United States PRE Breeders Association



Vaccine Reactions


Vaccination is a vital part of caring for horses and, while most vaccines do not cause problems, side effects can occur. The most common include general malaise, body aches, and a slight fever 24-48 hours after administration. Necks can be sore and injection sites can swell, but these signs usually resolve within a couple of days. Rarely, owners and practitioners have noted more severe effects, including anaphylactic reactions (some life-threatening), localized infection, scar tissue development, and generalized hypersensitivities. For these reasons, veterinarians generally do not advise competing or working horses for a few days post-vaccination. 

Potent vaccines, such as those protecting against tetanus and rabies, are particularly prone to triggering side effects. Intranasal vaccines commonly produce a mild sneeze or a slight clear drainage from the nostril post-administration. In more serious reaction cases veterinarians might administer a non-steroidal anti-inflammatory medication to help alleviate clinical signs.

Ideally, vaccines would not cause any side effects and would establish protective antibody titers (the concentration of antibodies in the blood that recognize a particular disease agent). In most instances horses respond well to vaccines, but it can be difficult to determine what antibody titer is protective in individual animals. In an ideal world, we would measure antibody titers to determine positive and negative responses to the vaccines, which might help us focus on the horses needing the most protection, but this is costly.

In my experience, nine out of 10 horses respond to vaccination with no noted side effects. But why does that tenth horse react negatively? Was it the vaccine? And what was the reaction: fever, general malaise, or worse? I question when a horse that I have vaccinated routinely, using the same brand with no side effect, suddenly reacts negatively. We can speculate why, but we don’t have any definitive answers.

Possible reasons for reactions include:

  • Hypersensitivity to one of the antigens, which are the substances added to induce a specific immune response; 

  • Hypersensitivity to an adjuvant, which is a substance included in inactivated vaccines to enhance immune response;

  • Concurrent illness; or

  • Inappropriate vaccine administration.

In small-animal clinics, practitioners perform a basic physical exam before administering vaccines to detect fever or pinpoint other health concerns. In equine practice we usually perform an abbreviated physical exam, especially on large farms, due to time constraints and cost. Often, we administer vaccines a horse at a time while working down a barn aisle, giving the animals a quick once-over, but sometimes overlooking details.

Remember, a vaccine’s purpose is to stimulate an immune response to a specific antigen. If a horse isn’t feeling well, his immune system might not respond appropriately and we might actually do harm. So, never vaccinate a sick animal. Wait until they recover and then vaccinate.

Other reasons horses might not respond appropriately include concurrent diseases that could impact immune response, including insulin resistance and Cushing’s disease. Older horses might not respond well due to debilitated immune systems.

Vaccines are essential to helping us control many diseases, but they aren’t without risk. Veterinarians should continue following current core and risk-based vaccine protocols in most situations, but thoroughly evaluate horses that react negatively to vaccines. We must question why these horses react and not just treat the clinical signs. Was it the brand or the injection site? Was the horse sick? Was it timing? At the very least, stagger vaccinations by two to three weeks, instead of potentially overloading horses with four or five antigens at once. This reduces antigen load and immune challenge and might help us identify which vaccine is causing a reaction.

In the end, vaccines are a crucial part of equine preventive care; however, make sure they are administered appropriately and at the right time and intervals. Take into consideration these variables when deciding which vaccines your horse needs.


By: Tom Schell, DVM