Probiotic supplementation containing Bacillus velezensis enhances expression of immune regulatory genes against pigeon circovirus in pigeons (Columba livia)

Aims In this study, we aimed to isolate and evaluate the efficacy of Bacillus velezensis as a probiotic and to assess its activity towards pigeons infected with pigeon circovirus (PiCV). Methods and Results Bacillus velezensis, isolated from pigeon faeces, was orally administered to pigeons for 60 days. After pigeons were challenged with PiCV, the PiCV viral load and expression of indicator genes for innate immunity were detected in spleen tissue and faeces of pigeons. Bacillus velezensis significantly reduced the PiCV viral load in the faeces and spleen of pigeons 5 days post‐challenge (dpc). The mRNA expression levels of treated pigeons showed that interferon‐gamma (IFN‐γ), myxovirus resistance 1 (Mx1), and signal transducers and activators of transcription 1 (STAT1) genes were upregulated, whereas no expression of interleukin‐4 (IL‐4) was detected. Moreover, toll‐like receptor 2 (TLR2) and 4 (TLR4) were significantly upregulated in probiotic‐treated pigeons (P