Efficacy of Dose Regimen and Observation of Herd Immunity from a Vaccine against Escherichia coliO157:H7 for Feedlot Cattle†

A clinical trial was conducted to test the effect of a vaccine product containing type III secreted proteins of Escherichia coliO157:H7 on the probability that feedlot steers shed E. coliO157:H7 in feces. Six hundred eight same-source steers were utilized. Of these, 480 steers were assigned randomly to 60 pens (eight head per pen) and to one of four vaccination treatments (120 cattle per treatment, two head per treatment per pen). The four treatments were (i) no vaccination; (ii) one dose, vaccinated once at reimplant (day 42); (iii) two doses, vaccinated on arrival (day 0) and again at reimplant (day 42); and (iv) three doses, vaccinated on arrival (day 0), on day 21, and again at reimplant (day 42). The remaining 128 steers were assigned randomly to 12 pens within the same feedlot to serve as unvaccinated external controls. The probability of detecting E. coliO157:H7 among cattle receiving different doses of vaccine was compared with that of unvaccinated external control cattle, accounting for clustering by repeated measures, block, and pen and fixed effects of vaccine, corn product, and test period. Vaccine efficacy of receiving one, two, and three doses of vaccine was 68, 66, and 73%, respectively, compared with cattle in pens not receiving vaccine. Cattle receiving three doses of vaccine were significantly less likely to shed E. coliO157:H7 than unvaccinated cattle within the same pen. Unvaccinated cattle housed with vaccinated cattle were 59% less likely to shed E. coliO157:H7 than cattle in pens not receiving vaccine, likely because they benefited from herd immunity. This study supports the hypothesis that vaccination with this vaccine product effectively reduces the probability for cattle to shed E. coliO157:H7. There was no indication that the vaccine affected performance or carcass quality. In addition, we found that vaccinating a majority of cattle within a pen offered a significant protective effect (herd immunity) to unvaccinated cattle within the same pen.