Strategies to Enhance Vaccine Efficiency

When it comes to protection from infectious diseases, the best defense is a good offense. Not only is a strategic vaccination program important, but the housing and handling of horses on a farm can enhance vaccination efficacy. A successful outcome (no disease) is best accomplished by applying intelligent management strategies for individual horses and proper herd health principles. Immunizations are available for many diseases, not all of them communicable from horse to horse. Some infectious microbes that have a huge impact on herd health include respiratory diseases like equine influenza, equine herpesvirus, and strangles.

In the following article, we’ll give you tips and strategies on protecting your horses from infectious and contagious diseases with proper management strategies.

Health Exam Prior to Entry

The best defensive strategy stops communicable diseases before they enter a property. This is accomplished by requiring health certificates on any horse coming to your farm, whether the horse remains indefinitely or is just passing through. When possible, every incoming horse should be examined thoroughly close to the time of entry to your facility. Many infectious diseases have an incubation period of up to 21 days. The standard requirement for a health certificate for interstate travel requires an exam to be conducted within 30 days of travel, although some states require a shorter, 10-day window prior to movement.

Consider that at 30 days out, it is difficult to identify those horses that might be incubating disease, or those that have not yet been exposed, but could be exposed within that 30-day window. There are also problems identifying horses that are spreading disease as inapparent carriers; they show no signs of clinical illness. This is particularly true for carrier horses with equine herpesvirus (EHV-1) and strangles.
John Timoney, MVB, PhD, DSc, MRCVS, is a leading authority on equine strangles, basing his research out of the Gluck Equine Research Center at the University of Kentucky. He has some sound advice to identify risky horses coming to a farm: “It is important to ask questions about a horse’s history: Where did he come from, had there previously been a disease outbreak, an unexplained cough, a unilateral nasal discharge, or asymmetrical swelling behind the jaw?”


In addition to requiring a preliminary health exam prior to entry on a farm, a newcomer should be quarantined in an isolated area for two to three weeks to allow monitoring for clinical signs associated with a health concern. Quarantine of new horses on a farm implies that there is no opportunity for these horses to touch each other, share food or water, or to co-mingle in any way. George Allen, PhD, from the Gluck Center, suggests, “The greatest danger for infectious disease dissemination within a herd lies in the introduction of new horses into already established groups, especially those with recent opportunities for exposure to large, intermingled assemblages of horses from diverse sources such as sales, shows, boarding stables, racetracks, competitions, and training centers.”

Many organisms are passed through respiratory secretions and aerosolized droplets thereof, so it’s important to house new horses far enough away from resident herd members. The new horses should be fed and handled, and their paddocks and stalls mucked, only after caring for the needs of the resident horses. All personnel, including visitors, farriers, and veterinarians who interact with horses on a farm, should respect quarantine principles.

The likelihood of a horse developing an infection is not only related to the immune system of each individual, but it is often related to the dose of virus or bacteria that a horse receives. By minimizing the amount of microbes that are shed from an infected individual, you can greatly reduce the number of other horses that get sick, and/or the degree of sickness that develops on a farm. With this in mind, stalls and the insides of horse trailers should be scrubbed with disinfectant, especially in the face of an outbreak. Water tanks should be cleaned daily.

Many infectious diseases cause a fever, so rectal temperature should be taken at least daily on quarantined horses. (A normal, adult horse has a rectal temperature ranging from 97 to 101.5ºF, while a temperature of 102ºF is considered to be within normal limits for a foal, provided he is acting normally.) The presence of any nasal or eye discharge or coughing should be noted. The horse’s appetite should be monitored closely. If a horse appears “out of sorts” in any way, a veterinary examination should be performed and possibly a blood sample submitted to the lab for a complete blood count to screen for systemic infection.

Before being released into a resident group, a horse should be examined by your veterinarian. This should include monitoring all vital signs, listening to the lungs with a stethoscope, and palpating the external lymph nodes. Other screening tests may be warranted if the quarantined horse has come from a premises known to have infectious disease.

Daily Practices

Implementing strategic management practices on a farm all the time is a sen-sible strategy. Allen advocates segregation of horses on the premises into small groups. Each group should be maintained as an isolated unit for purposes of stabling, pasture turnout, watering, and feeding. Allen elaborates on how to do this: “For maximal effectiveness, group size should be as small as the physical facilities will reasonably allow, with each group kept under conditions that limit the transmission of disease between established groups. Horses should be segregated into groups that are similar in age, gestation status, use, and frequency of removal of individual animals. Restrictions should be placed on movement of horses into and out of each established group, and contact with transient horses in particular should be avoided.”

