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Medical Index
First Posted Oct 6, 2009
Nov 20, 2011

Vaccination Side Effects Horses

by Debora Johnson

If your horse has never had a reaction from a vaccine consider yourself lucky. Although most horses do fine with their shot program, some do have adverse reactions. My horse, A Patchy, has had an adverse reaction from the Rhino/Flu vaccine two years running. Annual Horse Vaccines Therefore, we have stopped giving that shot. This past week, he had an adverse reaction from the West Nile Virus shot for the first time. He has received that shot many times in the past. This shot is not an option for us because West Nile Virus is in our area. I called the vet and asked three main questions: 1) Was Fort Dodge the manufacturer? 2)Was the vaccine killed? and 3)Can a reaction present 4 days later? The answer to all three questions was "yes."

Armed with that information I have decided to write this article. At the bottom of the page you will see links to various sources of the information. Please get a more in-depth understanding by following those links. Meanwhile, I will try to give you an outline of what I learned.


  • Muscle swelling
  • Mild fever
  • Swelling at the site of injection
  • Local tissue swelling
  • Stiffness
  • Off feed
  • Lethargy
  • Anaphylaxis
  • Infection

Try to Avoid Reactions

There is really no way to know if your horse is going to have adverse reactions to vaccines. However, if a horse has had a negative reaction in the past it is more likely to have an adverse reaction, again.

  • Use the vaccines that have worked in the past with no adverse consequences. Do not change manufacturers unless you have a problem.
  • Only vaccinate a horse that is healthy.
  • Give injections well in advance of any significant event or possible stressful situation. This may include competitions, long distance tailoring, a change in environment, heavy exercise or training, etc. At least two weeks before the potential stresser or two weeks after the potential stressor might be OK. Discuss this with your vet.
  • Spread out your shots. Do not give them all at one time, if possible. We wait at least 4 days between vaccines and still have had adverse reactions!
  • We now try to limit our vaccinations to only what is necessary: Eastern and Western equine encephalomyelitis (EEE and WEE), West Nile virus (WNV), tetanus, and rabies. Vaccination against these diseases is by far the best protection option for your horse, because he could still be exposed even if he does not have contact with other horses. We use to also give Potomac Fever but it has not been so prevalent in our area lately. We stopped giving Strangles because it often gives a reaction and our horses are not exposed often to other horses or large gatherings. Our herd is stable only Rusty, A Patchy and Oliver. Other horses do not come onto the property.

If Your Horse Has A Reaction

  • Know that horses who have had a reaction are more likely to develop reactions, again.
  • We give A Patchy bute before his shots, now, and after his shots for several days. We are careful to watch him for swelling and heat at the site of injection, make sure he is on his feed, look to see if he is listless, and check for fever.
  • Both Banamine (flunixin), bute and antihistamines are used to offset or prevent vaccine reactions.
  • Cool and warm compresses, topical DMSO gel, and gentle exercise may also be recommended by your vet. Always check with your vet!
  • Try to find another manufacturer other than the brand used when the adverse reaction occurred.
  • Ask your vet to see what adjuvant* was used and try to avoid it in the new brand.
  • When introducing a new vaccine, especially if from a different manufacturer, schedule these injections separately from other vaccines. That way you will be able to see if that particular shot caused an adverse reaction.

*In immunology, an adjuvant is an agent that may stimulate the immune system and increase the response to a vaccine, without having any specific antigenic effect in itself. The word "adjuvant" comes from the Latin word adjuvare, meaning to help or aid. "An immunologic adjuvant is defined as any substance that acts to accelerate, prolong, or enhance antigen-specific immune responses when used in combination with specific vaccine antigens."

Adjuvants have been whimsically called the dirty little secret of vaccines in the scientific community. This dates from the early days of commercial vaccine manufacture, when significant variations in the effectiveness of different batches of the same vaccine were observed, correctly assumed to be due to contamination of the reaction vessels. However, it was soon found that more scrupulous attention to cleanliness actually seemed to reduce the effectiveness of the vaccines, and that the contaminants - "dirt" - actually enhanced the immune response. There are many known adjuvants in widespread use, including oils, aluminum salts, and virosomes, although precisely how they work is still not entirely understood.

Immunologic adjuvant

The following links provide comprehensive information about side effects that your horse can have from shots, what your options are, and what steps can be taken to help with the problem.

Vaccination Side Effects
Vaccine Reaction: Calculated Risk?
Vaccine Reactions in Horses
"Vaccination Induced Immune Reactions"- The Equine Manual by Andrew HIggins, A.I. Wright

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