Vaccinations for Youngsters

Protecting tender immune systems against disease is the surest way to ensure the young horse doesn't fall victim to either temporarily uncomfortable or life-ending infections.

Even though foals receive some immunity by drinking colostrum within the first 12-24 hours of life, lack of exposure and the naiveté of the immune system sooner or later leave them susceptible to various disease-causing agents. Fortunately, many of these deficits can be addressed through vaccination.

The question is, against which diseases should you vaccinate the foal and at what age do you start?

Before the Beginning

Even before the foal hits the ground, newborn health can and should be assist-ed by vaccinating the broodmare, from whom protective antibodies pass to the newborn. Says Rebecca S. McConnico, DVM, PhD, Dipl. ACVIM, associate professor of equine medicine in the Equine Health Studies Program at Louisiana State University, "Broodmares should have booster vaccinations for tetanus, the encephalitis viruses (EEE, WEE, and WNV), equine herpesvirus 1 and 4, and equine influenza 1 and 2 about one month prior to foaling to ensure adequate antibody levels in the mare's colostrum." (To help protect against abortion, broodmares should be vaccinated against EHV 1 during the fifth, seventh, and ninth months of gestation as well.)

As those maternal antibodies wane in the growing foal, vaccinations are called upon to step in and continue the protection. Unfortunately, it's not exactly known when maternal antibody levels drop to a point where they cease to provide protection. Adding to the unknown, small amounts of maternal antibodies (even those below protective levels) can interfere with vaccines, leaving a window of opportunity for infectious disease. Given too soon, the vaccine is rendered ineffective; given too late, the foal is left exposed.
That's one reason why foal vaccinations are boostered at least once during the first year--vaccine potency. All vaccines need a second administration several weeks later in order to stimulate an adequate immune response.

There is some disagreement concerning timing of foal vaccinations. "Foals should have the first set of vaccinations between three and six months of age," McConnico states. "However, it is thought that even very low amounts of (maternal) passive IgG (colostral immunity required for healthy foal survival) remaining in the foal between three and five months of age may actually interfere some with the foal's ability to mount an immune response following vaccination. Therefore, booster vaccinations are necessary every four to eight weeks until one year of age."

McConnico notes that in some areas of the world such as Great Britain, veterinarians recommend waiting until the foal is six months of age to begin vaccination. "Timing of vaccinations for foals is still under intense investigation," she says.

Philip J. Johnson BVSc (Hons), MS, Dipl. ACVIM, MRCVS, professor of internal medicine and equine medicine and surgery at the University of Missouri, Columbia, recommends waiting until the foal is six months old, warning, "If vaccines are given much sooner than six months of age, their effectiveness might be inhibited by maternally derived antibodies (from colostrum) and the net result of the vaccine might not be so predictable. Simply said, giving a vaccine too early might cause nothing to happen, good or bad. Therefore, owners should not rely on early vaccines as they might for vaccines given to older horses.

"Also, foals don't have a well-developed immune responsiveness com-pared with adults," Johnson says. "Some have suggested that foals are, for several weeks or months following birth, under the influence of mare-derived signals that work in the mare (during pregnancy) to reduce immunity (in order that the mare can ‘tolerate' the developing fetus—which is technically an immunologically ‘foreign' object)."

Regardless of when the first shots are given to a young horse, Johnson strongly recommends that vaccinations be performed by veterinarians. "Vaccine failures can often be attributed to vaccine that hasn't been properly stored, is over-date, and so forth. Additionally, there is the very rare risk of an allergic reaction; if a horse does develop a reaction to the vaccine given by a horse owner, the horse might die from the reaction." A veterinarian at the scene could counteract that potential tragedy.

"It is also important to note that ‘active' vaccines (those that provoke the host to develop a helpful immune response) must be allowed time to work," Johnson says. "Giving a ‘shot' does not confer instant immunity." Even without interference from maternal antibodies, it can take two to three weeks before the initial vaccination and its booster can achieve maximum protection.

Choose Your Weapons

Vaccination recommendations for young horses are fairly uniform, with some variance according to degree of risk. For all youngsters, both Johnson and McConnico recommend protection against tetanus, influenza, Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), West Nile virus (WNV), rhinopneumonitis, and rabies.

"Young horses, especially those exposed to new horses, should also be vaccinated for respiratory tract diseases, especially equine influenza (flu) and equine herpesvirus," McConnico says. Vaccinate several weeks prior to introduction to other young horses of different (and uncertain) viral exposure backgrounds, Johnson suggests, to ensure the maximum immune response.
Johnson points out that much current thinking suggests that the influenza vaccines should not include equine type-1 (A1) influenza. "The current recommendations by the Influenza International Surveillance Panel from April 2005 recom-mend the current vaccines contain updated strains (A/eq/South Africa/4/03 or A/eq/Ohio/03, and A/eq/Newmarket/2/93, A/eq/Suffolk/89, or A/eq/Borlange/91)," he says.

