Duration of tetanus immunoglobulin G titres following basic immunisation of horses

Reasons for performing study: Recommendations for prophylactic vaccination against tetanus in horses vary greatly between countries and have scarce scientific support in the peer-reviewed literature. In human medicine, recommended booster vaccination intervals are also very variable, but are considerably longer than for horses. More information is needed about the duration of immunity induced by modern vaccines.

Objectives: To investigate if the duration of antibody titres previously determined to be protective against tetanus differ from what is indicated by recommended vaccination intervals for horses.

Study design: Prospective seroconversion study.

Methods: Thirty-four horses were enrolled for basic immunisation with an ISCOM Matrix-combination vaccine (Equilis Prequenza Te). Horses received the first vaccination at age 5-11 months, and the second dose 4 weeks later. A third vaccine dose was given 15-17 months after the second dose. Serum tetanus antibody titres were analysed by toxin-binding enzyme-linked immunosorbent assay 2 weeks as well as 14-16 months after the second dose. After the third vaccine dose, titres were checked once yearly for 3 years. Results were described by age and level of antibody titre at first sampling.

Results: Two weeks after the second dose, all horses (34/34) had antibody levels that exceeded the limit of detection, 0.04 iu/ml. After 16 months the levels were above 0.04 iu/ml in 28/33 horses, the remaining 5 horses potentially had suboptimal protection against tetanus. After the third vaccine dose antibody levels remained above 0.04 iu/ml in 25/26 horses for 1 year, 16/16 horses for 2 years, and 8/8 horses for 3 years.

Conclusions: Horses that undergo basic immunisation with 3 doses of vaccine after age 5 months are likely to have serum antibody titres consistent with protection against tetanus for more than 3 years. Current guidelines for tetanus prophylaxis should be revised.