Effect of cooling during the dry period on immune response after Streptococcus uberis intramammary infection challenge of dairy cows

Heat stress in the dry period affects the immune status of dairy cows in the subsequent lactation. We hypothesized that cooling during the dry period improves immune response to postpartum intramammary infection (IMI) by environmental pathogens such as Streptococcus uberis. Cows were dried off 46 d before expected calving and assigned to cooling (CL, n=15) or heat stress (HT, n=15). Cooled cows were housed with sprinklers, fans, and shade, whereas the HT group had only shade. All cows were cooled postpartum. Rectal temperature (RT) and respiration rate (RR) were recorded thrice weekly during the dry period. From −46 to 42 d relative to calving, dry matter intake was recorded daily, and both body weight (BW) and body condition score (BCS) weekly. Milk yield and composition were recorded daily after calving. Streptococcus uberis IMI was induced at 5 d postpartum in a subset of cows (CL, n=5; HT, n=5). Blood was collected at 0, 12, 18, 24, and 36h after IMI. Hematological analysis was performed, and neutrophils isolated for RNA extraction. Immune response genes (TLR2, IL1-β, IL6, IL8, IL10, and TNFα) were assessed by real-time, reverse transcription-PCR. Relative to HT cows, CL cows had lower RT and RR during the dry period. The CL cows also consumed more feed prepartum but not postpartum, gained more BW prepartum but lost more BW in lactation, and had higher BCS score prepartum and a lower BCS postpartum. During 40wk of lactation, CL cows produced more milk (33.8 vs. 30.0kg/d) than HT cows but milk composition was not affected. Cows in the CL group had greater white blood cell counts and more neutrophils than HT cows during IMI. From 0 to 36h post-IMI, TNFα mRNA expression decreased, whereas that of IL6 and IL8 increased in both treatments. Additionally, CL cows had lower IL10 mRNA expression at 18h post-IMI. Expression of TLR2 mRNA decreased over time in both treatments. However, CL cows had greater overall TLR2 mRNA expression than HT. No differences were detected for mRNA expression of IL1-β, IL6, IL8, or TNFα. Cooling cows during the dry period alters immune function and neutrophil response to IMI in early lactation.