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

BACKGROUND: Vaccination recommendations for foals against Clostridium tetani vary. Available studies assessing foal immune responses, maternal antibody interference, or individual variation often used small foal numbers. OBJECTIVES: To quantify antibody responses after tetanus vaccination in foals and explore associations with passively transferred tetanus antibodies. STUDY DESIGN: Longitudinal cohort study. METHODS: Sixty-two Warmblood foals from a single stud farm were followed from birth to 360 days of age. Foals received tetanus vaccinations at day 180 (d180) and d220 of life. Serum samples were collected at 19 time points during the study period and analysed using a fluorescent bead-based Tetanus Multiplex assay to quantify total and isotype-specific anti-tetanus antibodies. RESULTS: Passively transferred anti-tetanus antibodies in foal serum started to decline on d15 until d180. The first tetanus vaccination on d180 did not induce detectable antibodies. Following the second vaccination on d220, antibodies peaked on d230, before declining again to concentrations below pre-vaccination by d360. A weak correlation was observed between maternally derived antibodies on d5 and the vaccine-induced response on d230 (rsp = -0.36). Antibodies between pre- (d180) and post-vaccination (d230) did not correlate (rsp = -0.16). IgG1 contributed majorly to the initial vaccine-induced response, whereas IgM remained unaffected. The individual variability in antibody responses was high, with 38.7% of the foals showing low responsiveness to vaccination. MAIN LIMITATIONS: Only foals from a single farm were included; only one vaccine and tetanus vaccination schedule were evaluated, and a nonvaccinated control group or foals without maternal antibodies were not available. CONCLUSIONS: The variability in foal responses and the rapid post-vaccination antibody decline suggest that tetanus vaccination schedules for foals, including booster timing, may benefit from optimisation. Low vaccine responses at a time when passively transferred antibody concentrations had waned challenges the hypothesis of maternal antibodies as the sole reason for reduced antibody induction in foals.