Horses occasionally react adversely to vaccination. These reactions can be mild and temporary or serious and life threatening. Dr. Tia Nelson, a veterinarian in Helena, Montana, says that usually the most serious reactions (such as anaphylactic shock) occur very quickly. Generally the first sign of anaphylaxis is difficulty breathing, followed by collapse. In rare cases a horse has fallen dead immediately after the injection.
“I have also seen horses 3 or 4 weeks after a strangles or sometimes even a rhino vaccine develop purpura hemorrhagica, which is also a serious kind of reaction and can be fatal,” says Nelson. This condition is characterized by swollen legs, triggered by an allergic type reaction; the white blood cells release enzymes that damage blood vessels and they leak fluid into surrounding tissues.
"The subcutaneous edema appears primarily on the legs and lower areas of the abdomen and chest. On rare occasion the edema may follow upper respiratory tract infections like strangles—or a strangles vaccination if the horse already has high serum levels of antibodies. Immediate medical attention is needed, and it may take days or weeks of treatment to resolve this problem."
“The most common reaction to vaccination is swelling and soreness at the injection site for a few days. The horse doesn’t want to put its head down to eat or drink because the neck is sore. I now give most vaccinations in the thigh, instead, since the neck can be so sensitive afterward. Occasionally, however, the horse may not want to take a certain lead for a few days when cantering because the thigh aches—if that was the injection site. With this kind of local reaction at the injection site, we can give the horse a little bute or Banamine to ease the soreness,” she says.
Alcohol doesn’t do any good, and may actually create poor conditions for vaccination.
Some horses are sensitive to certain vaccines—reacting to the antigen or more commonly to the carrier or the adjuvant (the ingredient that enhances immune response by slowing absorption and making it longer-lasting). “If you know your horse is going to react, we can give administer Banamine intravenously before the vaccine, to minimize the inflammatory reaction,” says Nelson. In some cases your vet might recommend a different brand or type of vaccine to see if the horse has less reaction to that one.
“A few horses spike a fever the day after vaccination, but it resolves in another day or so. They are not sick, but just a little bit off or dull because of the fever. Most of these reactions are easily managed with non-steroidal anti-inflammatory medication,” she says. To be safe, don’t vaccinate horses just before a competition. Do it well ahead of any event or ride, just in case the horse has a fever for a few days.
Some horses break out in hives all over the body after vaccination, and this can be treated with Banamine or Benadryl, according to Nelson. “Most of these reactions are easy to manage or circumvent if you know the horse is sensitive to a certain vaccine.”
It helps to know the horse’s history, regarding any type of vaccination reaction. If it’s a new horse and you don’t know the history, or a horse that’s never been vaccinated before, it’s best to be cautious and spread out the vaccinations—not giving them all at the same time, and in different places. “Then if the horse reacts to one of them, you know which one it is,” she says.
It also helps to use proper vaccination technique and make sure everything is clean, and use sterile needles. You don’t want the horse to develop an abscess at the injection site. The skin should be clean and dry. “We don’t recommend scrubbing unless you leave the chlorhexidine or Betadine on the skin for at least 7 to 8 minutes before you give the vaccination. It’s contact time, rather than the scrub, that kills any pathogens that might be on the skin. Alcohol doesn’t do any good, and may actually create poor conditions for vaccination,” Nelson says.
You want the skin dry. The needle is more apt to take pathogens into the tissues with wet skin. “You don’t want to vaccinate through mud, but I don’t recommend scrubbing the site. It is better just to brush the horse and make sure the skin is dry.”
For intramuscular vaccination, use a 1½ inch needle, to go deeply into the muscle tissue, rather than a shorter one. This aids in proper absorption and less chance for reactions. Make sure the horse stands still. “If he moves while you are giving an injection, there is more tissue damage; the inserted needle creates more irritation in the muscle. There is more risk for a reaction because of potential tissue damage,” she says.
Horses should be monitored after any vaccination. Regarding reactions, keep in mind that anaphylaxis can occur immediately or up to a few hours afterward (and there is more chance to save the horse with medication like epinephrine and steroids the later it happens).
“Fifteen or 20 minutes after vaccination (up to a few hours later) the horse may get hives. Fever and soreness may show up 12 to 24 hours after vaccination. An abscess may not show up for several days to a week later. Anywhere from 1 to 4 weeks later a vaccinated horse may develop purpura hemorrhagica. This is why it’s important to keep monitoring the horse and not just assume that everything is ok after the first day.”
“If a horse suffers a severe reaction like anaphylaxis or purpura we do NOT vaccinate that horse ever again with that vaccine. A second exposure may kill the horse. I’ve seen horses get purpura after they’ve had strangles. The disease itself, or the vaccine, can trigger this,” says Nelson.
Strangles vaccine is especially controversial. “Your veterinarian may recommend vaccination in the face of an outbreak, but we frequently run blood tests to check titers—antibody levels—on horses before we vaccinate them. If they have good titers we don’t vaccinate them because they don’t need it (they are already protected) and the vaccine might trigger a serious reaction,” she says. Revaccination of horses that already have sufficient immunity (to any disease, but particularly strangles) does not add to their disease resistance and may increase their risk of post-vaccination reactions.
“Occasionally a horse will develop an abscess at the injection site. Even a properly given injection has the potential to create an abscess. The worst-case scenario is when a person gives a strangles intranasal vaccine as an injection rather than up the nostril. This can create a life-threatening injection site reaction abscess,” says Nelson.
“There are also reports of problems when people do multiple vaccinations at once and give the intranasal strangles vaccine first. If you get any strangles vaccine on your fingers, and then give another vaccination, there is potential for contamination of those injection sites, causing a strangles abscess at the injection site. Always give the strangles intranasal vaccine last. Don’t even touch it until you are done giving all the other injections,” she says.