VACCINATION
Dr. Angie Struck

Vaccinating your horse is just one of the steps you can take toward controlling infectious diseases in horses. You should develop a program that includes all of the important components of good managerial practices in the control of disease.

This program should 1) Reduce the horse’s exposure to infectious agents in its environment, 2) Minimize factors that reduce the horse’s resistance to disease, and 3) Enhance your horse’s immunity with vaccines.

Vaccination alone is often times not successful in preventing disease, but it may lessen the severity of the disease process and minimize the risk of infection. It is imperative that the vaccines be administered appropriately and at the time of most likely exposure. Not all horses are protected to the same degree or for the same duration after vaccination. Attempting to keep all horses in the same herd on a similar schedule will help increase herd resistance to infection. Accurate and precise vaccination records are very important. Once the vaccine has been administered following the manufacturer’s guidelines, there is a period of time before the horse’s body produces active immunity.

Immunity comes in both natural and acquired forms. From the time the horse is born, it begins developing acquired immunity. The term vaccine originated from immunization against smallpox virus with vaccinia virus. Vaccines work by introducing an antigen derived from an infectious agent into the animal’s system so that an animal will mount a specific immune response to the antigen and will achieve resistance to the agent. When you first introduce vaccines to a horse, you have to give at least two injections at least three weeks apart. The reason for this is so that the body can recognize the antigen the second time around, and build a more intense response. Once the initial program has been instituted, re-exposure to the antigen allows for a very strong immune response.

There are three methods of producing vaccines, which are live vaccines, modified-live vaccines, and killed vaccines. The live vaccines may stimulate a very good immune response, but pose risks of causing disease in the horse. The inactivated vaccines do not produce as good of immune response, but are very safe for animals. Administration of the vaccine does not always mean that the vaccine is going to be effective. If the animal is sub-clinically infected with the disease when the vaccine is given, the vaccine will not be protective. Animals with a suppressed immune system, such as heavily parasitized or malnourished animals, will not produce an appropriate immune response. Stress caused by fatigue and extreme heat or cold can also reduce the normal immune response.

There is the potential for adverse reaction to vaccination. After intramuscular injection, some horses can have swelling and pain at the injection site. Fever, anorexia, and lethargy are also potential complications. Severe injection site reactions can be quite difficult to treat, and because of this, a lot of people do not recommend vaccinating horses within 2 weeks before shows, sales, shipment, or other big events. Along with recovery from any unexpected adverse reactions, this time allows for production of an adequate immune response prior to high challenge period.

The recommendation that vaccines should be administered by, or under direct supervision of a veterinarian comes highly when considering the possibility of an anaphylactic reaction to a vaccine. Anaphylactic shock can occur within seconds after the vaccine enters the circulation. Horses will develop severe inflammation of the upper airways, such that extreme difficulty with breathing occurs. They can become extremely agitated, nervous, get hives, and begin acting colicy due to the edema that may form in their GI tract. There are times when the reaction is so severe that cardiovascular insult leads to death.

Administration of vaccines is via the intramuscular, subcutaneous, or intranasal route. In administering a vaccine, the most important aspect to remember and employ is safety!! Follow the manufacturer’s guidelines carefully. If the vaccine is intended for the intranasal route, DO NOT INJECT THE VACCINE INTO THE HORSE’S MUSCLE. If the vaccine is for intramuscular use, DO NOT INJECT THE VACCINE INTO YOUR HORSE’S VESSEL!!!

WHAT TO VACCINATE YOUR HORSE FOR:

Equine influenza:
The influenza virus of horses has been a hot topic in the equine world for the last few years. Influenza causes an acute, highly contagious, febrile respiratory disease. The immune status of the horse is very important in determining the clinical outcome. Epidemics can occur when infected horses are introduced into a group of naïve horses. It is most commonly transmitted through contact with infective secretions via nose to nose contact or inhalation. Clinical signs can begin anywhere from 18 hours-7days. Cough, fever, depression, anorexia, weakness, nasal discharge, and ocular discharge are the common symptoms. Vaccines for influenza come in both the intramuscular and intranasal form. Multiple articles have been published revealing the lack of efficacy using killed intramuscular influenza vaccines in horses. The intranasal vaccine has been proven effective. It comes both as a separate vaccine (Flu-Avert) and as a combination vaccine with rhinopneumonitis (Calvenza).
Flu-Avert intranasal: Labeled for vaccination of healthy horses 11 months of age or older. Re-vaccinate every 3-6 months depending upon exposure. FOLLOW THE MANUFACTURER GUIDELINES. Not intended for SQ or IM use, and not intended for use in pregnant animals.

