Vaccination Principles

Debra Sellon, DVM, PhD, associate professor of equine medicine at Washington State University's vet school, spoke about equine vaccination principles and strategies at the Western Performance Horse Forum held in Nampa, Idaho, on Feb. 15-17. She stressed two goals when considering immunization: 1) To prevent infectious disease; and 2) To comply with requirements for travel, competition, and breeding.

While vaccines are helpful in the prevention of infectious disease, an owner's implementation of strategies to minimize exposure and to maximize a horse's immune system, and be able to recognize signs of disease early to minimize exposure to others.

She recommends a "TROTS" approach:

  • Twenty-one- to 28-day isolation;
  • Reduce stress;
  • Observe and monitor for signs of disease;
  • Treat for parasites;
  • Sterilize and disinfect.

To adequately immunize, one must consider the disease being vaccinated against, the horse's immune response to the disease, the technology of the vaccine, how it elicits an immune response, and horse and owner requirements.

Sellon reviewed basic immunology, stressing that of the two arms of immunity (humoral antibodies and cell-mediated immunity or CMI), a viral vaccine response is best if skewed toward CMI protection and production of immunoglobululin IgGb. She reviewed the concept of vaccinating for individual immunity against non-contagious diseases, such as equine encephalitis, West Nile virus, and tetanus, as well as rabies. She also emphasized the importance of immunizing a herd against equine herpesvirus-1 and -4, influenza, and strangles.

Strategies for different needs

Pregnancy can change a mare's vaccination requirements. Sellon said pregnancy is, in a sense, like a foreign protein (antigen) eliciting some degree of immune suppression in the mare to maintain pregnancy. This results in an insufficient immune response by naïve mares (never before vaccinated) who are vaccinated during pregnancy. Vaccination prior to pregnancy facilitates an appropriate response to a booster given during pregnancy.

Stress conditions, such as pregnancy, reactivate latent infections such as equine herpesvirus and shedding of Salmonella or strangles bacteria. Increased exposure occurs in a herd due to co-mingling of mares and foals arriving at a farm for breeding or shipping of a mare to a breeding farm.

Foals are unable to develop their own immune response until four to six months of age, thus must depend on passive immunity received from colostrum (high in IgGb) directly after birth. A foal should receive quality colostrum and be blood tested for its level of transfer. It is best to delay vaccination of foals with killed vaccine until at least six months. If killed vaccines are given at frequent intervals starting at two to three months of age, a foal can develop a tolerance to the vaccine, creating a delayed response.

A young athletic competitor might be at increased risk of disease since such a horse often has not encountered specific pathogens in sufficient amounts to stimulate good protective responses. Coupling this with the stress of transport, training, competition, nutrition and diet changes, and changes in routine, increases the young horse's susceptibility to infectious disease. Not all horses congregating at an event are on a similar proactive vaccination schedule, and many that change ownership have an unknown vaccination history. Athletic horses, as a mobile population, have high exposure risk and potential for spread of disease through tack and equipment, hands, and clothing (fomites).

Formulas for different needs

Killed vaccines use an adjuvant (carrier) to improve the immune response, but this response is often skewed toward humoral antibody protection. In contrast, modified live virus (MLV) vaccines or chimera vaccines generate both CMI and humoral responses for better protection.

Adjuvants combined with an increase in antigenic mass are necessary to improve response to killed vaccine, and this increases the likelihood of adverse reactions, such as lethargy, fever, aching joints, eye inflammation, muscle soreness, and being off feed. Immunity induced by killed vaccine is not as long lasting, requiring more frequent boosters than a MLV vaccine, which more closely mimics natural infection to elicit a more appropriate immune response with a rapid onset and long duration of immunity.

Herd safety

No vaccine completely prevents disease, but the objective is to decrease the number of sick horses and the severity of the disease should a horse become infected.

Equine herpesvirus creates a latent state in which the virus sits in the lymphatic tissue not producing any proteins, therefore the horse does not "respond" against it. A stressor such as transport, weaning, castration, or foaling reactivates virus, and it is shed in respiratory secretions, not necessarily with any associated clinical signs. EHV-4 causes 80% of respiratory disease attributable to equine herpesvirus, whereas EHV-1 can cause respiratory disease, abortion, or neurologic disease.

It is thought that 80% of all horses are infected with equine herpesvirus by seven- to 10-days of age, so one might wonder: "Why vaccinate?" The goals are many:

  • To prevent recrudescence (return) of disease;
  • To suppress virus so it remains latent;
  • To stop shedding in nasal secretions in order to limit transmission to naïve horses.

Defense against equine herpesvirus relies on CMI response for best immunity. To date, no vaccine has a label claim against neurologic disease. In a study at the University of Kentucky, horses were infected with the neurologic strain of herpes and years later, it was found in the lymph nodes of all, waiting for reactivation and shedding. Following transport, 3% of horses shed herpesvirus although they did not necessarily show clinical signs. The best control relies on management techniques to control stress and exposure.

The biggest economic impact in the equine industry from viruses is related to outbreaks of equine influenza. Because flu is capable of antigenic changes, the goal is to immunize with a relevant strain of vaccine. Flu is highly communicable, and t is spread via aerosol and direct contact, with an incubation period of one to three days. Flu elicits a high fever, nasal discharge, and a dry cough. Killed influenza vaccine is best administered every four to six months, and MLV vaccine every six months to moderate or high-risk horses. A mare in late pregnancy must receive the intramuscular killed vaccine in order to develop colostral antibodies.

Equine strangles is a highly contagious bacterial respiratory disease caused by Streptococcus equi. Fever precedes nasal shedding, so new arrivals or exposed horses should be isolated and monitored to catch the disease early. Nasal shedding of strangles begins one to two days after onset of fever and can persist for two to three weeks, or can even continue indefinitely as a latent infection in the guttural pouch. As many as 75% of recovered horses develop a lasting immunity for up to five years, but 25% do not, so there is no guarantee of long-term immunity from clinical disease, also suggesting that it might be difficult to achieve lasting immunity from vaccine. To date, strangles vaccines do not provide consistent efficacy, although vaccine has been shown to decrease the number of cases and the severity of disease.

For more from the Western Performance Horse Forum click here.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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