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What is Bute Allergy?

Phenylbutazone, more commonly known as Bute, is a non-steroidal anti-inflammatory medication (NSAID) that is regularly used to treat pain, swelling, and fever in equine patients. Horses can develop allergies to Phenylbutazone after either just a few exposures to the drug or after many months or years of successful treatment with it. Allergic reactions to drugs like Phenylbutazone generally result in hives for horses and on rare occasions may trigger dangerous anaphylactic responses which can be fatal without prompt medical treatment. 

Bute, or phenylbutazone, is a commonly prescribed pain medication for horses. Equine allergies to medications generally present as hives, although anaphylactic shock is also a possible reaction.

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Symptoms of Bute Allergy in Horses

The most common allergic response to NSAID drugs like Bute in horses is hives, which generally show up twelve to fourteen hours after exposure to the antigens. In most cases, hives initially present either at the site of injection if the anti-inflammatory was injected or on the side of the neck, and then move across the throat and shoulders. Early welts may start as small as half an inch in diameter, however, they frequently increase in size and often converge as the reaction progresses. Welts may or may not be itchy, and the patient may also exhibit signs of depression and have a slightly increased temperature. 

  • Hives
  • Depression
  • Low fever

Types

Bute (Phenylbutazone) is not the only nonsteroidal anti-inflammatory medication available to equines, although it is one of the most popular drugs for pain, it is also more likely to cause ulcers when given over extended periods of time or in excessive doses. Other equine medications that may be included in the NSAID class are:

  • Acetylsalicylic acid - Aspirin can be utilized by horses as well, and although the relief is generally short-lived, it can be particularly useful in conditions where blood clots are likely to form
  • Diclofenac - Commonly known by the brand name Surpass, this is a topical anti-inflammatory that is generally used for osteoarthritis relief
  • Firocoxib - This drug may be more expensive than other NSAIDs, however, it has fewer detrimental GI effect than other medications and can be given to horses that may be sensitive to other pain medications
  • Flunixin - This medication, also know as Banamine, is particularly well suited for treating soft tissue damage and colic symptoms

Causes of Bute Allergy in Horses

Mast cells are the specialized cells of the immune system that are tasked with the role of protecting the body from dangerous pathogens. When the immune system is stimulated by a particular substance, such as the anti-inflammatory medication Phenylbutazone, it manufactures its own protein which triggers the mast cells to release histamine. Histamine, a naturally occurring compound that has an inflammatory effect on the tissues it comes into contact with, is the instigator of the red welts that are characteristic of this type of allergic reaction in equines. 

In rare cases, allergies that are related to medications may induce an anaphylactic reaction, in which the blood pressure drops dramatically, and the horse experiences respiratory distress due to swelling of the lung tissues. This type of response occurs rapidly and can become fatal if left untreated.

Diagnosis of Bute Allergy in Horses

Your horse’s doctor will typically start the visit by evaluating the animal’s physical conformation as well as ordering routine tests such as a biochemistry profile and a complete blood count. These tests will often expose increased levels of a specialized type of white blood cell that would be indicative of an allergic reaction known as eosinophils, although eosinophils may also be increased in horses that are hosting parasites in their gastrointestinal system. Both intradermal testing and serum testing are available to help define a particular horse’s sensitivities.

Intradermal testing involves sedating the patient and shaving a patch on their necks, then injecting under the skin a tiny amount of the allergens that the veterinarian suspects to be the cause of the reaction or allergens that are common to equines. These injections are placed in a grid pattern in order to quickly ascertain which of the specific allergens are triggers. This patch will be evaluated at several intervals, from fifteen minutes from injection until twenty-four hours after injection. Although serum testing is less invasive, simply requiring a sample of the horse’s blood for screening, it is not used as frequently as it is considered to be less reliable than the intradermal testing methods. 

Treatment of Bute Allergy in Horses

If the horse is in distress supportive treatments are likely to be started without delay. In some instances, equines experience an anaphylactic response to the medication, and breathing becomes obstructed due to swelling in the lungs membranes. When anaphylactic reactions do occur, the use of epinephrine injected directly into the animal’s jugular will be required to protect the horse’s health. The veterinarian may also recommend that an IV line be set up to provide the large volumes of water that are needed to adequately support the large animal’s circulatory system. An essential part of treating equine allergies to medications is to cease administering the drug to the animal. Accurate diagnosis is of particular importance when dealing with an animal that is allergic to a medication like a NSAID which is often used as a remedy for the swelling and the fever that are typically characteristic of allergic reactions.

Cold water is frequently used as an effective method to reduce allergy-related fever and inflammation until your veterinarian has been able to examine the animal to ascertain an appropriate treatment plan and although horses who are allergic to one type of non-steroidal anti-inflammatory medication are often prone to allergies triggered other NSAIDS, many horses are able to safely employ different types of the anti-inflammatory drugs without harm. Corticosteroids and antihistamines are helpful in reducing the symptoms of allergy but are more commonly used in treating allergies with allergens that are difficult to either avoid or diagnose.

Recovery of Bute Allergy in Horses

Anaphylactic shock is a serious and rare allergic reaction which requires prompt medical attention. If epinephrine is available in the proper dosage, it should be injected it directly into the horse’s jugular vein and will generally need to be repeated every fifteen minutes until a veterinarian is able to set up an IV to allow for further medications. If no epinephrine is on hand, general antihistamines are often helpful to reduce the symptoms long enough for emergency treatment to commence and general therapies for shock, such as keeping the patient quiet and calm, covering them with a loose coat or blanket, and allowing them to find the most comfortable position to rest in.