He qualifies the idea of quarantine even further: “The addition of any new horse into a closed group should be preceded by a 21-day period of isolation. A horse temporarily removed from a group for purposes that may involve prolonged transport or contact with other horses (e.g., breeding, showing, training, veterinary care, or sales) should also undergo a 21-day period in isolation and evaluation for signs of infection before returning to its resident group.”
While these strategies are applicable to containment of any infectious disease, Cormac Breathnach, PhD, of the Gluck Center, re-lates the value of immunization and management in reducing abortion or neurologic storms caused by equine herpesvirus infection (EHV-1): “Part of the reason for the reduction in storms was herd immunity arising from vaccination. But a large part was improved awareness and diligence of farm managers who more effectively quarantined and segregated sick or aborting mares. This remains a key to prevention of EHV-1 abortion or neurologic disease outbreaks. With major outbreaks of either (such as the EHV outbreak that occurred in Findlay, Ohio, in 2003), it seems you can always track the spread of the virus from the index case by retracing the move-ment of animals around that time. All of these large outbreaks that spread across premises appear to arise from cases where horses were not effectively quarantined or were discharged from quarantine too early. This is when an already serious problem becomes dis-astrous.”

Optimizing a Healthy Environment

Critically important as a management strategy is the reduction of stress to individuals within a herd. Allen says, “It is important to minimize stress caused to horses by crowding, poor nutritional state, heavy parasite infestation, lengthy transport, disruption of estab-lished social groups, inclement weather, en masse weaning, and other disease states.”
Common sense dictates that to obtain the best in health and performance, it is best to develop the most nurturing environment for every horse. Daily turnout is important for mental health as well as the positive effects it has on improving circulation and muscle tone. Horses should be turned out with, or stabled beside, compatible members of the herd. While this encourages horses to eat comfortably and to maintain good body condition, herd compatibility also minimizes fighting and the risk of injury. Good-quality hay maximizes inner health and the immune system, thus helping the horse avoid allergic airway disease that adversely affects respiratory tract immunity. Run-in sheds and other shelters shield horses from stress created by adverse weather.

Strategies that promote clean air have significant favorable effects on immune responses of a horse’s respiratory tract. Dust from hay should be kept to a minimum by not storing hay over the stalls and by moving hay storage to a different barn from where horses are housed. Hay should be covered to protect it from mold-producing moisture. Arena dust can be managed by periodic watering. Stalls should be bedded well with mats and/or shavings to eliminate aerosolized dust generated by kicking and stomping at flies.

Stabling should have adequate drainage for urine, and feces and urine-soaked materials should be removed regularly from stalls and paddocks. Decomposing bedding should be stored away from the stabling area to minimize exposure to proliferating mold spores or ammonia fumes. This will also help keep away flies that can carry disease. Ample ventilation within a barn minimizes ammonia fumes in the stalls while promoting air exchange to remove infectious organisms and to decrease humidity that favors formation of aerosolized microbe-containing droplets.
Craig Barnett, DVM, a technical service veterinarian for Intervet, notes that any stres-sor to a horse’s system can depress the im-mune system. For example, he comments about stress-induced viral abortion related to transport: “Unless absolutely nec-essary, I recommend avoiding long-distance hauling of mares during their last trimester, as hauling is known to stress horses and has a negative effect on the immune system. I know of late-term pregnant, vaccinated mares that have been hauled long distances only to abort herpesvirus-positive foals. In these situations the hauling had an immunocompromising effect on the mares, the latent virus reactivated, and the mares aborted. Even though they were vaccinated, the vaccine could not overcome the immunocompromising effects of the stress and hauling and the reactivation of the latent infection.”

Studies have also proven that stress induced by transport has significant adverse effects on a horse’s immune system. Horse trail-ers should be purchased with the comfort of the horse in mind to minimize the stresses related to hauling. Attention to individual quirkiness about drinking and eating on the road also maximizes a horse’s comfort en route and upon arrival. Many destinations are fraught with stimulation, anxious horses, and stressful energy. Fear is communicable from one horse to another, so efforts to remove a nervous horse from the presence of other excitable animals can go a long way toward creating a calm environment.

