Editor’s Note: This excerpt is from Understanding Equine First Aid by Michael Ball, DVM. 

Sometimes a horse has a reaction following a vaccination. The local and whole body reaction to a number of the equine immunization products is a well-known side effect of these necessary shots. Some horses demonstrate signs of systemic illness, such as depression, inappetence, and fever.

Localized inflammatory reactions (heat, pain, and swelling) usually arise within two to three days of vaccination. A common misconception is that such reactions result from a “dirty needle” or “poor injection technique.” These reactions are usually the result of the vaccine causing a reaction in the muscle tissue surrounding the injection site.  If the injection site becomes abscessed, it might need to be lanced and allowed to drain, but initially it should be hot packed. The application of heat can be extremely beneficial when trying to “mature” an abscess, or area of infection, in an effort to reduce the swelling and promote drainage. However, as with cold therapy, a heat treatment can be overdone. One of my favorite ways to apply heat is to place a wad of cotton in a zip-lock baggie, adding water that is just hot enough to allow you to hold the bag without it being unbearable. Hold the baggie hot pack against the abscessed area for 20 to 30 minutes. Repeat the heat treatment several times per day, ringing out and re-heating the cotton as necessary. Electric heating pads should not be used as they can easily burn the skin if the temperature is not monitored carefully. Of course, the good old hot water bottle works well, too. Your veterinarian should be consulted before using anti-inflammatory drugs such as phenylbutazone or applying topical anti-inflammatories such as DMSO.

Again, do not forget to evaluate the horse’s current level of tetanus protection with any wound (including a vaccine reaction on the chance that it could become infected)