CALVES FED WITH COLOSTRUM CONTAINING HIGH ESPB-, INTIMIN- AND STX2- IGG ANTIBODIES ARE NOT PROTECTED FROM E. COLI O157:H7 COLONIZATION AT WEANING

Mercado Elsa Cristina (INTA); Vilte Daniel Alejandro (INTA); Rabinovitz Bettina Carol (INTA); Tironi Farinati Carla (UBA); Abdala Alejandro (INTA); Larzábal Mariano (INTA); Galarza Roxana (INTA); Ibarra Cristina (UBA); Cataldi Angel (INTA)

The majority of enterohemorrhagic Escherichia coli (EHEC) infections are caused by E. coli O157:H7 and this serotype frequently colonize the intestine of cattle. Vaccination of pregnant cows with EHEC virulence factors could induce specific colostral antibodies that protect calves against intestinal colonization by EHEC O157:H7. We evaluated the susceptibility to EHEC O157:H7 colonization of weaned calves with passively acquired IgG titers against EspB, Gamma Intimin and Stx2 proteins. Recombinant proteins and inactivated Stx2 were used in combination with a W/O/W adjuvant to immunize Holstein pregnant cows. Animals were immunized at 60, 40 and 20 days prepartum. A control group received PBS in adjuvant alone. Calves were fed by their respective dams during 8-12 weeks, just until 2 weeks previous to receive 10⁹ colony forming units of a nalidixic acid-resistant strain of EHEC O157:H7 by the oral route. The presence of the bacteria in feces was monitored at two-day intervals over the following three weeks by plate counting onto sorbitol-MacConkey agar containing potassium tellurite and nalidixic-acid (TN-SMC) and by enrichment of recto-anal mucosal swabs followed by immunomagnetic separation and plating onto TN-SMC. Sorbitol-negative colonies were confirmed to be E.coli O157 by latex agglutination test and PCR. Colostrum and serum samples from cows and calves were assayed for IgG-specific antibodies by an enzyme-linked immunosorbent assay and confirmed by western blot. Anti-Stx2 titers were determined by neutralization in Vero cell cultures. All vaccinated cows mounted a significant serum and colostrum IgG response against Gamma Intimin and EspB and high Stx2-neutralizing titers that were transferred to colostrum-fed calves. At challenge, calves fed with hyperimmune colostrum exhibited still significant titers against Gamma Intimin and EspB proteins compared with control group. However, non significant reduction in total bacterial shedding or frequency of EHEC O157:H7 excretion between both groups of calves could be observed over the sampling period. Diarrheic calves were observed in both groups wihout isolation of recognized enteric pathogens. In conclusion, weaned 2 to 3-month-old calves were susceptible to EHEC O157:H7 intestinal colonization even in the presence of passively acquired Gamma Intimin-, EspB- and Stx2- specific serum IgG antibodies.