Reactions can range from a bump where vaccine was injected to anaphylactic shock and death

by Barbara Sheridan

THE SUDDEN death in 2002 of Spend a Buck, the 1985 Kentucky Derby (G1) winner and Horse of the Year, showed the many dangers a horse can face as a result of a routine treatment gone bad. While based in Brazil, the stallion was treated with an injection of penicillin to treat a cut above his eye, which resulted in his traumatic death from anaphylactic shock. As with people, horses also can develop an extreme allergic reaction to a seemingly innocent substance.

Anaphylactic shock, the most severe type of anaphylaxis, is the result of a severe allergic reaction that causes the body to begin to shut down. Anaphylaxis, an acute, life-threatening hypersensitivity reaction, is defined by a number of signs and symptoms, either alone or in combination, that occur after being exposed to some form of provoking substance.

"Anaphylactic shock is a type of hypersensitive reaction to some sort of antigen, which can occur in as little as a few seconds to several hours," said Betsy Greene, Ph.D., extension equine specialist at the University of Vermont. "In most cases, the horse has been exposed to the agent previously and has the severe reaction on the second exposure."

This severe reaction could have occurred during a routine vaccination, administration of a drug, or even from an insect bite. Upon the initial exposure, the immune system becomes sensitized

to a particular substance that is foreign to the body. Upon re-exposure to the same allergen, several

substances, including histamine, are released, resulting in the anaphylactic reaction, which causes the body to shut down.

"The horse's immune system is critical for protection against infection," Greene said. "However, sometimes the horse's system can work too well, and the body then develops a hypersensitivity to certain substances. This can be local, such as a severe sensitivity to insects or bee stings, or systemic, such as a reaction to certain drugs such as penicillin, dewormers, or anesthesia."

Greene said that, upon exposure, the body then will start an immediate immune response, which causes varying physiological changes. "Some of the physiological changes include blood vessel dilation with increased-permeability mucous [membranes], an increase in gastric acid, and smooth-muscle constriction," she said.

Local reactions

In most cases, anaphylactic reactions in horses are immune-mediated, which is the body's natural defense mechanism when it reacts to antibodies in defending against these invaders. Scientists refer to this as an IgE-mediated (immunoglobulin E-mediated) reaction, where the body has had a prior exposure to an antibody, causing it to sensitize the system and resulting in a worse reaction the next time it becomes exposed to it.

This reaction is similar to people with sensitivity to bee stings or poison ivy, but it also can be caused by mediations such as penicillin or a vaccine reaction. Mediators are basically chemical signals in the body that set off a variety of responses, and while different from an allergen, a mediator sets off a similar series of events that make it difficult to distinguish one from the other. However, non-allergic anaphylaxis, referred to as non-IgE-mediated, carries the same symptoms but derives from a different cause.

David T. Cross, D.V.M., Ph.D., clinical assistant professor of veterinary medicine and surgery at the University of Missouri, said anaphylactic reactions in horses can be both immune and nonimmune-mediated.

"Both are hypersensitivity reactions in the animal," Cross said. "However, non-IgE-mediated is caused by a variety of chemical reactions within the body and is usually called an anaphylactoic reaction or anaphylaxis. Unlike IgE-mediated, it doesn't need a prior exposure to get that negative response."

While the cause of non-IgE-mediated reactions can consist of certain nonsteroidal medications and anti-inflammatories, in some cases, this reaction can originate as a direct result of a drug injection, such as penicillin or a simple vaccination.

"In the case of non-IgE-mediated, there are different types of cells and chemical pathways in the body that can be activated by certain things to set off an anaphylactoic reaction," Cross said. "Many times it is due to an incorrect route of administration, such as getting a drug into a blood vessel when it should have been given in the muscle. An example would be hitting a vein when injecting procaine penicillin, which should only be given in the muscle. In this case, this reaction doesn't require pre-exposure to the drug or vaccine for the reaction to occur.

"In other cases, it may be a reaction not so much to the drug itself, but what's in it," he said. "All these drugs have some sort of carrier molecule or liquid vehicle [adjuvant] that can be the indirect cause."

Cross said it is not the antigen itself but the adjuvant--the medium in which the antigen is mixed or dissolved--that causes the problem. And while a horse can experience a bad reaction to an injection, vaccinations still should play an important role in disease prevention and keeping the horse healthy.

Because manufacturers typically do not use the same adjuvants, switching brands may be a solution.

"I've been in situations where a vaccine from one company can cause a reaction in a particular horse, but from another company it may not," Cross said.

Just as there are many contributing causes of anaphylaxis, there also are varying degrees, such as something as simple as a lump or abscess at the horse's vaccination site, signs of a skin irritation, or just a case of hives, any of which owners might overlook.

"The appearance of urticaria or hives can be a form of mild anaphylaxis," Cross said. "Whereas, in cases of severe anaphylactic shock, there can be bronchial constriction, pulmonary edema, and hypotension due to vasodilation [widening of blood vessels]. These can be severe enough to be fatal, and the reaction can happen rapidly."

Clinical signs can include difficulty breathing from constriction of the airway, sweating, sudden swelling of skin and mucous membranes (edema), and extreme itching and hives. The horse may colic and possibly show signs of laminitis from changes in blood flow.

"Just as people can have different reactions, horses can, as well," Greene said. "Most of the times that I've seen it, unfortunately, the horse owner had no idea and found out the hard way that the horse was allergic to whatever it was. It's not pleasant to watch a horse throwing itself violently around in the stall."

While the cause may be different, the effects of the reaction are the same and are generally treated in the same manner.

In cases of anaphylactic shock, it is important that symptoms be recognized quickly. Should the horse show signs of breathing difficulty, treatment would be given to assist or maintain the horse's respiratory system, as well as to remove the cause of the reaction if it has been identified.

With severe anaphylaxis, treatment would involve intravenous administration of epinephrine, Cross said. This helps to relax the horse's airways and reverse some of the body's response.

"If the reaction is milder, epinephrine can be given in the muscle," he said. "I've also used dexamethasone [a synthetic glucocorticoid] treatment for some horses that have a reaction to fly bites and develop urticaria [hives]. Glucocorticoids are also used in severe, acute reactions [to address inflammation], and [intravenous] fluids may be required, as well, in severe reactions to maintain blood pressure. If the horse is having trouble breathing, an emergency tracheotomy may have to be performed. I've seen this several times in the teaching hospital here."

Because anaphylaxis can occur quickly and unexpectedly, veterinarians usually are armed with an emergency kit containing epinephrine- and dexamethasone-loaded syringes to be accessed in a minute's notice. However, in some cases anaphylactic shock may not be noticeable until several hours after the veterinarian's visit, and horse owners should become familiar with its symptoms and the urgency of care.

As with most cases in the medical world, avoidance is the best form of treatment. While anaphylactic reactions can vary in degree, in the case of a severe anaphylactic reaction, death often results. Horse owners should be made aware of any hypersensitivity their horses may have to a drug, medication, or other offending mediator or allergen and keep a record of it for future reference.

Because racehorses can change hands often, it is important to make sure that horses' medical records go with them.

"People should always make sure they get the horse's medical records along with its vaccination schedule upon the sale of the horse, especially if he has had a prior history of anaphylaxis, no matter how mild," Greene said. "It's as simple as saying, 'Now that I've bought the mare, could you pass on her records?' This would certainly help to offset any potentially life-threatening situation."

Barbara Sheridan is a freelance writer based in Ancaster, Ontario.