Immunizations: Protect Your Horse Against Infectious Diseases

Few things will protect your horse from the ravages of disease as easily and effectively as immunizations. The vaccinations administered by your veterinarian to your horse place a protective barrier between the animal and a whole list of problems: tetanus, encephalomyelitis (sleeping sickness), influenza, rhinopneumonitis, rabies, strangles, West Nile virus and Potomac Horse Fever, to name the most common.

Vaccinations are a vital part of proper equine management. If incorporated into a program that includes regular deworming, an ample supply of clean water, a good nutrition program and a safe environment, you and your horse will be all set to enjoy many happy, healthy, productive years together.

WHAT TO EXPECT

A good immunization program is essential to responsible horse ownership, but just as in humans, vaccination does not guarantee 100 percent protection. In some situations, immunization may decrease the severity of disease but not prevent it completely. This is due to many complicated scientific reasons, such as differences in the virulence or severity of some diseases (such as influenza).

Vaccination involves the injection (with a sterile syringe and needle) of bacteria or viruses that are inactivated or modified to avoid causing actual disease in the horse. Recently intranasal vaccinations have been developed for immunization against influenza and strangles. Two or more doses are usually needed to initiate an adequate immune response.

Once the immunization procedure is completed, the protective antibodies in the blood stand guard against the invasion of specific diseases. Over time, however, these antibodies gradually decline. Therefore, a second vaccination is required to boost immunity to acceptable levels. Protection against some diseases such as tetanus and rabies can be accomplished by boostering once a year. Others require more frequent intervals to provide adequate protection.

VACCINATIONS NEEDED

The specific immunizations needed by a particular horse or horses depend upon several factors: environment, age, use, exposure risk, value, geographic location and general management. Your local equine veterinarian can help you determine the vaccination program best suited to your horse’s individual needs.

The following diseases are those most often vaccinated against. Again, your local veterinarian will know what is best for your horse.

Tetanus. Sometimes called “lockjaw,” tetanus is caused by toxin-producing bacteria present in the intestinal tract of many animals and found in abundance in the soil where horses live. Its spores can exist for years. The spores enter the body through wounds, lacerations or the umbilicus of newborn foals. Therefore, although not contagious from horse to horse, tetanus poses a constant threat to horses and humans alike.

Symptoms include muscle stiffness and rigidity, flared nostrils, hypersensitivity, prolapsed third eyelid and the legs stiffly held in a sawhorse stance. As the disease progresses, muscles in the jaw and face stiffen, preventing the animal from eating or drinking. More than 80percent of affected horses die.

All horses should be immunized annually against tetanus. Additional boosters for mares and foals may be recommended by your veterinarian. Available vaccines are inexpensive, safe and provide good protection.

Encephalomyelitis. More commonly known as “sleeping sickness,” this disease is caused by the Western Equine Encephalomyelitis (WEE) virus or the Eastern version (EEE). WEE has been noted throughout North America, while EEE appears only in the east and southeast. VEE, the Venezuelan variety, has not been seen in the United States for many years (however, a recent outbreak of VEE occurred in Mexico). Sleeping sickness is most often transmitted by mosquitoes, after the insects have acquired the virus from birds and rodents. Humans also are susceptible when bitten by an infected mosquito, but direct horse-to-horse or horse-to-human transmission is very rare.

Symptoms vary widely, but all result from the degeneration of the brain. Early signs include fever, depression and appetite loss. Later, a horse might stagger when it walks, and paralysis develops in later stages. About 20 to 50 percent of horses infected with WEE die, and the death rate is 75-100% of animals infected with EEE. The mortality rate for VEE is 40 to 80 percent.

All horses need an EEE and WEE vaccine at least annually. Pregnant mares and foals may require additional vaccinations. The best time to vaccinate is one month before mosquitoes become active. In the South and West it is necessary to administer a booster shot every 4 months to ensure extra protection all year-round.

Rabies. A frightening disease, which is more common in some areas than others. Horses are infected infrequently, but death always occurs. Rabies can be transmitted from horses to humans.

