03/30/2004 - Vaccination Reactions

Vet tip of the Day: When Vaccinations go Wrong
Key Words: Vaccine, adjuvant, immune system, clostridium

Vaccination season is drawing to a close, but I thought I would return to this subject one more time to talk about vaccination reactions.  Of course I would like to tell you that I have never had a horse demonstrate an adverse response to vaccination, but that would be a big fat lie.  Fortunately, I can tell you that I have never had a horse suffer a serious vaccination reaction that didn't respond promptly to appropriate treatment. There are several types of adverse reactions to vaccination, and it is important that you understand how they differ, both in onset of signs, and seriousness of consequences.


By far the most common adverse reaction to vaccination is the simple sore neck.  I vaccinate hundreds (literally) of horses every spring, and this year to date I know of only two horses which developed significant signs of neck pain following vaccination.  Typically the day after vaccination the owner notices that these horses are unwilling to move their heads, show signs of pain if one of the vaccination sites is touched, and exhibit swelling at the vaccination site.  Occasionally the discomfort will be so severe that horses will not lower their heads to eat or drink, or will pull back if pressure is applied to a lead rope when they are haltered.  Years ago I had a client call to tell me that her horse was having a seizure when in fact the horse's neck was so painful that when the owner tried to lead her forward she reared over backwards in response to the pain.  The vast majority of inflammatory reactions after vaccination resolve within 48 - 72 hours with palliative therapy including warm compresses and phenylbutazone.

This "sore neck" reaction to vaccination is NOT an allergic response to the vaccine.  It is an exagerrated inflammatory response to the ingredient in the vaccine that stimulates the horse's immune system.  This ingredient is called an adjuvant, and is very important in causing vaccines to elicit a strong antibody response by your horse's immune system.  Without this response, the vaccine will not be effective in preventing the disease against which it is directed.  Drug companies spend a lot of time and money developing different adjuvants.  Their goal is to find an adjuvant that is a potent stimulator of the immune system but does not cause severe local soreness.  Over the years I have used many brands of vaccines, and have come to be a staunch supporter of Intervet vaccines.  I believe that their adjuvant causes very few advers reactions, and I have been impressed with the company's dedication to client education and with their committment to research and development of new products to protect horses' health, such as Prevenile, their DNA based West Nile vaccine.

When a horse develops a sore neck after vaccination, it is important to notice whether or not the horse is systemically ill.  Specifically, will the horse refuse food and water even when it is placed so that the horse can reach it without lowering its head? Is the horse's temperature over 102.5 degrees?  Is the swelling at the vaccination site severe and increasing over time?  When the swelling is palpated, is there a crackling feeling underneath the skin?  Does the horse appear markedly depressed?  If the answer to any of these questions is yes, then the horse should be examined by a veterinarian.  As stated earlier, the vast majority of inflammatory reactions after vaccination are not serious and resolve within 48 - 72 hours with palliative therapy including warm compresses and  phenylbutazone.  However, in rare cases, it is possible for a bacteria called Clostridium to grow deep in muscle tissue at a vaccination site. If this unlikely event does occur, it can be life-threatening.
Clostridial bacteria exist normally in the environment in a spore form which can only grow in the abscence of oxygen.  Even when a clean needle and syringe are used and the vaccination is administered correctly, it is possible for Clostridial spore sitting on the skin to be carried deep into the muscle tissue by the needle during vaccination.  When this happens it is a random, extremely unlucky event, and does not mean that the vaccine was administered improperly.  Clostridial infections can be life threatening and require prompt and aggressive treatment. The Clostridium bacteria grow rapidly in the abscence of oxygen and produce several toxins which invade the horse's blood stream and cause severe systemic illness which can be fatal. Therefore, it is always a good idea to contact your veterinarian if your horse has an adverse reaction to a vaccine and discuss your horse's specific clinical signs so that you and your vet can decide if your horse needs to be examined.  If you take your horse's temperature and do a physical examination (see blog on this!) and carefully observe your horse's behavior before you call your vet you will be best able to provide important information in making that decision.

In tomorrow's Vet tip of the Day I will discuss Allergic reactions to vaccination and the importance of careful storage and administration of  vaccines, for those of you who vaccinate your own horses.

Until then,

I believe that education is the key to evolution.
I believe that animals are the key to compassion.
I believe the learning never stops.