Immunizations: Protect Your Horse Against Infectious Diseases
A Bayer Animal Health Brochure
Revised April 2008
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.
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.
INFLUENZA. This is one of the most common respiratory diseases in the horse. Highly contagious, the virus can be transmitted by aerosol from horse to horse over distances as far as 30 yards (for example, by snorting or coughing).
Signs to watch for are similar to those in a human with a cold (i.e. dry cough, nasal discharge, fever, depression and loss of appetite). With proper care, most horses recover in about 10 days. Some, however, may show symptoms for weeks, especially if put back to work too soon. Influenza is not only expensive to treat, but results in a lot of "down time" and indirect financial loss, not to mention discomfort to your horse.
Unfortunately, influenza viruses constantly change in an effort to bypass the horse's immune defense. Therefore, duration of protection is short-lived and revaccination is recommended every 3 to 6 months depending on the vaccine administered.
Not all horses need influenza vaccination. However, animals that travel or are exposed to other horses should be regularly immunized against influenza. Follow your veterinarian's advice as to whether your horse needs an influenza vaccine.
RHINOPNEUMONITIS. Two distinct viruses, equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4), cause two different diseases, both of which are known as rhinopneumonitis. Both cause respiratory tract problems, and EHV-1 may also cause abortion, foal death, and neurological signs, which may eventually cause paralysis. Infected horses may be feverish and lethargic as well as lose appetite and experience nasal discharge and a cough. Young horses suffer most from respiratory tract infections and may develop pneumonia secondary to EHV-1.
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.
All pregnant mares must be immunized. Foals, weanlings, yearlings and young horses under stress also should be vaccinated. Immune protection is short. Therefore, pregnant mares are vaccinated at a minimum during the 5th, 7th and 9th months of gestation, and youngsters at high risk need a booster at least every three to four months.
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.
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.
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.
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.
IN A NUTSHELL
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 at 10 to 12 months of age 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 mare: First dose: 6 months. Second dose: 7 months. Third dose: 8 months, then at 3-month intervals. Intranasal: Administer either a single dose of the MLV intranasal vaccine or a primary series of 3 doses of inactivated virus vaccine at 6 months of age (see above), unless there is an unusual threat that recommends earlier vaccination. Because some maternal anti-influenza antibody is still likely to be present, a complete series of primary vaccinations should still be given after 6 months of age. Foals from vaccinated mare: First dose: Administer either a single dose of the MLV intranasal vaccine or a primary series of 3 doses of inactivated-virus vaccine beginning at 6 months of age. The recommended intervals between these vaccinations are 4 to 6 weeks between the first and the second vaccinations. The third dose should be administered between 10 and 12 months of age. 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.
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. Second dose: 1 month later (plus 3rd dose at 6 months in endemic areas). 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. 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.
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.
Limited studies have been performed examining maternal antibody inference and inhibition of protection against WNV disease. The only data currently available is for the inactivated product in which foals were demonstrated to produce antibody in response to vaccination despite the presence of maternal antibody. No studies have been performed evaluating protection from disease in foals vaccinated in the face of maternal immunity.
Foals of vaccinated mares
Inactivated 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: No data is available for the vaccination of foals.
Administration of a 3-dose primary vaccination series beginning at 5 to 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 between the first and second doses. The third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.
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.
Modified live flavivirus chimera vaccine: This vaccine is labeled for the administration of a single dose to foals 5 months of age or older. A second dose is recommended at 10 to 12 months of age prior to the onset of the next vector season.
There is no data regarding administration of this product to younger foals. It is recommended that the above described schedule be followed to completion should this vaccine be administered to increased-risk foals < 5 months of age. Animals may be vaccinated more frequently with the product if risk assessment warrants.
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 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.
Recombinant canary pox vaccine: No data are available for the vaccination of foals and scheduling of the administration of the primary vaccination is based on the assumption that foals at 5 to 6 months of age respond to vaccination similarly to adults. A second dose, given at a 3 to 4 week interval after the first dose, may be warranted to ensure protective immunity. Animals may be vaccinated more frequently with this product if risk assessment warrants.
Modified live flavivirus chimera vaccine: There is no data regarding administration of this product to foals younger than 5 months of age. Due to presence of vectors and risk of disease, vaccination is warranted at earlier than 5 months of age and the use of this product is likely more appropriate for revaccination of older juveniles having already been administered a primary series.
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.
For more information, contact your equine veterinarian.
Or visit www.aaep.org/vaccination_guidelines.htm, for the complete AAEP Vaccination Guidelines.
American Association of Equine Practitioners (AAEP)
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Lexington, KY 40511