Vaccinations play a vital role in the health of our horses. Vaccines are particularly important for those that are routinely exposed to other horses at large barns, or through training and competition.
Vaccines are generally regarded as safe and effective for most horses, though, like all medical interventions, they carry a small risk of side effects. Severe reactions are uncommon, with most vaccinated horses experiencing only mild, short-term reactions, such as soreness at the injection site, temporary lethargy, or a slight fever.
While these side effects typically resolve on their own, it’s important for horse owners to recognize potential adverse reactions and understand when to seek veterinary assistance.
In this article, we discuss common vaccine side effects in horses, tips for managing horses after receiving a vaccination, and guidance on when to contact a veterinarian.
Types of Equine Vaccines
Vaccination is one of the most important advances in modern medical technology. In both humans and animals, vaccines help to prevent the spread of disease and reduce the severity and mortality associated with infectious illnesses. [1][2]
In some instances, vaccines have even led to the complete eradication of specific diseases through of herd immunity. [1][2]
Equine vaccine recommendations vary by country. In North America, the American Association of Equine Practitioners (AAEP) has established core vaccines for all horses. Your veterinarian can help determine if your horse would also benefit from non-core, or risk-based, vaccinations based on factors like lifestyle and regional disease risks. [3]
There are three main types of vaccines used in horses: [2][4]
- Inactivated (killed) vaccines, including whole pathogen and protein-based vaccines
- Live-attenuated (modified-live) vaccines
- Recombinant vaccines
Inactivated (Killed) Vaccines
Inactivated vaccines use a non-living strain of the pathogen to elicit an immune response, leading to the production of pathogen-specific antibodies. Importantly, inactivated vaccines cannot cause disease and cannot replicate or spread between hosts. [2][4]
Inactivated vaccines generally require a series of doses and regular boosters. There are two main types: [2][4]
- Inactivated/killed pathogen vaccines: Contain whole pathogens that have been inactivated.
- Protein vaccines: se naturally derived pathogen components. These proteins are typically non-pathogenic and may lead to fewer injection site reactions than whole-pathogen vaccines.
Live-attenuated (Modified-live) Vaccines
Live-attenuated vaccines are created from weakened strains of viruses or bacteria, often achieved through laboratory culture. In some cases, they are developed using pathogen variants from a similar species or temperature-sensitive mutations. [2][4]
These vaccines stimulate a robust immune response and the production of pathogen-specific antibodies, typically resulting in a longer duration of immunity with fewer doses required. [2][4]
Recombinant Vaccines
Recombinant vaccines leverage advanced technologies to stimulate immunity and include: [2][4]
- Live Attenuated Vector Vaccines: Produced by incorporating a pathogen’s antigenic peptides into a harmless carrier virus or bacteria. This effectively disguises the harmless bacteria as the pathogen, stimulating an immune response without causing clinical disease.
- Chimeric Vaccines: Produced by substituting genes from the target pathogen for similar genes in a safe, but closely related organism.
- DNA Vaccines: Consist of a DNA plasmid encoding a viral gene that can be expressed inside cells of the animal to be immunized. Once the immunized animal’s cells start producing the viral gene, it stimulates an immune response, building antibodies to the source virus.
Vaccine Ingredients
Horse owners often have questions about the safety of vaccine ingredients. By familiarizing yourself with some of the most important active ingredients, you can better understand veterinary recommendations about your horse’s vaccine schedule.
Common components found in most horse vaccines include: [5][6][7][8]
- Antigens: The primary component of any vaccine. These molecules are specific to the pathogen the horse is being immunized against. Antigens stimulate the horse’s immune response, promoting amplification and long-term survival of antigen-specific T lymphocytes, which recognize and attack the pathogen if the horse is exposed in the future.
- Adjuvants: Substances designed to increase the size, efficacy, and persistence of immune response. Adjuvants work to promote long-lasting immunity.
- Stabilizers: Added to live attenuated vaccines to enhance virus stability and increase antibody response to the vaccine. The main ingredients of stabilizers include hydrolyzed gelatin, sucrose, sorbitol, and other sugars.
- Antibiotics: Used in some vaccines for preservation purposes. However, the level of antibiotics permitted in vaccines is well below a therapeutic dose; antibiotics are not included for the purpose of treating bacterial infections.
- Preservatives: Used to prevent fungal and/or bacterial contamination of the vaccine. Preservatives are not present in all vaccines. They were originally introduced for use in multi-use vaccine vials. Modern vaccine production mostly uses single-dose packaging, reducing the reliance on preservatives in these formulations.