West Nile virus (WNV). A neurological disease that affects horses throughout the continental United States and is transmitted through the bite of an infected mosquito. States with more persistent mosquito populations may require more aggressive vaccinations along with aggressive mosquito control techniques. Follow your veterinarian’s recommendations.

OTHER DISEASE THREATS

Several other diseases are common, although the need for vaccination against them is a highly individual one. Rely on your veterinarian to guide you.

Other diseases include:

Strangles. A highly contagious and rarely fatal disease. There may be some side effects associated with vaccination; therefore, it is important to discuss the risks versus benefits of vaccination with your veterinarian.

Botulism. Known as “shaker foal syndrome” in young horses, this disease can be serious. Botulism in adult horses, “forage poisoning,” also can be fatal. Vaccines are not available for all types of botulism, but pregnant mares can be vaccinated in endemic areas.

Equine viral arteritis (EVA). A complicated disease, which can result in some breeding restrictions and export problems. Follow your veterinarian’s recommendations.

Potomac Horse Fever. A seasonal problem with geographic factors. The mortality rate varies from 5 to 30 percent. Contact your veterinarian.

Rhinopneumonitis is spread by aerosol and by direct contact with secretions, utensils or drinking water. The virus may be present but inapparent in carrier animals.

For primary immunization, an initial vaccination is required, followed by a repeat dose in 3 to 6 weeks. The following is a handy reference guide for scheduling your horse’s immunizations:

Tetanus. All horses. Tetanus antitoxin: Foals from nonvaccinated mares should receive a dose of tetanus antitoxin soon after birth.  This induces immediate passive protection that usually lasts 2 to 3 wks.  Therefore, another dose of tetanus antitoxin may be indicated on rare occasions where a foal has a penetrating injury or surgery prior to starting the tetanus toxoid series and > 3 wks after receiving the initial tetanus antitoxin.  Tetanus antitoxin should never be given to horses currently vaccinated for tetanus or foals born to correctly tetanus vaccinated mares.It may be worth noting that a small but significant number of horses experience serum sickness and fatal hepatic failure (serum hepatitis) several weeks following vaccination with tetanus antitoxin.  Therefore, tetanus antitoxin should not be given without discretion.  Providing there has been adequate passive transfer of antibodies via the colostrum, foals born to vaccinated mares should have adequate levels of tetanus antibody and should not need tetanus antitoxin.

Tetanus Toxoid: Foals from nonvaccinated mares: Administer a primary 3-dose series of toxoid beginning at 1 to 4 months of age with 4-week intervals between doses. Serologic data indicates that a 3- dose initial series produces a more consistent immune response in all foals, regardless of the age at which the series is initiated.  Tetanus antitoxin is indicated to provide passive immunity in situations where a foal is born to a non-vaccinated mare and is at risk of tetanus infection.

Foals from vaccinated mares: Administer a primary 3-dose series of tetanus toxoid beginning at 4 to 6 months of age.  A 4 to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age.

Encephalomyelitis. All horses. Foals from nonvaccinated mares: Administer a primary 3-dose series beginning at 3 to 4 months of age. A 4-week interval between the first and second doses is recommended. The third dose should be administered on a 60 day interval after 2nd dose before the onset of the next mosquito season.

Foals from vaccinated mares: Brood mares at 4 to 6 weeks before foaling. Administer a primary three-dose series beginning at 4 to 6 months of age. A 4- to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

Influenza. Most horses. Inactivated injectable: Foals from nonvaccinated/ vaccinated mare: First dose: 6 months. Second dose: 3-4 weeks after 1st dose. Third dose: 10-12 months of age. Modified Live injectable: Foals from nonvaccinated/vaccinated mare: First dose: 6-7 months of age. Second dose:  11-12 months of age. Canary Pox vector injectable: Foals from nonvaccinated/vaccinated mare: First dose: 6 months of age. Second dose: given 5 weeks after 1st dose. Broodmares: biannually, plus booster 4-6 weeks pre-foaling. Consult with your veterinarian regarding the vaccine schedule for the intranasal modified live version.

