When it comes to vaccines, the needs and requirements of horses are always changing. Learn what’s shifted over the past few years and how you can protect your equine companion without over-vaccinating.
Articles on equine vaccines authored by myself and a colleague first appeared in Equine Wellness in early 2014. Since then, both veterinary communities and the public have become increasingly aware of the potential risks associated with companion animal and livestock vaccines. The accumulated evidence indicates that vaccination protocols should no longer be considered a “one size fits all” program. At the same time, the risk of disease has been effectively reduced by the widespread use of vaccination programs. So how can you ensure your horse is protected without risking over-vaccination?
Alternatives to annual boosters
Appropriate alternative options to routinely giving booster vaccinations include:
- measuring serum antibody titers
- avoiding unnecessary vaccines or over-vaccinating
- deferring vaccination of sick or febrile individuals
- tailoring specific minimal vaccination protocols for equine breed types or families known to be at increased risk for vaccinosis
- starting the vaccination series later when the immune system is more robust
- monitoring behavior and overall health after boosters
- avoiding revaccination of individuals already experiencing a significant adverse event.
A recent landmark study of rabies vaccine serological immunity in horses found that a rabies vaccination interval of more than a year should be protective, but not for horses vaccinated only once. In this study from 2016, 48 horses had blood samples collected prior to vaccination, three to seven weeks after vaccination, and at six-month intervals for two to three years. Serum rabies virus–neutralizing antibody (RVNA) values were measured, and a value of ≥ 0.5 IU/mL was used to define a predicted protective immune response on the basis of the World Health Organization recommendations for humans. Values were compared between horses over 20 years of age and those 20 or younger, and between horses believed to be previously vaccinated and those inferred to be immunologically naïve. Protective RVNA titers were maintained for two to three years in the horses previously vaccinated on the basis of pre-vaccination RVNA values. No significant difference in horse ages was evident in response to rabies vaccination or duration of protective RVNA values. Seven horses were poor responders to the vaccination. There were significant differences between the previously vaccinated horses and those inferred to be naïve prior to the study.
Current AAEP Equine Vaccination Guidelines (extracted from 2012 and 2015)
The professional judgment of those involved with developing these guidelines may differ from the manufacturer’s recommendation. Ultimately, it’s up to you and your veterinarian to decide on vaccine usage based on his/her professional experience, and the circumstances of each unique situation.
The information provided herein addresses only those products licensed by the United States Department of Agriculture (USDA) and Agriculture Canada for use in horses (including draft and pony breeds). Data regarding the use of vaccines in other Equidae (i.e. asses, donkeys, mules, miniature horses and zebra) is limited; vaccination of these animals is left to the discretion of the attending veterinarian.
Injection site selection should include a consideration of potential adverse reactions (see sidebar at left). Injection in the gluteal muscles/hip region is not recommended, as gravitational drainage along fascial planes can be obscured. Should an abscess develop, considerable tissue damage can occur, resulting in eruptions in undesirable locations with lesions that require a prolonged time to heal.
Core equine vaccination guidelines – tetanus, EEE/WEE, WNV and rabies
Tetanus toxoid
Adult horses: An initial two-dose series at a three- to four-week interval followed by a yearly booster. Horses that sustain a wound or undergo surgery six or more months after their previous tetanus booster should be revaccinated with tetanus toxoid immediately at the time of injury or surgery.
Foals of mares vaccinated against tetanus in prepartum period: A three-dose series beginning at four to six months of age with a four- to six-week interval between the first and second doses and the third dose administered at ten to 12 months of age.
Foals of unvaccinated mares or unknown vaccination history: A three-dose series beginning at one to four months of age with a fopur-week interval between doses.
EEE/WEE
Adult horses: An initial two-dose series at a four- to six-week interval, followed by a yearly booster prior to the vector season. In high-risk animals, and in areas with year-round vectors, more frequent vaccination is recommended during periods of likely exposure (twice yearly).
Foals of mares vaccinated against EEE/WEE in prepartum period: A three-dose series beginning at four to six months of age with a four- to six-week interval between the first and second dose. The third dose is administered at ten to 12 months of age.
Foals of unvaccinated mares or having unknown vaccinal history: A primary series of three doses beginning at three to four months of age, with a 30-day interval between the first and second doses and a 60-day interval between the second and third doses. If the primary series is initiated during the mosquito vector season, an interval of three to four weeks between the second and third doses is preferable to the above interval of eight weeks.
West Nile virus
Adult horses: An initial two-dose series at a three- to six-week interval is recommended with a yearly booster prior to the vector season. In high-risk animals, and in areas with year-round vectors, more frequent vaccination (with any currently licensed product) may be needed to meet the vaccination needs of these horses.
Foals of vaccinated mares: A primary three-dose series beginning at four to six months of age with four to six-week interval between the first and second dose. The third dose should be administered at ten to 12 months of age prior to the onset of the next mosquito season.
Foals of unvaccinated mares or having unknown vaccinal history: A primary series of three doses beginning at three to four months of age, with a 30-day interval between the first and second dose and a 60-day interval between the second and third dose. If the primary series is initiated during the mosquito vector season, an interval of three to four weeks between the second and third dose is preferable to the above interval of eight weeks.
Rabies
Rabies virus vaccine is an excellent immunogen; these vaccines typically induce a strong serologic response after just a single dose. (Note the more recent 2016 paper, which differs as discussed above.)
Adult horses: Following an initial single-dose administration, rabies vaccines are administered as a yearly booster.
Foals of mares vaccinated against rabies: A primary two-dose series. The first dose of vaccine should be administered no earlier than six months of age. The second dose should be given four to six weeks later. The recommendation is to revaccinate annually thereafter, although the more recent 2016 paper cited below indicates sustained protection beyond a year, if more than one dose of rabies vaccine has been given.
Foals of mares not vaccinated against rabies: Should be administered according to label directions. The first dose should be administered at three to four months of age. Revaccinate at least once and annually thereafter, but see above comments with regard to frequency of subsequent boosters.
Risk-based equine vaccination guidelines – anthrax, botulism, EHV, EVA, equine influenza, Potomac horse fever, rotaviral diarrhea and strangles
These vaccine products are used following a risk-benefit appraisal by the attending veterinarian. Their use may vary regionally, from population to population within an area, or between individual horses within a given population.
Summary
Based on published literature and clinical experience to date, there is little scientific and medical reason to introduce unnecessary antigen, adjuvant and other excipients (fetal calf serum, egg protein, tissue culture remnants) as well as preservatives (aluminum and thimerosal salts) by routinely administering booster vaccines. By measuring serum antibody titers triennially or more often, as needed or desired, your vet can assess whether your horse needs a vaccine booster.
References
American Association of Equine Practitioners (AAEP). Vaccination Guidelines Review Group, 2015 and Executive Summary, 2012.
aaep.org/guidelines/vaccination-guidelines
Dodds WJ. “Vaccine issues and the World Small Animal Veterinary Association (WSAVA) guidelines (2015-2017)”. Israel J Vet Med 2018; 73(2): 3-10.
Dodds WJ. Rabies virus protection issues and therapy. Global Vaccines Immunol 2016; 1: 51-54.
Harvey AM, Watson JL, Brault SA et al. Duration of serum antibody response to rabies vaccination in horses. J Am Vet Med Assoc 2016;249:411–418.
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