Injection Objection: Adverse Vaccine Reactions

Injection Objection: Adverse Vaccine Reactions

Understanding how to minimize adverse reactions and having a plan in mind in case of an adverse reaction can help your horse stay comfortable following vaccination.

Photo: Anne M. Eberhardt/The Horse

We do it with the best of intentions; we arrange a mere moment of discomfort that will protect our horses against any number of painful, debilitating, and potentially deadly diseases. A vaccine is our first line of defense against illness, but it can, and occasionally does, backfire with resounding force.

Adverse vaccination reactions can take many forms, from mild cases such as a stiff neck to the extreme of anaphylactic shock and death. Understanding how to minimize adverse reactions and having a plan in mind in case of an adverse reaction can help your horse stay comfortable following vaccination.

What is an Adverse Reaction?

To understand an adverse reaction, we must first understand vaccines. The theory is that by giving a little of the causative agent (i.e., virus, bacteria), we give a horse an antibody reaction and immune system memory that will help protect him in case he is faced with the real disease.

The causative agents used in vaccines are called antigens, which can be live or killed pathogens, including bacteria, viruses, protozoa, and fungi. For example, in the case of tetanus vaccines, the pathogen by-product is used.

The other component of a vaccine is the adjuvant, which is the base, or vehicle, of the vaccine in which the antigen is mixed. Adjuvants enhance the stimulation of the immune system.

Kenneth Hinchcliff, BVSc, MS, PhD, Dipl. ACVIM, assigns adverse reactions to two categories: immunologically based reactions to the antigen, and non-immune (local) reactions.

Immunologic reactions occur when the horse's immune system reacts to the vaccine's antigen atypically. 

One example of an immunologic reaction is purpura hemorrhagica, a syndrome that can be associated with strangles vaccination. In this syndrome, antibodies and antigen proteins combine to cause an allergic reaction resulting in vasculitis (inflammation of blood vessel walls that makes them leak) and edema (fluid swelling) of the head, abdomen, limbs, and scrotum. Horses demonstrating facial edema associated with purpura hemorrhagica can take on caricature-like appearances with grossly distended muzzles.

Another rare, but serious immunologic reaction is anaphylactic shock. This can occur within seconds of vaccination and up to 15-20 minutes afterward. As anaphylactic shock sets in, the bronchial airways constrict and blood pools in the lungs, resulting in extreme respiratory distress. If left untreated, veins dilate and blood seeps into the intestines and liver, leading to secondary complications. To avoid fatal consequences, anaphylactic shock must be treated immediately with the intravenous (IV) administration of epinephrine, an adrenal hormone that acts as a heart stimulant and muscle relaxant.

Further immune-mediated reactions can be observed hours after vaccination and include signs such as depression, poor appetite, sweating, mild colic, and diarrhea.

The second type of adverse reaction is a nonspecific response to the vaccine's antigen or adjuvant, or bacteria that might have been unintentionally introduced via the needle during injection. These local reactions are much more common, and they are generally far less severe, than immunologic reactions.

Julia Wilson, DVM, Dipl. ACVIM, says, "For intramuscular vaccines, the most typical reaction is local soreness. In moderate to severe cases, particularly if the horse is vaccinated in the neck, the animal may be reluctant to move and appear uncoordinated. Some will have concurrent mild fevers and may experience reduced appetite, usually the day following vaccination."

These nonspecific reactions can result in swelling and soreness. Occasionally, an abscess might form at the site of injection as well.

"Because the vaccine is a foreign body, like a splinter, it becomes an area where bacteria can grow," explains Hinchcliff.

As nonspecific adverse reactions tend to be localized, their impact is not usually as severe as immunologic reactions. However, there is one reaction that can cause massive damage, clostridial myonecrosis, also known as gas gangrene. This occurs when the local reaction to the vaccine creates inflammation that allows the growth of bacterial spores that already exist within the muscle (not introduced as a result of the injection), says Hinchcliff. The result can be edema and the eventual sloughing of massive tissue areas. Lacking medical treatment, many horses die within 48 hours.

Being Prepared

As the types and severity of adverse reaction vary, so do their clinical signs, which can give you an idea of what's brewing.

