Allergic Reactions in Your Horse

Allergies and the symptoms they create vary with the allergen encountered and the body system affected.

Common equine allergies and their treatments.

By Dr. Thomas R. Lenz

The term “allergy” was first used by an Austrian physician in 1906 to describe a situation where the body’s immune system overreacts and results in self-injury. The allergic reaction is in response to a specific substance (an allergen) and affects some individuals but not others, which differentiates it from infections. Today, the term hypersensitivity is used synonymously with allergic reaction and occurs when an allergen (dust, mold, insect saliva, wool, etc.) enters or contacts the animal’s body. In response to the allergen, a component of the immune system called IgE is activated by the horse’s body and starts a cascade of chemical events that results in the release of histamine, which then stimulates the development of a number of allergic reactions.

Allergies are a frequent problem in horses. To learn more about how to keep your equine companion healthy, order the Common Horse Health Issues report.

Allergies and the symptoms they create vary with the allergen encountered and the body system affected. The horse can develop a hypersensitivity reaction to hundreds of allergens, but we’ll focus on the major types that are commonly encountered by horse owners.

  • Chronic obstructive pulmonary disease (also called heaves, broken wind and equine asthma) is similar to asthma in people. It is a progressive illness and is caused by the horse developing hypersensitivity to inhaled allergens such as pollens, molds and fungal spores. The hypersensitivity reaction causes nasal discharge and narrowing of the lung airways. The condition, if left undiagnosed and untreated, can cause the lungs to be irreversibly damaged. The most common symptoms of heaves are chronic cough, mucus discharge from the nose and forced expiration. With time, the horse develops a “heave line” diagonally across the ribs as muscles thicken in an attempt to force air out of the lungs. If untreated, the condition can lead to decreased performance and death. Treatment is aimed at removing the causative agent, as well as reducing lung inflammation and dilating the horse’s bronchioles.
  • Sweet itch (summer eczema), is a horse’s reaction to the saliva from the bite of a number of insects including mosquitoes, midges and “no see ums.” The intense itching caused by the reaction and the subsequent rubbing to obtain relief causes hair loss and wounds on the neck, mane, poll, back and base of the tail. The affected skin thickens and eventually develops a gray, scaly appearance. These horses are extremely uncomfortable and constantly rub against fence posts or stall walls in an effort to relieve the intense itching. Treatment is aimed at removing the insect causing the reaction and the administration of anti-inflammatory drugs.
  • Uticaria is the medical term for hives or nettle rash. The cause is an allergic reaction of the skin to a variety of substances such as food (molds in hay and straw, proteins in high-protein feeds), drugs, vaccines or pollens. The reaction causes the skin to swell into raised plaques of variable size, which become itchy and might ooze serum. Swelling of the eyelids and nostrils is common. Treatment is aimed at identifying the causative allergen and eliminating it. Supportive anti-inflammatory and antibiotic drugs are also indicated in serious cases.Finally, sweat eczema (contact dermatitis) is a hypersensitivity reaction that causes loss of hair and hives where the saddle or harness contacts the skin. The reaction is to substances contained in the leather, saddle cloth, wool saddle blankets, tanning agents in the leather, or ointments and lotions. Treatment consists of removing the causative agent and implementing supportive therapy. Saddles and harness can also spread bacteria or fungi that cause skin infections and require disinfection of the tack and treatment of the horse with antibiotics.
  • The most difficult problem in all of these allergic conditions is determining the allergen or agent that is initiating the response in the horse. A complete examination and diagnosis by a veterinarian is the first step before treatment is attempted. The diagnosis relies on history, physical examination and a battery of diagnostic tests. Blood tests to measure IgE are conducted to determine whether an allergic reaction is occurring. Skin biopsies are often indicated in horses suffering from allergic skin reactions. In addition, skin tests using a battery of allergens can be conducted to specifically determine the causative agent. The test is conducted by shaving an area on the horse’s neck. Tiny amounts of a number of allergens are then injected just under the skin. An allergic response is demonstrated by the appearance of a small swelling at the injection site. Once the causative agent has been determined, it can be removed from the horse’s area or the horse can be given decreasing doses of the allergen in an attempt to desensitize them to the agent.

Your horse’s health is important to you. Learn more about equine ailments in the Common Horse Health Issues report.

Allergies are a major problem in many horses and very difficult to diagnose. If your horse is suffering from any of the conditions discussed in this column or has a recurring problem that doesn’t resolve, contact your veterinarian. Be prepared to give a detailed history on the horse’s surroundings, types of feed, tack used and when the condition appears.

