All horses are at risk of development of tetanus, an often fatal disease caused by a potent neurotoxin elaborated by the anaerobic, spore-forming bacterium, Clostridium tetani. Tetanus toxoid is a core equine vaccine and is indicated in the immunization program for all horses.
Clostridium tetani organisms are present in the intestinal tract and feces of horses, other animals and humans, and are abundant as well as ubiquitous in soil. Spores of Cl. tetani survive in the environment for many years, resulting in an ever-present risk of exposure of horses and people on equine facilities. Tetanus is not a contagious disease but is the result of Cl. tetani infection of puncture wounds (particularly those involving the foot or muscle), open lacerations, surgical incisions, exposed tissues such as the umbilicus of foals and reproductive tract of the postpartum mare (especially in the event of trauma or retained placenta).
Vaccines currently available are formalin-inactivated, adjuvanted toxoids. Tetanus toxoid is a potent antigen that rapidly induces strong serological responses. Circulating antibody is able to mediate complete protection against tetanus. It is generally accepted that tetanus toxoid administered per manufacturer recommendations is both safe and effective.
A 6-month study comparing serologic responses of equids to commercial vaccines demonstrated significant IgG response for the duration of the study. The end point for antibody persistence was not explored and may potentially be longer than the 6 months stated in the study. Extending the revaccination interval beyond the manufacturer’s recommendation for annual revaccination is not advisable due to a veterinarian’s liability if label recommendations are not followed.
There are no challenge studies that have been published to document the onset or duration of immunity induced by tetanus toxoid products available for use in horses in the USA. Conclusions regarding the efficacy of products used in the USA are based on serologic response in laboratory animals and field experience. This may be accepted as evidence of vaccine efficacy as antibody alone can mediate protection. Tetanus has rarely been documented in vaccinated horses in the USA, illustrating the variability of response of equids to any biologic product. Note: Survival of horses with tetanus was strongly associated with previous vaccination.
Adult horses, previously vaccinated against tetanus: Vaccinate annually. Horses that sustain a wound or undergo surgery 6 or more months after their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery. Note: The severity of the wound does not predict the risk for development of tetanus. Superficial wounds have resulted in clinical tetanus in horses.
Adult horses, previously unvaccinated against tetanus, or of unknown vaccinal history: Administer a primary 2-dose series of tetanus toxoid with a 4- to 6-week interval between doses. Protective concentrations of immunoglobulin are usually attained within 14 days of the second dose of vaccine. Vaccinate annually thereafter.
Tetanus antitoxin is indicated to provide passive immunity in situations where the horse is at risk of tetanus infection and has not been immunized according to labeled recommendations for tetanus. If the veterinarian determines that administration of tetanus antitoxin is indicated, then it should be administered in one site and the initial dose of a priming series of tetanus toxoid vaccinations should be administered at a distant muscular site. The rare, but fatal, risk of Theiler’s disease consequent to the use of tetanus antitoxin needs to be taken into consideration when determining if use is indicated.
Pregnant mares previously vaccinated against tetanus: Vaccinate annually 4 to 6 weeks before foaling, both to protect the mare should foaling-induced trauma or retained placenta occur and to enhance concentrations of colostral immunoglobulins.
Pregnant mares unvaccinated against tetanus or of unknown vaccinal history: Administer a 2-dose primary series of tetanus toxoid with a 4- to 6-week interval between doses. Revaccinate 4 to 6 weeks before foaling.
Foals of mares vaccinated against tetanus in the pre-partum period: Administer a primary 3-dose series of tetanus toxoid beginning at 4 to 6 months of age. A 4- to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age.
Foals of unvaccinated mares or mares of unknown vaccinal history: Administer a primary 3-dose series of toxoid beginning at 3-4 months of age. A 4- to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age. Serologic data indicates that a 3-dose initial series produces a more consistent anamnestic response in all foals, regardless of the age at which the series is initiated. Tetanus antitoxin is indicated to provide passive immunity in situations where a foal is born to a non-vaccinated mare and is at risk of tetanus infection. (See Tetanus antitoxin above.)
Horses having been naturally infected with tetanus and recovered: Revaccinate annually.