Most of the time vaccines are given to horses with no ill effect and no sign that the horse even received an injection. But on occasion, some horses demonstrate mild to moderate reactions to specific antigens (foreign proteins) or adjuvants (substance added to vaccines to enhance the immune response) within the vaccine.
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As an example, one common vaccine that I have seen cause reactions is one that targets equine herpesvirus (rhinopneumonitis).
It is good to keep a record of your horse's vaccinations and any reactions the horse has each year.
What Might You See?
Observing your horse and understanding his normal behavior can help you spot abnormal behavior.
A horse with a vaccine reaction is often feeling “blue.” His posture might look "deflated" and his attitude and energy levels flat. His appetite might be decreased, and he might have a fever.
He might seem to be muscle sore all over, or just show muscle soreness at the injection site. The horse might move in a stilted manner, particularly if the neck muscles are affected.
An adverse reaction in the neck might make it uncomfortable for the horse to reach for food and water that is low to the ground. A hay rack would be helpful to ease distress.
If muscle soreness occurs with an injection in the hind end, then there might be associated lameness.
Swelling sometimes occurs around an injection site, ranging from very slight to moderate, with the horse resentful if you attempt to press on that area.
What to Do About It
Such reactions are often transient, usually resolving spontaneously within 48-96 hours.
Many horses respond well to one or two treatments with a low dose of an NSAID such as phenylbutazone or flunixin. NSAIDs usually provide relief within just a few hours.
Even if managed with “benign neglect,” i.e., doing nothing, most adverse reactions will run their course and abate.
On a rare occasion, a horse can have an anaphylactic reaction such as a person might experience who is allergic to bee stings, peanuts or shellfish. This is life threatening and requires immediate intervention.
In 31 years of practice, I have never witnessed anything even remotely resembling an acute allergic response to any vaccine.
Most equine vaccines are extremely safe, but there could be one individual horse that is extremely sensitive, so it is prudent to always observe your horse for problems following immunization.
If you know your horse is sensitive to a particular vaccine and has experienced an adverse reaction, then it helps to pre-medicate with an NSAID just before or at the time of vaccination. This usually prevents or reduces the adverse response.
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