poultry, innate immune system, immune system responses


Heat stress (HS) is a growing concern in broiler production. Little is known regarding the effect of HS on immune function. To examine the effects of HS on innate immunity, the local- and systemic-inflammatory responses to lipopolysaccharide (LPS) were examined in Cobb 500 male broiler chicks reared under thermoneutral (TN) or cyclic HS conditions. Beginning at four days of age, HS birds were subjected to 35 °C from 8:00 a.m. to 10:00 p.m. and TN temperatures from 10:00 p.m. to 8:00 a.m. At 37 days of age, four groups of broilers were formed: LPS-TN (8 broilers), phosphate-buffered saline (PBS)-TN (4 broilers), LPS-HS (8 broilers), and PBS-HS (4 broilers), with each broiler receiving LPS- (100 μg/mL) or PBS-treatments by intradermal pulp-injection of 12 growing feathers (GF; 10 μL/GF). Blood and GF were collected before (0 h) and at 6 and 24 h post-injection to determine leukocyte population changes. Locally, LPS-HS broilers had lower (P ≤ 0.05) levels (% pulp cells) of infiltrating heterophils and macrophages in GF-pulps at 6 and 24 h, respectively, compared to LPS-TN birds. In the blood, TN and HS broilers had similar baseline (0 h) concentrations (cells/μL) of heterophils, monocytes, eosinophils, and basophils, but HS broilers had lower (P ≤ 0.05) T- and B-lymphocyte levels. Concentrations of heterophils and monocytes were greatly elevated (P ≤ 0.05) at 6 and 24 h, respectively, only in LPS-TN broilers. Overall, results indicated that cyclic HS reduced both the local and systemic acute inflammatory responses to LPS in broilers, likely impairing their innate defense against microbial infection.