Allergies & Intolerances


Dr Teresa Hollands, Nutritionist Dodson & Horrell Ltd Specialist lecturer Animal Nutrition, School of Veterinary Medicine and Surgery, University of Nottingham

How Do I Know If My Horse Has An Allergy?

There are distinct reactions that your horse will get depending upon the type of allergy that he has. So for example, if he is allergic to hay dust, he may develop respiratory problems as described by Professor Bruce McGorum; if he is stung by a wasp and is allergic to wasp venom, then he may come out in lumps and have trouble breathing; if he is allergic to a drug, he may also develop breathing problems, have an increased temperature and even develop anaphylactic shock. Hives or Urticaria is an allergic reaction in horses and it is often difficult to diagnose what has triggered the allergic reaction.

Hives Or Urticaria

Hives (also called weals or welts) are large, itchy red bumps on the skin. