Previous research in our laboratory demonstrated hormones known to respond to changing day length can influence fecal shedding of E. coli O157:H7 in cattle. Continuing with this research, we examined the effect of serum prolactin concentrations on fecal shedding of E. coli O157:H7 and cellular immune response. 2?Bromo???ergocryptine methanesulfonate salt (BROMO) a dopamine agonist and sulpiride a D?2 dopamine receptor blocker were administered to decrease and increase prolactin levels, respectively. Fifteen Holstein steers experimentally infected with E. coli O157:H7 were randomly assigned to receive BROMO (0.05 mg/ kg BW), sulpiride (0.05 mg/kg BW) or control (ethanol) via s.c. injection, twice daily. Fecal samples were collected daily and shedding of E. coli O157:H7 was determined via an immunomagnetic separation technique. Blood samples were collected via jugular venipuncture for analysis of serum prolactin concentrations and circulating neutrophils were isolated from peripheral blood on d 7 and 14 and degranulation and oxidative burst (OB) assays conducted. When examined over the 14-d experimental period, BROMO decreased (P = 0.0001) the percentage of cattle shedding E. coli O157:H7 (56% vs. 25.33% for control and BROMO treatments, respectively) while sulpiride had no effect (P > 0.10). BROMO decreased (P 0.10). Oxidative burst by neutrophils for BROMO vs. control showed no significant difference on d 7, however, on d 14 OB by neutrophils tended to be higher for BROMO vs. controls (P = 0.08). Serum prolactin concentrations tended to be negatively correlated with OB (P = 0.09). No significant differences were observed for degranulation in BROMO vs. control on either d 7 or14. These results support our hypothesis that hormones influenced by day length are responsible for the seasonality of E. coli O157:H7
