allergic reaction

Local allergic reactions can occur for a variety of reasons and can pose a formidable challenge with respect to identifying the cause.

Allergic reactions are complicated and somewhat unpredictable events that are triggered by various environmental factors or other physiologic abnormalities. There are several special cells and blood proteins that mediate most allergic reactions. Of the immune proteins (IgG, IgE, and so on), Immunoglobulin "E" is the one most often involved with allergic reactions. There are several special cells in the body that play a role in allergic reactions: the "mast" cell and the "eosinophil." The eosinophil is a white blood cell that looks like a giant raspberry when viewed under the microscope. The mast cell resides mostly in various body tissues.

The major basis for many of the allergic reactions is the release of a variety of chemicals from either the eosinophil or mast cell. The stimulus for the release of such chemicals from either of these cells is the entry of some substance into the body to which the body is allergic. The substance can be inhaled--such as pollen, dust, molds, or even little mites--or "injected" via insect bites, punctures, and so on. In addition, there are many substances that can trigger allergic reactions when they contact skin or when they are ingested (food allergies). The substance that enters the body and triggers an allergic reaction is called an allergen. So what happens once the allergen enters the body? Allergies are very strange phenomena and sometimes come and go at different stages of life or with changing environments. What it is specifically that causes the body suddenly to become sensitized to an allergen one day is largely unknown, but for the millions of people and animals that are allergy sufferers, it can be a terrible affliction.

In routine allergic reactions, once the allergen enters the body it is recognized by the IgE protein molecule, which then binds to the allergen. The reaction depends on there being prior "sensitization" of the body to the offending allergen, and therefore the presence of the correct IgE to "match" the allergen. Once the allergen/IgE combination contacts the eosinophil or mast cell, it binds to it and causes the release of chemicals that mediate the clinical signs of the allergic reaction. There are various chemicals, with "histamine" being one of the major players in allergic reactions--hence the use of antihistamines in controlling allergic reactions.

The chemicals cause dilation of blood vessels and the leakage of fluid into the surrounding tissue. This is what causes the raised/puffy tissue when a "hive" forms, the swollen obstructed nasal passages, and some of the difficulty in breathing if the lungs are affected. In addition to the fluid buildup, there is an influx of other inflammatory cells into the various tissues affected. This is an extremely oversimplified explanation of how allergic reactions develop, but should serve to give you the basic idea of what's going on.

The diagnosis of an allergic condition relies on history, physical examination, and sometimes a bit of guesswork. In addition, the use of ancillary testing--such as blood analysis, tissue biopsy, and trans-tracheal washing (see The Horse of March 1997)--can be of assistance in confirming the diagnosis.

There also are special tests available to determine the specific cause of the reaction; these tests are either a special blood analysis or skin testing. The skin testing, similar to that done in people, is a procedure where a great number of known potential allergens are injected into the skin and observations are made regarding a reaction or lack thereof. In my experience and based on the experiences of several of my colleagues, the skin testing and special blood analysis are somewhat unpredictable and difficult to interpret. The main purpose of these tests is to identify the exact causes of the allergy so an effort can be made to remove the offending material from the horse's environment (often impractical or impossible) or to provide information for the manufacture of desensitization products. Although my experiences with these tests and procedures have not been significantly rewarding, there are reports of positive experiences that might be of benefit to certain individually selected cases.

The most common allergic reaction that horses experience typically manifests as hives on the skin all over the body. The causative agent of an episode of hives generally goes unknown, but it can be one of a wide variety of things. Among causes of hives are toxic/irritating compounds in a variety of plants, the stings or bites of various insects, and a variety of chemical compounds. In addition, reactions from ingested or inhaled substances to which the animal is sensitive can be a cause of hives, or more severe allergic reactions.

Generally, the development of hives causes no health danger and is of no significance (unless, of course, they are all over a conformation horse the night before the show!). It often is a one-time occurrence and frequently will resolve spontaneously without treatment. On rare occasions, hives will be a leading sign of a more severe allergic reaction that will require immediate medical attention. A horse with hives should be evaluated carefully and have its temperature, heart rate, and respiratory rate determined and sequentially monitored. The respiratory rate and heart rate are particularly important, and if elevated, could indicate a more severe allergic reaction. Veterinary assistance should be sought immediately.