24 thoughts on “Allergic Reactions in Your Horse”

  1. my horse has a skin reaction to the ,”brace,” or chin strap of his bridle. He gets sores from the leather chin strap and brakes out causing reddened sores. Is this common and what would you suggest as treatment?

  2. warro
    April 28th, 2010 at 6:47 am
    my horse has a skin reaction to the ,”brace,” or chin strap of his bridle. He gets sores from the leather chin strap and brakes out causing reddened sores. Is this common and what would you suggest as treatment?

  3. Sowarro: The problem you are having with your horse breaking out is not a common problem. I have been in the horse business for almost 50 years and have never heard of a horse being allergic to leather, curb strap or otherwise.

    I would consult a veterinarian and rule out all other possibilities.

    AQHA Professional Horseman AL Dunning

  4. Ive had my grey warmblood mare for 10yrs & used same bridle & oil all this time but recently she started having reaction on her face ie hair loss & scabs where the bridle lays. I even went so far as to emerse in hot water & scrub thinking its the leather oil. I wud also sponge sweat area after riding but it didnt help. I then borrowed a webbing bridle & face improved & hair regrew. Used leather bridle once & face broke out the next day. Can she have developed a leather allergy? Im very confused as I use leather breastplate but no reaction on her chest. Does anybody have any insight,similar problem?

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  6. My horse itches like crazy every time I put his leather head stall on. I do believe horses can be alergic to leather. Why not??

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  8. My horse has wound on back leg from barbed wire. Happened. About a month ago. Vet said give sulphur drugs. Gave 1st round 10 days. Noticed his muzzle was very swollen then knots under jaw…got another round of sulphur..muzzle went down..knots under jaw went away..swelled a couple of other places all went away.then his eye swelled bulging blood horrible looking..looks like the eyelid ruptured…stopped sulphur per vet….started giving Dex..I’m really afraid his immune system is going haywire….had a mare that died of purpura.I pray this will not develop into purpura..has anyone ever had a horses eye to swell and bleed?

  9. My 21-yr old mare broke out in welts beginning on right hind qtrs & progressed all over her body. Vet gave her Cortisone shot & gave me a syringe to continue small dosage for 5 days. She became normal for about two weeks & it happened again. Vet called in a prescription 20 cc’s first, & 5 cc’s after & lessening it until none. She again is fine, BUT she is now wearing a fly sheet each day! I spray her before turning her out in the AM & put the sheet + fly mask on & out she goes dressed for the day! So far, so good.
    I have two other horses who have not been affected.

  10. my horse had a severe reaction to something when he was out in the field yesterday all his head neck and chest filled up with fluid the vet give him a steroid and antibiotics he his still full of fluid this morning vet said he could have had a bad reaction to a wasp sting but wasnt sure have you heard of anything else that would cause this to happen and how it was delt with please

  11. My horse stood on a nail yesterday (sunday), vet has been and exposed the wound and given bute and a long acting anti biotic. Within one hour of the anti biotic she began to bite at her shoulders, rub her face on her forelegs and she has also been pawing her stable floor frantically and lashing out with her back legs. She’s two hours post injection now and calmer but she is covered in bumps. I got bitten by an insect once and came out in hives this seems similar. Poor horse, now I’m scanning the net looking for advise!!!

  12. Sounds like allergic reaction to Antibiotic, it happens quick , did you ask your vet about a allergic reaction to Antibiotic! My advice check with vet and avoid that group of antibiotics ! Like PCN you can take all your life, then have a severe relation ! My brother did one time ! LPN nurse Pam

  13. Does anyone have any idea on what to help give my QH mare some relief from being itchy. I have had her allergy tested and it is to pollens mostly, her belly is broke out and now her face, mane and tail is getting rubbed. I have her on a needle program since last August 2012. Right now I also have been giving her 10 ml of hydroxyzine twice daily.
    Any advice would be greatly appreicated.

  14. Dear Dena, thank you so much for posting on here because you have helped me out..My gelding has broken out in welts all over including on the side of his sheath and a vet came out looked at him told me that all the welts ranging in size from a dime to two half dollars put together were an allergic reaction to something but the one on his sheath was supposedly a tumor even though it looks identical to the other welts. So I am thinking of switching vets and now thanks to you I know that I need to immediately buy him a fly sheet and mask as well as get him the shots since this only occurs when bugs are out and about 🙂 so thanks again!

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