3/31/2010 - Vaccination Reactions II

Vet tip of the Day: "Allergic" reactions to vaccination
Key Words: Urticaria, hives, hypersensitivity, anaphylaxis

Most importantly, remember that severe vaccination reactions are rare.  That's why it was difficult to find a decent picture - sorry for the quality of this one.  True hypersensitivity or anaphylactic reactions to vaccines, while rare, do occur and require immediate attention.  The physiology behind these reactions is very complex and poorly understood in horses. 

Anaphylaxis refers to a shock reaction by the body to some foreign substance.  In the case of vaccines, anaphylaxis can occur the first time a vaccine is administered.  An anaphylactic response can vary from mild hives that appear hours after a vaccine is given, to acute life-threatening cardiovascular collapse which can occur within minutes of vaccination.  Epinephrine is the preferrred initial treatment for severe anaphylasis.  It works by counteracting the immediately life-threatening components of the body's severe inflammatory response.  It is usually administered by intramuscular injection.  In the most severe cases, it may be given intravenously.  Administration may be repeated every 15-20  minutes if necessary.

Hypersensitivity and immune-mediated reactions to vaccines also occur and may have similar clinical signs to anaphylaxis.  When these reactions occur, they usually are less dramatic than acute, severe anaphylaxis.  Signs may include swelling of the muzzle, face and throat region, which can lead to respiratory distress, or may manifest as hives, or urticaria.  Depending on the severity and progression of the clinical signs, treatment may include steroidal or non-steroidal (phenylbutazone, flunixin/banamine) anti-inflammtory agents, and/or anti-histamines.

When a horse experiences an adverse reaction to vaccination, the question always arises, should the horse be vaccinated again?  If the reaction is a very sore neck, sometimes changing vaccine brands, and thus the adjuvant used, will help.  If the horse has a true anaphylactic or severe hypersensitivity response to vaccination, the problem is more complex and the decision to repeat vaccination should be made based on the severity of the reaction and consideration of the true risk of the disease against which vaccination is directed.

Finally, for those of you who adminster your own vaccines, be aware that storage conditions are extremely important for vaccines.  Vaccines that are not stored at proper temperatures are significantly more likely to cause adverse reactions than those kept cool at all times. Also, correct administration of intramuscular shots, both with respect to location of the injection and injection technique, play important roles in the safety of injections.  If you choose to vaccinate your own horses, be sure to obtain instruction from a licensed veterinarian or licensed veterinary technician concerning the correct administration of intramuscular injections.  And be sure that your vaccines have been properly stored before you purchase them, check the expiration date, and keep them carefully stored until they are administered.  Better yet, take advantage of the chance to visit with your vet and have a spring wellness check up and make an appointment with your veterinarian to give your spring vaccinations!

It's cold and snowy on March 31 in Northern Nevada.  What's it like where you are?  Looking forward to spring and good riding weather.