Proper storage, handling, and administration are essential to maintain vaccine efficacy and minimize the risk of adverse effects. [9]
Core Vaccines
The core vaccines for horses defined by the AAEP, provide protection against endemic diseases and help control highly contagious, severe diseases, safeguarding both equine and public health. [2]
In North America, the core vaccines for horses include the following: [2]
- Tetanus
- Rabies
- Viral encephalitis from Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE)
- West Nile Virus
Core vaccines should be administered to all horses, with veterinarians using appropriate re-vaccination (booster) schedules to maintain strong immunity. Titer testing can help determine if a horse has sufficient antibodies before a booster is given, offering a personalized approach to vaccination. [10] Veterinarians may also recommend titer checks when assessing a horse’s need for risk-based vaccines. [2]
If you choose to personally administer vaccines to your horse(s), be aware that AAEP provides protocol guidelines for equine practitioners, which may differ from manufacturer instructions. [2] However, the safest option for both you and your horse is to have all vaccines administered by a veterinarian.
Non-Core Vaccines
In addition to core vaccines, your veterinarian may recommend other immunizations based on your horse’s specific risk factors. These are known as non-core, or risk-based, vaccines.
Risk-based vaccines for horses include: [9]
- Equine influenza (EIV)
- Equine Herpesvirus 1 and 4 (EHV 1 and 4)
- Strangles
- Potomac Horse Fever
- Anthrax
- Botulism
- Leptospirosis
- Equine Viral Arteritis
It’s important to note that the international governing body for equestrian sports, the Fédération Èquestre Internationale (FEI), mandates that horses be vaccinated against equine influenza within 6 months and 21 days before participating in any level of sanctioned competition. [3]
Administration of all other non-core vaccines depends on: [9]
- Horse history
- Herd group
- Occupation
- Level of exposure to horses outside the herd
- Region
When considering non-core vaccines for your horse, consult with your veterinarian to consider the following factors:
- Risk of disease to the horse and/or herd
- Consequences of the disease itself
- Efficacy of the vaccine
- Impact of disease versus cost, efficacy, and risk of vaccination
- Potential risk of adverse reactions associated with vaccine
- Timing of immunity onset
- Individual horse and possible risk factors (i.e. age, health status, recent immunosuppressive medications, stress, poor nutrition)
Administration Guidelines
To minimize the risk of side effects, all vaccines should be administered by a veterinarian or qualified healthcare practitioner, following AAEP-recommended protocols for safe and effective vaccination.
Considerations for vaccine administration include: [11]
- Administrator safety – Ensuring proper handling and technique
- Minimizing side effects – Using best practices to reduce adverse reactions
- Injection site selection – Avoiding vital structures like bones, nerves, blood vessels, and ligaments
For many vaccines, a large muscle mass actively used by the horse is the best place to give a vaccine. Appropriate injection sites include: [11]
- Base of neck: Triangular area bordered by the nuchal ligament (along the top of the neck), the cervical vertebrae (along the bottom half of the neck, from poll to the point of shoulder), and the scapula (shoulder blade).
- Hamstrings: Muscle mass below the point of buttocks.
Injecting vaccines in the gluteal muscle/hip region is not recommended. If an abscess develops after vaccination, these areas may not drain properly, leading to tissue damage and considerable healing time. [11]
Vaccine Schedule
The recommended interval between vaccine doses depends on immunologic memory. This describes the immune system’s ability to respond more quickly and effectively to pathogens that have previously been encountered. [10]
The duration of immunologic memory depends on several factors including: [10]
- Characteristics of the antigen
- Live vs. killed source of antigen
- Which adjuvants are in the vaccine
- Route of administration
For example, some vaccines result in lifelong immunity while others may need to be given every 2 – 3 years. Keep in mind that the duration of immunity stated by the vaccine manufacturer is the minimum duration supported by data at that time. [10] Veterinarians make an individual assessment when determining the frequency of re-vaccination for each horse.
