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Title: Pre-natal heat load affects bacterial levels and innate immunity in neonatal calves
Journal: J Dairy Sci
Accepted date: 2014 May 21
Interpretive summary: Heat stress suppresses immunity, making animals more susceptible to bacterial infections. Additionally, field observations suggest that calves have greater morbidity and mortality when they are born after a heat event. However, scientific evidence is still lacking, limiting the development of targeted interventions. Therefore, a study was conducted with the objective of determining effects of pre-natal seasonal heat load on bacterial levels and innate immunity in neonatal calves. Calves born in late winter (February and March) and summer (June to August) of 2010 on 2 commercial dairies (dairy 1 and 2) located in Hawaii were studied. Hide and udder swabs from dams on dairy 1 were taken shortly after calving. Rectal and nasal swabs from calves on the dairy 1 were taken 1, 2, and 3 week after birth. Total aerobes (total bacteria) and total coliforms (suggesting fecal contamination) were determined for all samples. Jugular blood samples from calves on dairy 2 were collected on week 1, 2, 3, and 4 after birth to determine blood leukocyte RNA expression of a cell surface protein involved in detection of bacteria, toll-like receptor 4, and an inflammatory protein, tumor necrosis factor-alpha. Colostrum samples from dams on both dairy farms were collected within 48 hours after parturition, and bacterial population and antibody concentration were determined. Compared to the winter cows, summer cows had greater hide and udder total aerobe counts, but coliform counts did not differ. Both aerobes and coliforms in colostrum samples did not differ between winter and summer cows. Winter calves had fewer nasal aerobe counts than summer calves at week 1 and 3, but no differences were found in rectal aerobe counts between winter and summer calves. Winter calves had greater nasal coliform counts at week 2. Rectal coliform counts in winter calves were also greater than in summer calves throughout the 3-week sampling. The greater heat load did not affect colostrum antibody concentrations. No difference in toll-like receptor 4 expression was detected between winter and summer calves. However, tumor necrosis factor-alpha expression in summer calves was less at week 2 compared to winter calves. Initial activity and oral behaviors were least for the summer calves, but were greatest for summer calves by the end of the study. The reverse was true for lying behavior. These results support recent recommendations for cooling of dry cows in summer months, but also demonstrate areas to focus change to improve profitability of dairies by improving calf morbidity and mortality rates.