Management Strategies to Enhance Vaccine Efficacy

With summer just around the corner, many of us are getting ready to enjoy the prime time for equine activities. Horse shows, rodeos, roping events, jumping, dressage, and trail riding are just a few of the many events that we take part in with our horses. Along with our horses' increased activity and travel, however, comes an increased threat of infectious disease--in particular, infectious upper respiratory diseases such as equine influenza, equine herpesvirus (rhinopneumonitis), and strangles.

The key component of an infectious disease control program is a good vaccination program developed in conjunction with your veterinarian. However, owners should recognize that vaccination programs will not succeed without appropriate horse management practices. Vaccine efficacy is dependent not only on the vaccine itself, but also on your horse's individual immune system, how you manage him, and his level of exposure to infectious agents or disease. Vaccines are not 100% effective, and many variables affect how your horse responds to the vaccine, how well it works, and how well he is protected following vaccination. As horse owners and veterinarians, we tend to think that following vaccination the horse is immediately protected and that the protection is 100% complete. Unfortunately, this is not the case.

Following vaccination, the horse must mount an immunologic response to the vaccine (i.e., develop antibodies and/or cellular immune responses against the infectious agent for which you are vaccinating). Response to vaccination is not immediate, so time must be allowed (generally at least 14 days) for the horse to respond to the vaccine. Most vaccines require two doses at four- to six-week intervals for primary immunization (the first time the horse is vaccinated). Therefore, if your horse has never been vaccinated against the disease, optimum response to vaccination will not occur until approximately two weeks after the second dose.

Also, not all horses will mount an immunological response to vaccination. Furthermore, of those that do respond there is a great deal of variation between responses (i.e., some horses respond very well and some horses respond poorly). In most situations about 10% of vaccinated horses respond poorly to vaccination and, therefore, might not be adequately protected against disease. So what can owners do to enhance their horses' response to vaccination, enhance vaccine efficacy, and better protect horses against infectious and contagious diseases? Answer: Apply intelligent and appropriate management strategies.

On the road--There are several things you can do to help decrease the chances of your horse getting sick from an infectious or contagious disease while traveling:

  • Whenever possible, use your own trailer and avoid shipping your horse with animals from other farms.
  • Haul your horse in a clean, well-ventilated trailer. Poorly ventilated, dirty trailers significantly increase deleterious effects on the respiratory system and increase the chances of the horse developing a respiratory infection.
  • When possible, avoid hauling for long distances. Studies have proven that transport-induced stress has significant adverse effects on the horse's immune system.
  • Avoid letting your horse touch other horses, especially nose to nose. Most infectious respiratory diseases are transmitted by nasal secretions. Even though your horse may be properly vaccinated, if he comes in contact with another horse that is shedding virus or bacteria, the "challenge dose" of virus or bacteria that your horse is exposed to may be significant enough to override or overwhelm the immunity from vaccination. It's also important to remember that some horses can be asymptomatic shedders (i.e., they show no signs of having a disease, yet they are shedding virus or bacteria). This is particularly true of carrier horses with equine herpesvirus and strangles. So while the horse that just touched noses with your animal appears to be healthy, he may still be shedding virus or bacteria.
  • Avoid allowing your horse to use community water tanks, and don't share equipment (e.g. water buckets, feed pans, brushes, etc.). Water is a great media for virus and bacteria growth.
  • Spray or disinfect stalls at show grounds prior to allowing your horse access, especially if they have recently been occupied by other horses. A hand-held, garden-type pump sprayer filled with disinfectant works well for this procedure.
  • Wash your hands after handling other horses before handling your own horse.

At home-Regardless of how careful you are, there is always the possibility that your horse might bring home an infectious disease. Therefore, all horses on the premises should be on the same vaccination schedule as the horses that are traveling. This is especially important with regard to respiratory diseases. Although you might still encounter an infectious disease outbreak, if all horses on the farm are properly immunized, the incidence, severity, and duration of disease should decrease dramatically.