Rhinopneumonitis. Foals first dose: Administer a primary series of 3 doses of inactivated EHV-1/EHV-4 vaccine or modified-live EHV-1 vaccine, beginning at 4 to 6 months of age and with a 4- to 6-week interval between the first and second doses. Administer the third dose at 10 to 12 months of age. Immunity following vaccination appears to be short-lived and it is recommended that foals and young horses be revaccinated at 6-month intervals.

Rabies. All horses. Foals born to non-vaccinated mares: Administer a primary series of 3 doses. The first dose of vaccine should be administered at 3 to 4 months of age. The second dose should be given 4 weeks later. The third dose should be given at 10 to 12 months of age. Revaccinate annually thereafter.

Foals born to vaccinated mares: First dose: Administer a primary series of 3 doses. The first dose of vaccine should be administered no earlier than 6 months of age. The second dose should be given 4 to 6 weeks later. The third dose should be given at 10 to 12 months of age. Revaccinate annually thereafter. This schedule avoids interference with antibody production in the foal due to presence of colostrum-derived antibodies.

Strangles. Foals: Injectable: For foals at high risk for exposure to strangles, administer a 3-dose primary series of an M-protein product beginning at 4 to 6 months of age. An interval of 4 to 6 weeks between doses is recommended.

Intranasal: Administer intranasally at 6 to 9 months of age a 2-dose primary series with a 3-week interval between doses. This vaccine has been safely administered to foals as young as 6 weeks of age when there is a high risk of infection, such as occurs during an outbreak, but the efficacy of its use in very young foals has not been adequately studied. If administered to young foals in this manner, a third dose of the modified live vaccine should be administered 2 to 4 weeks before the foal is weaned to optimize protection during that time of high risk of infection.

Note: The risk of vaccine-associated adverse events is increased when the modified-live product is administered to young foals.

Broodmares: Administer primary series of killed vaccine containing M-protein (see above, Adult horses unvaccinated) with final dose to be administered 4 to 6 weeks pre-partum.

Potomac Horse Fever. Foals first dose: Due to the low risk of clinical disease in young foals and the possible maternal antibody interference, primary immunization for most foals can begin after 5 months of age. The manufacturer’s recommendation is for a 2-dose series administered at a 3- to 4-week interval. However, as with other killed products, a third dose at 12 months of age is recommended. If the primary series is initiated when foals are less than 5 months of age, additional doses should be administered at monthly intervals up to 6 months of age to ensure that an immunologic response is achieved.

Semiannual for older horses. Broodmares: semiannually with one dose at 4 to 6 weeks pre-foaling. Consult with your veterinarian if you are in an endemic area.

West Nile virus (WNV). All Horses. Foals first dose: 3 to 4 months for foals of unvaccinated mares. 1st dose for foals of vaccinated mares is 4-6 months of age. Second dose: Foals of vaccinated and nonvaccinated mares: 4 weeks after 1st dose. Third dose: at 10-12 months of age, prior to the onset of the next vector season to foals of vaccinated mares. 3rd dose for foals of nonvaccinated mares at a 60 day interval after 2nd dose. Broodmares: 4 to 6 weeks prepartum. Annually, thereafter prior to expected risk. In endemic areas vaccinate 2 to 3 times a year depending on risk. Adult horses previously vaccinated, vaccinate annually in the spring, prior to the onset of the insect vector season.

For animals at high risk or with limited immunity, more frequent vaccination or appropriately timed revaccination is recommended in order to induce protective immunity during periods of likely exposure. For instance, juvenile horses (<5 years of age) appear to be more susceptible than adult horses that have likely been vaccinated and/or had subclinical exposure. Geriatric horses (>15 years of age) have been demonstrated to enhance susceptibility to WNV disease. Therefore, more frequent vaccination is recommended to meet the vaccination needs of these horses.

Booster vaccinations are warranted according to local disease or exposure risk. Only the modified live chimera WNV vaccine has been tested for protection against signs of clinical disease but protection against disease for 12 months is likely with all WNV vaccines. However, more frequent vaccination may be indicated with any of these products depending on risk assessment.

Inactivated whole virus vaccine: A primary series of 2 doses is administered to previously unvaccinated horses. A 4- to 6-week interval between doses is recommended. The label recommended revaccination interval is 12 months.

