of the Ma5 (GI-1) vaccine strain, as well as their impact on ileal microbiota in specific pathogen-free (SPF) chickens. The results showed that the Ma5 vaccine strain induced a higher proportion of inflammatory lesions in the trachea, kidney, and ileum. It also caused a significant reduction in villus height and the villus-to-crypt ratio in the intestinal mucosa. Additionally, higher viral load was detectable in the cecal tonsils in chickens immunized with the Ma5 strain. Regarding microbiota, vaccinated animals displayed greater alpha diversity and clear differentiation in beta diversity analyses compared to the control and A13 groups. In contrast, the A13 isolate did not produce notable lesions or significant alterations in gut microbial composition. These findings underscore the importance of evaluating not only the protective efficacy of vaccine strains, but also their safety profile and broader biological effects compared to circulating field strains. Furthermore, the results imply that despite its connection to a high-mortality outbreak, the A13 induced fewer alterations than Ma5 under the experimental conditions and at the time point evaluated, suggesting that this strain may be relevant for future studies aimed at vaccine development.