Equine Veterinary Journal Early View September 2015
By Heather Ferguson
Duration of tetanus immunoglobulin G titres following basic immunisation of horses
Kendall, A., Anagrius, K., Ganheim, A., Rosanowski, S.M. and Bergstrom, K.
Tetanus vaccination protocols vary widely worldwide, and there is no evidence-based consensus for the frequency of tetanus boosters. This study aimed to determine the efficacy of a commercial tetanus vaccine (Equilis Prequenza TE) at providing protective levels of immunity over a 3-year study period. Thirty-four horses received the first dose of vaccine aged between 5 and 11 months, a second dose 4 weeks later and a third between 15 and 17 months after the second dose.
Serum samples were obtained before the first dose, 2 weeks after the second dose and 14-16 months after the second dose and then yearly for 3 years following the third dose. Serum antibody titres to tetanus were determined by a tetanus-toxin binding ELISA, the limit of detection for which was 0.04 iu/ml. The recognised minimum IgG titre for protection in horses is 0.01 iu/ml, based on studies in other species. Therefore the test could not determine whether immunity was adequate (>0.01 iu/ml) in one horse with an antibody titre of less than 0.04iu/ml after the complete vaccination course, so this horse was excluded from further study and vaccinated.
Before the first dose, 38% of the group (median age of 7.5 months) had no detectable antibody titres. The rest (median age 6 months) had detectable titres related to maternally derived antibodies. All horses had detectable antibodies when sampled 2 weeks after the second dose. Horses with no detectable antibodies pre-vaccination mounted a greater immune response compared to those that had detectable pre-vaccination antibodies suggesting that maternally derived antibodies in younger animals may interfere with the development of a strong vaccine response.
For reasons unrelated to the study, the number of horses declined throughout the study. When sampled 14-16 months after the second dose, 28 of 33 (85%) had detectable antibody titres. One year after the third dose, 96% of 26 horses had detectable antibodies. Two years after the third dose, 16 of 16 horses had detectable antibodies and at the end of the study (3 years after the third dose) 8 of 8 horses remaining had detectable antibodies suggesting that after a primary course of 3 doses, protective levels of immunity to tetanus are maintained for 3 years.
After a primary course of 3 vaccinations according to the described protocol, levels of immunity well above those considered to be protective against tetanus are maintained for 3 years. Current booster guidelines may require revision.