Infectious Bronchitis Virus (IBV) strain M41 exhibits a broad tissue tropism, but its potential to persist in non-traditional sites remains poorly understood. This study investigated the pathogenesis of IBV M41 in specific pathogen-free (SPF) chicks, focusing on both expected (trachea, lung, kidney, bursa of Fabricius, rectum, and caecal tonsil) and unexpected (brain, thymus, bone marrow, and testes) tissues. One-day-old chicks were inoculated intranasally and intraocularly with IBV M41 (4.9 log10 EID50/mL) and monitored for clinical signs, viral distribution, and immune responses over 14 days. Clinical signs, including respiratory distress and depression, peaked by day 10, with recovery by day 14. Post-mortem lesions revealed tracheal congestion, renal swelling, and lung congestion. Virus isolation and RT-PCR detected IBV in all expected tissues up to day 10, while the caecal tonsil and testes remained positive until day 14. Immunofluorescence confirmed active viral replication in tracheal, renal, and bursal epithelia, but bone marrow was consistently negative. Serological analysis demonstrated specific IgM and IgG responses by day 14. These findings highlight the caecal tonsil and testes as potential sites for prolonged IBV presence, warranting further investigation into their roles in viral persistence. The study underscores the utility of combining virological, molecular, and serological methods to elucidate IBV pathogenesis and informs future research on tissue-specific viral dynamics.

Infectious Bronchitis Virus (IBV)

Keywords: Tissue tropism, Pathogenesis, Caecal tonsil, Viral persistence, Immune responses.