He also notes that the neurologic form of EHV-1 seems to be on the rise: "There have been several epidemics of the neurologic form of EHV-1 (encephalomyelopathy) in the past coup-le of years."

Recent research suggests that the modified live EHV vaccine, although it isn't labeled against neurologic disease, might offer better protection against the neurologic form of EHV-1 than killed preparations.

Additionally, McConnico reports that vaccination against equine protozoal myelitis (EPM) should be considered. "This potentially debilitating and fatal disease occurs more commonly in young horses in training," she says.

Vaccinating foals intended to be breeding stallions against equine viral arteritis is also a good idea, Johnson says.

Protection against botulism and Venezuelan equine encephalomyelitis (VEE) is generally recommended only in areas where there's a threat. "I recommend vaccinating against type B botulism on those farms that have experienced botulism, which is not very common in many parts of the U.S.," states Johnson.

The strangles vaccine, used to protect against infection from Streptococcus equi, remains controversial. Explains McConnico, "Although effective in stimulating a protective immune response, this vaccine carries the possibility of complications including fever, anorexia, muscle pain, and abscess formation. This vaccination is often considered for young horses, since they are more likely to become infected with moderate to severe forms of the disease. However, the American College of Veterinary Internal Medicine does not recommend strangles vaccination in horses that have had recent exposure (in the face of an outbreak)."

Although vaccines exist to protect horses against Potomac horse fever (PHF), endotoxins, equine viral arteritis, and anthrax, they aren't routinely recommended. "Vaccination against these diseases may be useful in areas where these diseases pose a threat," says McConnico.

However, Johnson adds, "I've been unimpressed with published data regarding efficacy for the PHF vaccine; it appears that there are several immunologically different strains of the causative organism and the vaccine only contains one strain. That said, we do not have high incidence of this disease on which to make a judgment regarding the effectiveness of the vaccine based on field use. There's a lack of evidence that providing vaccination against endotoxin is helpful for horses (and humans) that are affected with endotoxin diseases. Anthrax fortunately remains a very rare disease in horses."

Johnson reminds horse owners that it is important that vaccines be given in a manner that appropriately follows the manufacturers' administration recommendations. Doing so provides the best protection for any horse, with a minimum of risk.

COMMON DISEASES AFFECTING YOUNG HORSES

Tetanus  This severe disease progresses very quickly. Affected horses experience stiffness, rigidness, overreaction to noise and stimuli, inability to open the mouth, difficulty breathing, and recumbency. Fatal if untreated and sometimes despite early, aggressive treatment. Usually occurs consequent to a wound in a non-vaccinated horse.
Encephalomyelitis 
(EEE, WEE, WNV, VEE)
Spread by mosquitoes, these viruses affect the brain and spinal cord. Infected horses have severe depression, weakness, incoordination, ataxia, stiffness, fever, difficulty eating, and abnormal behavior.
WEE is fatal in about 25% of cases. EEE is nearly always fatal. WNV fatalities are relatively rare (about 30% of horses with neurological signs of WNV infection will die) although recovered horses might or might not retain neurologic deficits. VEE is often fatal.
Rhinopneumonitis 
(Equine Herpesvirus I and IV)
This virus is extremely well dispersed. Type 1 is commonly associated with respiratory disease, weak foals, and abortion (and rarely neurologic disease); type 4 is primarily associated with respiratory disease (and rarely weak foals and abortion). The neurologic form of the disease (EHV-1) appears to be on the rise recently; it causes horses (sometimes in groups) to lose control of their hind legs and bladder (and other things). There have been several epidemics of the neurologic form in the past couple of years; vaccination is likely not protective against the neurologic form.
Influenza  Influenza is usually not life-threatening, but it increases vulnerability to other diseases, including pneumonia. Clinical signs include fever, lethargy, cough, nasal discharge, muscle aches, and inappetence.
Rabies  Clinical signs include weakness in the limbs, loss of neurologic control of limbs, loss of ability to swallow, profound depression, or furious states where the animal aggressively attacks objects or people. Always fatal.
Potomac Horse Fever  This disease can cause severe diarrhea, severe laminitis or founder, and abortion.
Equine Protozoal Myeloencephalitis (EPM) Clinical signs can include weakness, lameness, incoordination, inability to move correctly (especially in the hindquarters) or to stand up, seizures, weight loss, blindness, loss of balance, disuse of a single limb, and/or inappropriate sweating. Lack of treatment can lead to permanent nerve damage and death.
Strangles  Easily transmitted by other horses and by intermediaries such as people, buckets, and tack. Early clinical signs include nasal discharge, cough, inappetence, and fever. Later, the horse often develops swellings in the throat, between the jaws, and/or under the ears. Occasionally, abscesses affect other parts of the body, causing colic or signs of neurological disease.
Information provided by Philip J. Johnson, BVSc (Hons), MS, Dipl. ACVIM, MRCVS, professor of internal medicine and equine medicine and surgery at the University of Missouri, Columbia.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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