Antibody kinetics against tetanus toxoid before and after primary vaccination of foals

Background : Vaccination guidelines for foals against C. tetani vary, and existing evidence is outdated or lacks statistical power to assess foal immune responses, maternal antibody interference, or individual variation. Objectives : To quantify antibody responses after tetanus vaccination in foals and explore associations with passively transferred tetanus antibodies. Study design : Longitudinal prospective cohort study. Methods : Sixty-two Warmblood foals from a single stud farm were randomly assigned to two vaccination groups. Group 1 (n=32) received tetanus-influenza (ProteqFlu®-Te) and equine herpesvirus (Equip® EHV1,4) vaccines at day 180 (d180) and d220 of life. Group 2 (n=30) received an additional influenza and EHV1/4 vaccination at d150, followed by the same protocol as Group 1. Serum samples collected at 19 time points from birth to d360 were analysed using a fluorescent bead-based Tetanus Multiplex assay to quantify total and isotype-specific anti-tetanus antibodies. Results : No differences were observed between vaccination groups, allowing combined analysis of all foals (n=62). Passively transferred anti-tetanus antibodies declined from d15 and reached minimal concentrations by d180 (P < 0.0001). Vaccination on d180 and d220 induced an antibody increase by d230 (P < 0.0001), followed by a decline to below pre-vaccination levels by d360. No correlations were observed between maternally derived antibody concentrations (d5) and the vaccine-induced response on d230 (rsp = –0.36), or between pre-vaccination levels (d180) and d230 (rsp = –0.16). IgG1 contributed most to the initial vaccine-induced response, whereas IgM concentrations remained unaffected. Considerable inter-individual variability was observed, with approximately half of the foals showing low responsiveness. Main limitations : A single vaccine formulation and tetanus immunisation schedule were explored. Conclusions : Vaccination of mares during late gestation effectively transfers protective anti-tetanus antibodies to foals. The variability in foal responses and rapid post-vaccination antibody decline suggest that tetanus vaccination schedules for foals, including booster timing, may require optimisation to ensure early-life protection.