Impact of heat stress on immune status of sheep

Abstract Text: The objective of this study was to examine the effects of heat stress on sheep physiology. Australian Merino wethers (n=144; 44.02 ± 0.32 kg) were randomly allocated to treatment in a climate controlled facility (CCF: 4 rooms; 3 pens/room; 6 sheep/pen; pen = 2.32 m²). The treatments were: hot (HOT) and thermoneutral (TN). Treatment were replicated 4 times and each replication ran for 29 d. Dry bulb temperature (DBT) and relative humidity was obtained every 10 min. From these data a temperature humidity index (THI) was calculated: THI = DBT – {(0.31 – 0.31× RH/100) × (DBT – 14.4)}. During HOT means for DBT, RH and THI were: 32.5 ± 0.40 ^oC, 62.5 ± 7.22 % and 30.5 ± 0.55 respectively. During TN the values were 19.7 ± 0.40 ^oC, 79.9 ± 7.22 % and 19.4 ± 0.55. Respiration rate (RR) were obtained 3 hourly between 0800 and 1700 h. Rumen temperature (TRUM) was recorded at 10 min intervals using RFID rumen temperature boluses. Blood (2 × 10 mL) was collected by jugular venipuncture on entry and then every 7 d. Plasma concentrations of interleukin-1β (IL-1β), interferon-gamma, tumour necrosis factor α, lipopolysaccharides (LPS), haptoglobin, alkaline phosphate (ALP), gamma-glutamyl transpeptidase (GGT), creatine kinase (CK) and creatinine were determined. The MIXED procedure (SAS Inst. Inc. Cary, NC) was used. The model fitted included terms for replicate and treatment, a term for collection and a collection × treatment interaction. Mean RR of HOT was greater (P < 0.01) at 140.0 ± 3.55 breaths per minute (bpm) compared with TN at 74.9 ± 3.55 bpm. The HOT group had a greater (P < 0.05) TRUM (40.4 ± 0.03 ^oC) than the TN group (39.9 ± 0.04 ^oC). ALP (P = 0.0003), GGT (P = 0.0158) and IL-1β (P < 0.05) were all lower in the HOT sheep. Creatinine concentration (P = 0.0038) and CK (P > 0.05) were higher in the HOT sheep. LPS concentration was greater (P < 0.05) in HOT compared with TN. The remaining parameters were not affected (P > 0.05) by treatment. Elevated CK, creatinine and TRUM suggest that the HOT sheep were heat stressed. There is some evidence pointing to impaired immune status. However, the data is equivocal, and in some cases confounding (e.g. greater IL-1β expression in TN, but greater LPS in HOT).

Keywords: Sheep, Heat stress, Blood parameters