Hay Allergy in Horses

Hay Allergy In Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost
Hay Allergy In Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

What is Hay Allergy ?

Horses that are allergic to hay dust and molds often show fewer signs in the summer when they can be out in the fresh air for longer periods of time. Affected horses find it much more difficult to breathe in the fall and winter, when they are kept indoors more often and fed more hay. However, some horses that are fed hay throughout the year may not show any seasonal changes, but may have symptoms flare up in response to repeated exposures to hay or molds.

A respiratory disease can result from an allergic reaction to the hay dust or mold that can constrict the small airways in the lungs. Not only does this produce excessive fluid in the lungs, but it can also thicken the airway walls, which is why an affected horse has difficulty breathing. This condition is often referred to as acute recurrent airway obstruction (RAO), heaves, or chronic obstructive pulmonary disease (COPD).

Horses that suffer an allergy to hay can be allergic to either the hay dust or to the mold that can grow in the hay. Signs of wheezing, sneezing and coughing are indications that your horse is having an allergic reaction to airborne dust or mold. Your horse could also suffer a food allergy from eating hay, which can cause diarrhea and weight loss.

Symptoms of Hay Allergy in Horses

Symptoms of a hay allergy can vary, and depend on the type of allergy your horse has and the amount of allergen he has been exposed to. Signs could begin as mild coughing or a little clear nasal discharge, but over time can progress to difficulties in breathing, especially during exercise. Symptoms of this kind of allergy include:

  • Exercise intolerance
  • Tires easily during exercise 
  • Coughing
  • Sneezing
  • Wheezing 
  • Nasal discharge that can be clear, white, or yellow 
  • Labored breathing 
  • Faster breathing
  • Nostril flare 
  • Abdominal strain with breathing, often causing a heave line from the muscles in the abdomen increasing in mass 
  • Itchy, watery eyes 
  • Itchy skin
  • Reddened skin
  • Skin swelling
  • Hives
  • Skin crusts
  • Hair loss
  • Diarrhea
  • Chronic colic 
  • Weight loss
  • Appetite loss


There are two types of hay allergy in horses.

Airborne Allergy

This is an allergic response to the dust produced from hay, or mold spores that are present in the hay. Symptoms of this kind of allergic response include respiratory complaints and skin issues. The presence of mold in the hay also indicates that there may be mycotoxins present, which can cause many additional symptoms, such as lethargy, reduced fertility and abnormal endocrine functions.

Food Allergy 

This is when eating hay causes an allergic reaction, manifested as gastrointestinal distress. Any mold or mycotoxins present can also cause a reaction internally as well.



Causes of Hay Allergy in Horses

An allergic reaction occurs when the immune system of a horse becomes sensitive to a particular allergen, in this case, hay or mold. When your horse is repeatedly exposed to this allergen, his immune system overproduces antibodies to fight it. These antibodies then release extra amounts of histamine, a chemical responsible for the symptoms seen in your horse.

In cases of a food allergy, as hay passes through the intestinal tract, it can come into contact with the antibodies, though this generally does not occur in healthy animals. Other factors such as age, the condition of the gut, and the presence of other bacteria or parasites can compromise the intestinal mucosal barrier which normally separates the antibodies from the eaten allergens, causing a reaction to occur.



Diagnosis of Hay Allergy in Horses

A diagnosis of a hay allergy can be difficult, as many of the symptoms are similar to other types of allergies and diseases. After a physical exam, any symptoms and their history, as well as details about your horse’s environment and feed, can help your veterinarian begin to narrow down a diagnosis, though more tests are often needed.

Your veterinarian will likely listen to your horse’s lungs, and may perform a lung function test. Fluid samples from the lungs or airways may be taken by endoscopy to be analyzed. Chest ultrasounds or X-rays can also be useful in determining the state of your horse’s lungs in the case of respiratory distress.

The most helpful testing method in determining if a specific allergy is present is through a blood sample. Blood and serum testing can reveal antibodies that are specific to a particular allergen. A skin allergy test may also be performed, which involves injecting various allergens just under the skin. Any swellings that appear indicate an allergy to that specific allergen. Both kinds of testing can give a definitive diagnosis, which is highly valuable in the formation of a specific immunotherapy for your horse.



Treatment of Hay Allergy in Horses

Based on the results of diagnostic testing, a specifically formulated immunotherapy injection can be made to help desensitize your horse to hay or mold. This will need to be given multiple times, and can take up to a year for results to be apparent, however, some horses will respond more quickly.

Your veterinarian may also prescribe inhalant medications, such as bronchodilators like albuterol, to be used directly into the nose or with face masks. These can relax the airways in the lungs and allow your horse to breathe easier. Antihistamines may be prescribed to control the allergic response. Corticosteroids are often given to reduce the allergic reaction and help to control inflammation, but are often not recommended to use for long periods of time due to side effects. Inhaled steroids in low doses may be recommended for long term use. Fatty acids may be added to your horse’s diet as well.

The most important aspect to keeping your horse’s allergy to hay under control is to limit his exposure to this allergen. Ideally, this is best accomplished by increasing turn out time and feeding your horse grasses and other substitutes for hay. Substitutes for hay include alfalfa pellets, alfalfa cubes, denji, and senior feeds. Hay has high levels of dust and molds, even if it appears clean. If you must feed your horse hay, completely submerge it in water for at least 30 minutes before feeding to your horse to reduce the allergen load. 

Minimize your horse’s exposure to hay dust by storing hay away from your horse’s stall, and turning your horse out when the stall is cleaned. Bedding should be replaced with shavings, paper or non-organic material to reduce the dust in your horse’s area. Keeping your horse’s stall ventilated can also help to reduce his inhalation of allergens.



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Recovery of Hay Allergy in Horses

An allergy to hay is not curable, but it is manageable with proper maintenance and treatments. Your veterinarian will likely send you home with medicines to help control your horse’s breathing and allergic reactions.

Proper management techniques to reduce your horse’s exposure to hay, and possibly mold, can significantly reduce symptoms. These include:

  • Monitoring hay and feeds for mold, and discarding moldy hay when found
  • Replacing hay feeds with other appropriate foodstuffs
  • Increasing turnout times
  • Using dust free beddings
  • Feeding your horse at ground level to reduce dust inhalation through the nose
  • Soaking hay before feeding
  • Storing hay away from your horse’s stall
  • Ensuring good ventilation in the stable or barn


Hay Allergy Questions and Advice from Veterinary Professionals




Irish Sport Horse


11 Years


0 found helpful


0 found helpful

Has Symptoms


My horse is very allergic to house dust mites, farinae mites, fleas, copra mites, grain mites and hay mites to name but a few. Had a specialist do lots of tests and she is undergoing immune therapy. I have her on haylage which was fine in winter as I bought big bales and shared with others. Now I’m using small bales and finding it very expensive. If I use hay and soaked it for 30 minuets, would it be safe for her with her allergies? Thank you, Nikki Briars.

June 4, 2018

Charlie's Owner


Dr. Michele K. DVM


0 Recommendations

Without knowing more about Charlie's test results, I'm not sure if soaking the hay will help, although I doubt that it will. It would be worth trying, while he is undergoing allergy therapy, to see if it helps, even though I am not sure that it will.

June 5, 2018

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