Managing Non-Communicable Diseases

Some diseases can be mostly prevented through vaccination, but management has a large role to play in minimizing risk. For ex-ample, although a horse that contracts tetanus is not contagious to any others, he still suffers a gruesome sickness, which is often fatal. Even the smallest puncture wound has the potential to create tetanus. Also, a program should be in place to immunize annually with tetanus toxoid, so it is important to check at least daily for wounds, and to administer first-aid care immediately; this will dramatically reduce the risk of a horse developing tetanus.

Other neurologic diseases, such as equine encephalitis viruses (Eastern, Western, and Venezuelan encephalitis, and West Nile vi-rus), are transmitted by infected mosquitoes. Prevention relies on immunization against these viruses at least once or twice a year following an initial primary series of two injections. But management strategies, such as eliminating breeding sites for virus-transmitting mosquitoes, have a significant effect on reducing exposure.

Mosquitoes can hatch in stagnant water that is there only four days; to limit such reservoirs, remove containers that might fill with even small amounts of water. Some items are not so obvious, such as flower pots, bird baths, rain gutters, wading pools, wheelbar-rows, stock tanks, clogged roof gutters, discarded tires, swimming pool covers, boat covers, discarded cans, or paint buckets. Open containers should be turned facedown so water cannot accumulate, or holes should be drilled in container bottoms for water to drain through. More obvious sites of water collection include ditches, creeks, or ponds. Chlorine in swimming pools kills the larvae, but a swimming pool cover and other sites of freestanding water should be cleaned regularly. Stock tanks should be cleaned at least weekly to remove algae and debris. Ornamental pools can be aerated or stocked with fish that eat insect larvae to avoid developing mosquito breeding grounds.

On properties with bodies of freestanding or slow-moving water, additional strategies will need to be applied to each horse. Fly sheets and leg nets limit the surface area of a horse’s body that’s exposed to mosquito bites, but cannot cover an entire horse. Insect repellents are somewhat useful, and efforts should be made to apply them to all body parts. Permethrin-containing repellents are safe for use in horses and offer some protection against mosquitoes. However, repellents retain only a limited duration of protection, and rain or sweat or rolling in the dirt removes them from a horse’s coat. Screened barns protect horses from mosquitoes during their feed-ing times.

Different species of mosquitoes transmit viruses, with each displaying a different feeding habit: Some feed during the daytime, while others feed only at dawn or dusk. This makes it difficult to predict which part of the day horses should be left in the barn. Mosquitoes also lurk in corners and cracks in the barn, so spray these areas with insecticides. The use of fans to keep air moving in the stalls is helpful to prevent mosquitoes from landing on and biting horses (mosquitoes don’t fly well in moving air).

Botulism is yet another neurologic disease that can infect one or many horses in a herd, although it is not communicable from horse to horse. Besides vaccination against botulism type B in high-risk areas, careful monitoring of the feed is essential for prevention. Horse feed can be a source of botulism toxin if it’s contaminated by a decomposing animal, such as a rodent or bird, that was inadver-tently bound in the baler. Decomposing plant matter (including hay or grain) is also a possible source of the organism and subsequent production of toxin. Round bales are commonly implicated as sources of botulism. Conditions that favor growth of mold in feed also create an environment that is favorable for botulism. This applies not only to baled hay or grain, but especially to silage or feed bagged in plastic.

Getting the Most from Vaccines

Keeping accurate records and a calendar plan ensures that an effective vaccine schedule is followed to provide optimal protection. Viral vaccines should be up-to-date to protect against current strains of disease. Vaccines should be stored to manufacturer’s recom-mendations and administered to those specifications. Timing immunization prior to the highest likelihood of exposure is a key factor in minimizing the incidence or severity of infection. As an example, a horse that is frequently exposed to large congregations of horses should be immunized against respiratory viral vaccines approximately three to four times per year. And immunizing against mosquito-borne viruses should coincide with the period just prior to mosquito season.

To gain the best effect from vaccine boosters, they should be administered at least two weeks prior to expected exposure to give the immune system time to develop protective antibodies against a specific disease. Similarly, if vaccinating a horse for the first time against a pathogen, the two- or three-dose regimen should be completed one to two weeks prior to the beginning of the period when a horse will be at greatest risk of exposure to that pathogen. These offensive strategies give your horse the best defense against dis-ease.

Take-Home Message

Vaccines are an important part of man-aging the health of your horse. However, vaccinations alone are not enough. Good man-agement practices can considerably lower the risk of disease in your horses, and can mitigate outbreaks if they do occur. Consult with your veterinarian and develop a plan to manage your horses to avoid disease.

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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