LAUSR

This objective of this study was to evaluate the use of tulathromycin on the timing of appearance and number of four indicator organisms representing the gastrointestinal microbial community, the incidence of diarrhea and a measure of the systemic inflammatory profile in Holstein heifers. Twenty-six Holstein heifer calves were distributed between receiving (ATB+) or not receiving (ATB-) tulathromycin at a dose of 2.5 mg/kg by 12 h of age. Samples from the calves were collected at six times during the neonatal period. Stool samples were used to determine the dry matter content and quantitative analysis of specific indicator bacterial populations. Samples of whole blood and serum were collected to determine the total number of neutrophils, the number of CD62L+ neutrophils, quantity of haptoglobin, and to allow for ex vivo measurement of reactive oxygen species. A higher frequency of diarrhea was detected in the ATB+ calves (84.6%) than ATB- (53.8%) on days 13-15 (P = 0.084). ATB- calves had a greater number of Bifidobacterium in stool on day 3-5 (P = 0.002), and on days 7-9 (P = 0.018). The ATB+ calves tended to have a higher number of Escherichia coli in stool on days 20-23 and days 27-30 (P = 0.052 and P = 0.072). Both the total number of neutrophils (P = 0.013) and the capacity for ROS production was higher in ATB- (P = 0.038) than ATB+ calves at all points tested. ATB+ calves had higher levels of haptoglobin (P = 0.032) on days 13-15. Administration of tulathromycin appeared to negatively impact the establishment of a normal microbiome and to modulate the development of innate immune function.