"For an immunologic reaction, depending on the specific type, the first abnormality can be rapid respiration and sweating," says Wilson. "The owner may interpret this as colic or anxiety. Other horses may develop hives, which typically begin as small skin lumps over the neck or trunk, progressing to larger swellings."

If your horse has an immunologic reaction, including anaphylaxis, call a veterinarian immediately. Wilson advises keeping the horse calm to avoid an increase in blood pressure, which could further circulate the infection.

Local reactions are common and often present as stiffness at the injection site, which will resolve without complications. For a local reaction, Wilson recommends ice and exercise to help disseminate the ache. Many intramuscular vaccine labels specifically suggest exercise directly after administration to minimize reactions.

"Topical preparations that have anti-inflammatory properties and components, such as arnica or diclofenac, may be useful, too, but the owner should discuss this with their veterinarian first," says Wilson.

Local reactions typically improve over 48 hours. If no improvement is noted, if the swelling increases, or if the horse is not eating or has a fever, consult your veterinarian. Vigilant observation following vaccine administration can help minimize the damage if a reaction takes place.

Calculated Risk

Hinchcliff refers to vaccination protocols as "a balance between efficacy and safety." While vaccines are necessary to prevent a host of diseases, over-vaccinating is a legitimate concern.

Wilson suggests owners carefully weigh the necessity of each vaccine they plan for their horse against the risk it can pose. The two questions she suggests owners ask themselves before vaccinating are whether there is an actual risk for that disease in the area, and what is the likelihood of exposure for that horse or another horse at its facility?

"So often an owner decides to start taking the horse places without thinking about the impact on infectious disease risks," Wilson says. "Similarly, for the horse that is boarded out, the group risk thinking is very important as the other horses may bring something back from a show or park, without the owner's horses ever having ventured off the property." This typically happens with respiratory diseases such as strangles or influenza.

Wilson cautions strongly against horse owners administering vaccines without their veterinarian's consent and input into diseases posing a risk for their horse.

"Self-vaccinators should not take the decision to do their own horses lightly," Wilson says. "No vaccine is 100% safe. If the horse has an adverse reaction, will the self-vaccinator recognize the abnormalities and know how to treat the animal? Will they have the right medications with which to treat the animal (almost all of which are prescription products)?"

Other issues to consider when deciding whether to self-vaccinate include the knowledge needed for optimum selection of vaccines for each individual, storage and handling concerns, how to mix them, and how, when, and where they are administered. For example, even a brief moment of freezing can damage a vaccine and present an adverse reaction.

Additionally, vaccinating another person's horse (as in a boarding situation) presents serious liability issues if something should go wrong, Wilson says.

While giving vaccines yourself might seem like a good way to save money on veterinary bills, the cost of the potentially resulting emergency care following an adverse reaction from mishandled vaccines far outweighs the cost of a visit from your veterinarian.


What should you do if your horse reacted to a necessary vaccine last year and is coming due again? Keeping him in a bubble to protect him from disease isn't feasible, but there are steps you can take to prevent another problem.

Wilson suggests that, along with talking with your horse's veterinarian, you try to ensure that the triggering vaccine is given at a different time than the other shots in your horse's regime. Some veterinarians will also give a non-steroidal anti-inflammatory drug or antihistamine prior to vaccination to dampen the immune response.

Wilson says some anecdotal reports have shown that Zylexis, an immunomodulator, can be helpful in buffering adverse reactions, although this use has not yet been approved by the Food and Drug Administration (FDA).

Additionally, trying a vaccine with a different adjuvant, or a non-adjuvanted vaccine (such as intranasal), might produce a different result.

Be aware that a horse that reacts to a vaccine once is more likely to react to it in the future. To report an adverse vaccine reaction, visit the the USDA Center for Veterinary Biologics's website for instructions. 

Take-Home Message

While vaccines can cause unforeseen problems, owners can minimize these risks by being vigilant in looking for the early warning signs of a reaction, communicating clearly with their veterinarians, and taking care in the selection, handling, and administration of vaccines.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care. She owns a portly gray gelding named Duncan and dabbles in several equestrian disciplines, with an emphasis on dressage.

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