Equine Herpes Virus I and IV (Rhinopneumonitis)
EHV-I: Abortions, some respiratory disease, myeloencephelopathy
EHV-IV: Respiratory disease, early abortion

The goal of vaccination, just like with most, is to reduce the severity of disease. Influenza and rhino both set up the respiratory system for failure leading to bacterial pneumonia and pleuropneumonia type situations. Foals can be infected first at just a few months of age and recrudescence with stress is common. Different vaccine for pregnant mares! Vaccinating foals can begin at 4-6 months with and initial series of three shots, 4-6 weeks apart. Performance horses or horses at high risk for exposure should be revaccinated at 3-4 month intervals. Pregnant mares should be vaccinated for EHV-1 (Pneumabort-K, Prodigy both killed vaccines) at 3,5,7, and 9 months of gestation. They should be boostered with EHV-1 and EHV-4 vaccine 4 weeks before foaling. The vaccine comes in both intramuscular and intranasal form. The intranasal form is the combination rhino/influenza (Calvenza). The calvenza is labeled for healthy horses 6 months or older, including pregnant mares. The initial series of Calvenza includes three series of vaccine administration 4-6 weeks apart. The first two are given intramuscular, and the third is given intranasal.

Equine encephalitides (sleeping sickness)
These viruses are in the class Arbovirus, which means that they are arthropod-borne viruses. Eastern, Western, and Venezuelan Encephalomyelitis. Most of these are transmitted via mosquitoes, but also can be other biting insects. Birds and rodents are reservoirs for the virus. This virus affects the brain and spinal cord. Humans are susceptible. Horses are vaccinated depending upon their location. Here in Central Oregon, at this time we find it sufficient to vaccinate yearly for EEE and WEE. If horses are traveling to other parts of the United States on a regular basis, or are coming into contact with many new horses from other parts of the world, they may need to be vaccinated for EEE, WEE, and VEE and more often. This vaccine is intramuscular. Foals should start at 6 months of age, and have three initial shots at 4-6 weeks apart. Revaccinate yearly. Mares should be vaccinated 4 weeks prior to foaling to have maximum levels in colostrum.

Tetanus
Tetanus is an often fatal disease of horses caused by neurotoxins released from a bacteria called clostridia tetani. It lives in the soil, manure, and GI tract of horses. It gains access to the body usually via thing like puncture wounds, lacerations, foot wounds, incision sites, and the umbilicus. ALL HORSES SHOULD BE VACCINATED. Because the immunity in horses varies in terms of length, we vaccinate yearly. If a horse gets injured and has not been vaccinated in the last 6 months, I booster the vaccine. Foals that receive colostrum from vaccinated mares shouldn’t be vaccinated until they are 6 months of age and started in a three dose series 4-6 weeks apart. If mare is unvaccinated, the foal should be started at 3-4 months of age with the three dose series. Adult non-vaccinated horses should be treated like foals. Pregnant mares should be vaccinated 4 weeks prior to foaling to allow proper levels of antibody in the colostrum.

Streptococcus equi (strangles):
Strangles is a bacterial infection that is most common in youngsters (weanlings/ yearlings), but any age can be affected. It is transmitted via direct contact with infected horses or carrier horses, or indirect contact with troughs, bunks, pastures, stall, tack, etc… The bacteria can live in the environment for 3 months or longer. The clinical symptoms in horses are often related to the upper respiratory tract, but the bacteria can invade the bloodstream and cause abscessation in the thoracic cavity or abdominal cavity as well. It is called strangles because of its propensity for causing severe lymph node inflammation and abscessation in the throatlatch area causing the horse to have severe difficulty swallowing and breathing. The bacteria can invade horses guttaral pouch, where they can sit quietly and be a persistent source of infection for animals around them. Horses can become very systemically ill with this disease process. Vaccines come in intramuscular and intranasal form. The intramuscular form is an inactivated bacterin that can cause very significant reactions. It does NOT prevent disease, but hopefully will minimize severity. The high likelihood of vaccine reaction has to be considered heavily before using this vaccine. The intranasal vaccine may be more efficient in terms of minimizing severity, but also has its special handling requirements!!!! Your horse can be inadvertently contaminated with strangles if this vaccine is not handled properly. It is not routinely recommended that horses be vaccinated for this disease unless they are on a premises where it is an endemic persistent problem or they are at very high risk for exposure. Foals shouldn’t be given the vaccine until 9 months of age and then given 2 shot series at 3 week interval(INTRANASAL). Reactions to the vaccine include purpura hemorrhagica, which is an acute non-contagious syndrome characterized by extreme vasculitis, urticaria, pitting edema, sloughing of skin, and hemorrhage.

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