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