Chronic Heat Stress Is Associated with Brain Injury, Blood-Brain Barrier Impairment, and Neuroinflammatory Gene Expression in Broilers.

This study investigated the effects of chronic heat stress (HS) on brain injury in broilers and the associated molecular changes. A chronic HS model was established by exposing broilers to 35 °C from 08:00 to 20:00 daily from 21 to 42 days of age, and samples were collected at 28, 35, and 42 days of age. Chronic HS significantly impaired growth performance and was associated with histopathological and ultrastructural alterations in brain tissue. Serum antioxidant enzyme activities and the total antioxidant capacity were significantly reduced, whereas malondialdehyde levels were significantly increased, indicating sustained oxidative stress (OS). Blood-brain barrier (BBB) permeability, assessed by Evans blue extravasation, was significantly higher in HS birds and was accompanied by reduced mRNA expression of the tight junction-related genes ZO-1 and Claudin-5. In addition, chronic HS was associated with increased mRNA expression in genes related to cellular stress, oxidative stress, and inflammation, including key components of the TLR4/MyD88/NF-κB/NLRP3 pathway, as well as decreased expression of IL-4. These findings suggest that chronic HS is associated with enhanced OS, altered neuroinflammatory gene expression, and BBB impairment in the broiler brain. Overall, this study provides evidence that chronic HS is associated with brain injury in broilers and highlights a potential link among OS, inflammation-related transcriptional changes, and BBB dysfunction, thereby offering a basis for further mechanistic and interventional studies.