Risk of Vaccine Side Effects
While all vaccines carry a small risk of side effects, most reactions are mild and temporary. However, it’s important for horse owners to recognize signs of adverse reactions, even mild ones, and report them to their veterinarian promptly. [3]
Possible side effects of vaccines in horses include:
- Self-limiting reactions
- Local Arthus reaction
- Granulomas
- Latent infection
- Allergic reactions
Self-Limiting Reactions
Self-limiting reactions are mild, short-lived side effects that commonly follow vaccination. Symptoms such as mild fever, reluctance to eat, and lethargy typically resolve on their own within one to two days. [11]
Arthus Reaction
The Arthus reaction is a mild response and is one of the most common reactions to vaccines, usually occuring within 24 hours after administration. This reaction involves the injection site becoming swollen and painful. [1][11]
This response happens due to circulating non-specific antibodies. When additional antigen is injected, these antibodies form immune complexes around blood vessels in the skin, resulting in localized inflammation and pain. [1]
Granulomas
Less commonly, delayed hypersensitivity reactions, like granulomas, can develop at the injection site. This reaction leads to a small lump forming due to a cluster of immune cells. Granulomas typically form in response to adjuvants used in vaccines. [4]
Latent Infection
Another rare side effect of vaccination is latent infection activation. In these cases, the stress from vaccination may trigger previously dormant infections. For instance, equine herpesvirus has been reported in a horse after vaccination against African horse sickness. [12]
Hypersensitivity/Allergic Reaction
Allergic reactions to vaccines are unusual. These reactions can be acute or delayed, but typically occur within minutes or up to four hours after the vaccine is administered. Most acute-onset reactions occur in response to a vaccine component such as egg protein, gelatin, or other additives. [13]
Hypersensitivity is also more commonly associated with multiple injections of an antigen from repeated use of inactivated products. [12] Viral vaccines such as the West Nile Virus vaccine may cause this type of reaction. [1]
Though not common, horses can also experience more severe, systemic reaction due to hypersensitivity. Some of these include: [11][13][14][15]
- Urticaria (hives): Welts that appear on the skin and are often itchy. This condition may resolve within 24 hours, but it could also indicate anaphylaxis. Therefore, hives that develop after vaccination should always be reported to your veterinarian right away.
- Purpura hemorrhagica: This sensitivity reaction causes necrotizing vasculitis (inflammation of blood vessels). It tends to occur in older horses and may occur as a side effect of the strangles (S equi) vaccination, particularly in horses that have previously had strangles.
- Angioedema: A potentially life-threatening reaction that involves edema (fluid accumulation) in deeper tissues, often affecting the face or oropharynx. It can be painful and may take 24–48 hours to resolve. This reaction can be acute or delayed.
- Colic: Colic is a rare reaction to vaccines, but most commonly associated with the rabies vaccine in horses.
- Anaphylaxis: A rare but life-threatening allergic reaction with rapid onset. Anaphylaxis affects multiple organ systems and may include symptoms such as hives, wheezing, swelling of the mouth, tongue, and throat, difficulty breathing, diarrhea, low blood pressure, loss of consciousness, and shock. The risk of anaphylaxis after vaccination is rare, estimated at around 1.31 per million vaccine doses administered in people.
Special Considerations for Vaccination
In some cases, vaccination may need to be delayed or adapted to an individualized protocol. Horses with specific immunological risks require tailored vaccination programs, highlighting the importance of consulting with a veterinarian when making immunization decisions.
Broodmares
Vaccinating broodmares is recommended for two primary reasons:[2]
- To protect the mare from certain abortive diseases and tetanus
- To stimulate the production of antibodies in colostrum, which confers passive immunity to protect the newborn foal
With that said, vaccination of pregnant mares does not come without risk. [2]
Research suggests that repeating vaccination in pregnant mares might increase risk for local and systemic disease. As a result, current practice is to avoid vaccination of the broodmare during the first 60 days of pregnancy. [2]
The AAEP recommends administering boosters of core vaccines 4-6 weeks before foaling. However, research shows that giving the modified-live vaccine against equine viral arteritis (EVA) during the last two months of pregnancy increases risk of abortion. [2]
The safety of administering any vaccine to broodmares should be carefully considered and discussed with your veterinarian. [2]
Foals
Vaccination protocols for growing foals differ significantly from those of adult horses due to their developing immune systems. When first born, rely on passive immunity gained from colostrum intake, which contains antibodies from the dam.
The scheduling of the first vaccination depends on factors such as:
- Whether the foal’s dam was vaccinated during pregnancy
- The disease risk in the foal’s environment
- The foal’s overall health status and immune development
In foals born to vaccinated mares, the initial core vaccines are usually administered around 4 – 6 months of age. For foals from unvaccinated mares or those with unknown vaccination history, early vaccination may be required.
