Protection of piglets against neonatal colibacillosis based on antitoxic immunity

Substantial protection against colibacillosis in neonatal piglets was provided by transfer of colostral antitoxic antibody to suckling piglets from dams immunized with procholeragenoid. The physical properties of procholeragenoid, an aggregate with a high-molecular weight formed during the heating of cholera toxin, were dependent upon the buffer and the duration of heating used for production. Procholeragenoid possessed a higher molar ratio of subunit A to subunit B than native cholera toxin. The biological activity of procholeragenoid was greatly decreased in several assay systems as compared to cholera toxin. Immunization of pregnant dams with 50 micrograms of procholeragenoid 5 weeks and 2 weeks prior to delivery elicited high titers of antitoxic-IgG and toxin-neutralizing antibody in the serum and colostrum at parturition. In field trials, immunization with procholeragenoid markedly decreased the incidence of diarrhea (85%) and death (84%) due to colibacillosis in neonatal piglets. Reimmunization of dams with procholeragenoid during the following gestation period afforded comparable levels of protection against death in piglets (mortality rate of 0.86% compared to 0.77% following the first immunization schedule). These results demonstrate that only antitoxic immunity can afford a high degree of protection against colibacillosis in neonatal piglets.