Vaccine Reactions
Another situation that can pose a potential danger is a severe, acute (and potentially fatal) allergic reaction from a simple vaccination. Have you ever wondered why your physician makes you sit in the office for 20 minutes after getting a vaccination? It is so you don't die from an anaphylactic reaction on the drive home. Although rare, the development of anaphylactic shock following vaccination for almost any disease process can occur and is listed as a potential complication on the package insert of most vaccine products. This is something to be taken seriously, because if your horse is the unlucky one-in-a-million, he most likely will die if medical treatment is not available.

For those of you who have your veterinarian administer your horses' vaccinations, there generally is no problem since he or she immediately can treat any reaction if it occurs. But, if you're in the ever-growing population of "self-vaccinators" due to the availability of "over the counter" equine vaccines and the desire to save some money, you might experience a horrible tragedy someday. I am not an advocate of "self-vaccination." To me, it is playing Russian roulette with your horse's life. So, if you are going to give your own vaccinations, at least be aware of the potential complications and consult with your veterinarian on what treatments to have on hand and what to do with them should an anaphylactic reaction occur--you won't have time in the middle of one to make a call and have your horse survive! Local Vaccine Reactions Although not technically an allergic reaction, there are many vaccines that cause a local reaction and even cause some systemic effects. As many of you might have experienced with the flu vaccine (human), it is not uncommon to spend a day or two of "not feeling quite right" after receiving a vaccination. Horses are no different. They can become slightly depressed or develop a poor appetite for 24-48 hours after vaccination. The important thing here is that they should not develop an abnormal temperature (up to 101° Fahrenheit can be caused by vaccine stimulus, but higher should be further evaluated), heart rate, or respiratory rate. If the vital parameters are abnormal or the mild depression/inappetence continues for more than 48 hours, the horse should be seen by a veterinarian. The development of a sore/stiff neck also is not uncommon. If a lump forms or the muscle around the injection site becomes very hard, the area can be "hot packed" with a hot towel. (Placing a hot bran mash inside a Zip-lock bag works very well for hot-packing.) This should be done four to six times per day for 10-15 minutes each time. Occasionally, an abscess will form at the site of injection. This can be a sterile reaction to the vaccine (there is no infection) and might or might not burst through the skin and drain.

There also is the possibility of infection being the cause of a reaction (and abscess formation). Either type of abscess might require a veterinarian to "lance" the skin to facilitate drainage. If these horses are showing signs of depression, loss of appetite, fever, or elevated heart or respiratory rate, they should be seen by a veterinarian as soon as possible.

A sign of a more serious infection is the presence of air under the skin. Some types of bacteria (particularly nasty ones, in fact) produce gas that collects under the skin. You can feel this as a crackle-type feeling, sort of as if there was some of the bubble-wrap packing under the skin. If you feel this and the horse is acting sick, it constitutes a veterinary emergency.

Local Allergic Reactions
Local allergic reactions can occur for a variety of reasons and can pose a formidable challenge with respect to identifying the cause. (Although if it is horse fly season and your horse is pelted with lumps, the cause might not be so inconspicuous.) There is a well-documented allergy to the sandfly (Culicoides spp). Due to this little pest's favorite biting places, the common sites of swelling and irritation are the poll, mane, and tail. Many times these types of allergic reactions can be controlled by using cold therapy (cold water or ice in a Zip-lock bag) and the local application of an over-the-counter human allergy cream, such as hydrocortisone. Should the allergy progress with the irritation becoming severe, your veterinarian might need to administer systemic drug therapy to control the inflammation.

A word of caution. It sometimes is easy to assume that a swelling or abnormal area of skin is caused by an allergic reaction. There are many diseases that can cause swelling of the legs or other areas of the body, so you must be careful. If there is any change in the horse's demeanor or appetite, or if there are abnormalities in the vital signs, veterinary consultation should be sought. The respiratory condition heaves (chronic obstructive pulmonary disease) is in most cases related to an allergic phenomenon and was discussed in great detail in The Horse of November 1997. There are some allergic conditions that affect the gastrointestinal system and typically present as chronic diarrhea. These conditions are extremely difficult to diagnose and even more difficult to treat; fortunately, they are somewhat rare in occurrence.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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