Given their increased vulnerability, consultation with a veterinarian is key to determine the most suitable vaccination schedule for each foal.
Immunocompromised Horses
There is a higher risk of adverse effects when vaccinating horses with compromised immune systems such as those with immunodeficiency or PPID. This is also true for horses that are on immunosuppressive medications such as corticosteroids. For these horses, it’s advised to consider the benefit versus the risk of all vaccinations. [11]
In recent years, there has been much discussion about the connection between excessive vaccination and overstimulation of the immune response in many species, including horses. While there is currently lack of evidence of this, several studies have attempted to establish a connection. Further investigation is needed in this area. [1]
With that said, the current consensus is that vaccinations alone do not cause an autoimmune response. It is more likely a combination of genetics, environmental factors, and overstimulation of the immune system that may lead to the development of many autoimmune diseases. [1]
Preventing Complications
To reduce your horse’s risk of vaccination side effects, work closely with your veterinarian to determine if non-core vaccines or re-vaccination are necessary. Vaccination may not be advisable for pregnant mares, immunocompromised horses, or those with a history of vaccine reactions.
[2][11]
It’s also important to avoid vaccinating animals during high-stress periods such as during extreme heat or cold. [12]
Having your veterinarian administer vaccines also reduces the risk of adverse reactions since they are trained on giving vaccinations safely and correctly. Monitor your horse closely for the first 24 hours after vaccination and make sure to report any signs of side effects to your veterinarian promptly.
Here are some frequently asked questions about the risk of vaccine side effects in horses:
Most side effects of equine vaccines are mild and self-limiting, meaning they resolve on their own within a few days. [2]
Mild reactions to equine vaccines include lethargy, inappetence and fever. [11] Some horses may have swelling or pain at the injection site. More serious reactions are extremely rare. [2][11]
Yes. It is common for horses to experience temporary lethargy after receiving a vaccine. [11] If your horse seems lethargic for more than a day or two after vaccination, contact your veterinarian.
Summary
Equine vaccinations are important in preventing serious disease in not only individual horses, but also the general population. Always weigh the risk versus the benefit of giving any vaccine to a horse.
- Vaccines are generally safe when administered properly, and any side effects seen are usually mild.
- Core vaccines are required for all horses while the administration of non-core vaccines should be discussed with your veterinarian.
- Though rare, there some side effects that can also occur from vaccines including lethargy, fever, hives, purpura hemmorrahgica, colic, and anaphylaxis.
- To reduce risk of adverse side effects from vaccines, allow a veterinarian to administer all vaccines, don’t over-vaccinate, and use caution and consult with your veterinarian about vaccinating pregnant mares, immune-compromised horses, or elderly horses.
References
- Gershwin. L. J., Adverse Reactions to Vaccination: From Anaphylaxis to Autoimmunity. The Veterinary Clinics of North America. Small Animal Practice. 2018. View Summary
- Desanti-Consoli. H. et al., Equids’ Core Vaccines Guidelines in North America: Considerations and Prospective. Vaccines. 2022. View Summary
- Entenfellner. J. et al., Response of Sport Horses to Different Formulations of Equine Influenza Vaccine. Vaccines. 2020. View Summary
- Vaccine Technology. American Association of Equine Practitioners.
- Carnet. F. et al., An Inventory of Adjuvants Used for Vaccination in Horses: The Past, the Present and the Future. Veterinary Research. 2023. View Summary
- Kamali-Jamil. R. et al., The Effect of Various Stabilizers on Preserving Immunogenicity of Lyophilized Mumps Vaccines. Journal of Research in Health Sciences. 2017.
- Board Recommendations for _Preservative Use in Vaccines. U.S. Department of Agriculture.
- Vaccine Components Fact Sheet. National Centre for Immunisation Research & Surveillance.
- Vaccination. Equine Disease Communication Center.
- Tizard. I. R., Administration of Vaccines in Animals. Merck Veterinary Manual. 2023.
- Vaccine Adverse Reactions. American Association of Equine Practitioners.
- Tizard. I. R., Vaccine Failure and Other Adverse Events in Animals. Merck Veterinary Manual. 2023.
- McNeil. M. M. and DeStefano. F., Vaccine-Associated Hypersensitivity. The Journal of allergy and clinical immunology. 2018.
- Boyle. A. G., Streptococcus Equi Subspecies Equi. Veterinary Clinics of North America: Equine Practice. 2023. View Summary
- Carte. S., Fall Vaccines and Deworming. Veterinary Teaching Hospital.
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