  • If possible, keep the horses that were off the farm and exposed to other horses isolated for two to three weeks (preferably three weeks) upon returning to the farm. This is not always practical, but it is optimal.
  • Segregate horses, if possible. For example, keep show horses segregated from resident horses and, if possible, keep all horses segregated from breeding stock.
  • Isolate any horse that is showing signs of infectious disease (lethargy, anorexia, fever, cough, nasal or ocular discharge, etc.), and call your veterinarian to examine the horse. Consult your veterinarian about booster vaccinations in the face of an infectious disease outbreak. In certain situations booster vaccinations given to healthy horses on the farm may be warranted. For example, if your veterinarian determines that the sick horse has influenza, booster vaccinating the rest of the herd with an intranasal influenza vaccine that has a very rapid onset of immunity might be justified and could help control the spread of the disease.

Also, develop a policy for accepting new horses on your farm. The greatest danger for infectious disease introduction into a herd lies in the introduction of a new horse, especially if the horse is coming from an area like a sales facility, show ground, boarding stable, racetrack, competition, or training center where an increased risk of exposure to infectious disease exists. The new horse requirements should include:

  • The horse must be current on vaccinations recommended by your farm prior to arrival.
  • The horse must be dewormed within two weeks of departing the farm of origin.
  • The horse must be accompanied by a proof of a negative Coggins test (which tests for equine infectious anemia).
  • The horse should have a health certificate of veterinarian inspection. This examination should be done close to the time of entry onto the farm. The standard interstate requirement for health certificates is within 30 days prior to travel. For many infectious diseases the incubation period (time from exposure until showing signs) is much shorter than 30 days. For example, the incubation period for influenza is three to five days, and for strangles it is generally three to 14 days. Therefore, an examination that was done 30 days ago may have been done while the horse was incubating the disease or even before the horse was exposed. The horse might have been healthy and not shedding virus or bacteria then, but he might be shedding and/or showing clinical signs today.
  • Whenever possible, ask questions and get a good history with regard to infectious diseases that might have been on the farm of origin.
  • Quarantine or isolate all new incoming horses. A minimum isolation period is two weeks, although three weeks is preferred. This allows time to monitor the new incoming horse for signs of disease. Remember, the new horse might appear healthy upon arrival but could be incubating a disease such as influenza, strangles, or equine herpesvirus. The incubation period for equine herpesvirus may be as long as three weeks. The horse's temperature should be taken daily while in quarantine. In addition, watch for other disease signs such as lethargy, anorexia, nasal discharge, ocular discharge, or coughing.

Take-Home Message

Always avoid or minimize factors that might cause stress and/or have an adverse effect on the horse's immune system. Provide adequate and proper nutrition, as a malnourished horse will not respond appropriately to vaccination and will have a weakened immune system incapable of responding appropriately to exposure to infectious disease. Avoid overcrowding and provide shelter to limit stress caused by adverse weather conditions.

We often speculate that all of these management practices are impractical and not worthy of our time and effort. However, once you have experienced an infectious disease outbreak on your farm, your perception of these recommendations will likely change. Remember, an infectious disease outbreak on a farm can be devastating and can present a significant loss in time training, showing, and having fun with our horses. The economic impact of treating and controlling an outbreak also can be significant.

Vaccines are an important part of managing your horse's health. However, an effective infectious disease prevention program not only depends on an effective vaccination program but also on good management practices aimed at lowering disease risk. Take proper precautions while traveling, isolate new arrivals, prevent stress and overcrowding, and be alert to recognize and take appropriate actions to deal with disease when it occurs. Consult your veterinarian and develop a management plan along with your vaccination program to help protect your horse from infectious and contagious diseases.

About the Author

D. Craig Barnett, DVM

D. Craig Barnett, DVM, serves as a senior equine technical services veterinarian with Merck Animal Health, where he provides continuing education to equine practitioners and conducts presentations on the use of vaccines and pharmaceuticals to horse owners and veterinarians.

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