Recombinant canary pox vector vaccine: A primary series of 2 doses is administered to previously unvaccinated horses with a 4- to 6-week interval between doses. Revaccinate prior to the onset of the next vector season. The label recommended revaccination interval is 12 months.

Modified live flavivirus chimera vaccine: Primary immunization is by a single dose administered to horses 5 or more months of age. The label recommended revaccination interval is 12 months.

Pregnant mares

Limited studies have been performed that examine vaccinal protection against WNV disease in pregnant mares. While none of the licensed vaccines are specifically labeled for administration to pregnant mares at this time, veterinarians have vaccinated pregnant mares due to the risk of natural infection. It is an accepted practice by many veterinarians to administer WNV vaccines to pregnant mares as the risk of adverse consequences of WNV infection outweighs any reported adverse effects of use of vaccine.

Pregnant mare previously vaccinated

Vaccinate at 4 to 6 weeks before foaling.

Pregnant mares previously unvaccinated

Initiate a primary vaccination series (see Adult horses previously unvaccinated) immediately. Limited antibody response was demonstrated in pregnant mares vaccinated for the first time with the killed vaccine. It is unknown if this is true for the other products. Vaccination of naïve mares while open is a preferred strategy.

Foals of vaccinated mares

Inactivated whole virus vaccine: Administer a primary 3-dose series beginning at 4 to 6 months of age. A 4- to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

Data indicates that maternal antibodies do not interfere with this product; however protection from clinical disease has not been provocatively tested in foals.

Animals may be vaccinated more frequently with this product if risk assessment warrants.

Recombinant canary pox vector vaccine: Administration of a 3-dose primary vaccination series beginning at 4-6 months of age is based on the assumption that foals of that age respond to vaccination similarly to adults. There should be a 4-week interval after the 1st dose. The 3rd dose is given at 10-12 months of age.

There is no data for this product regarding maternal antibody interference. Protection from clinical disease has not been provocatively tested in foals.  Animals may be vaccinated more frequently with this product if risk assessment warrants.

Inactivated flavivirus chimera vaccine: This vaccine is labeled for the administration of a 3-dose series beginning at 4-6 months of age. Second dose is given 4-6 weeks after 1st dose. Third dose is given at 10-12 months of age, prior to the onset of the next vector season.

Foals in the Southeastern USA: Due to early seasonal vector presence, the primary vaccination series should be initiated earlier with the addition of a dose at 3 months of age.

Foals of unvaccinated mares

The primary series of vaccinations should be initiated at 3 months of age and, where possible, be completed prior to the onset of the high-risk insect vector season.

Inactivated whole virus vaccine: Administer a primary series of 3 doses with a 30-day interval between the first and second doses and a 60-day interval between the second and third doses. If the primary series is initiated during the mosquito vector season, an interval of 3 to 4 weeks between the second and third doses is preferable to the above described interval of 8 weeks.

Note: Primary vaccination series scheduling may be amended with vaccinations administered to younger foals that are at increased risk of exposure due to the presence of vectors.

Foals born during the vector season may warrant inititation of the primary vaccination series at an earlier age than a foal born prior to the vector season.

Data indicates that maternal antibodies do not interfere with the originally licensed inactivated whole virus vaccine product; however, protection from clinical disease has not been provocatively tested in foals less than 6 months of age with any of the currently licensed West Nile Virus products.

Recombinant canary pox vaccine: First dose given at 3-4 months of age. A second dose, given 30 days after 1st initial dose. Third dose given 60 days after 2nd dose. Animals may be vaccinated more frequently with this product if risk assessment warrants.

Inactivated flavivirus chimera vaccine: First dose given at 3-4 months of age. Second dose given 30 days after 1st dose. Third dose given 60 days after 2nd dose.

Foals in the Southeastern USA: Due to early seasonal vector presence, the primary vaccination series should be intitiated at 3 months of age.

Many combination vaccinations are available. Please check with your local equine veterinarian.

Appropriate vaccinations are the best and most cost-effective weapon you have against common infectious diseases of the horse. A program designed with the help and advice of your local veterinarian will keep your horses — and you — happy and healthy for many years to come.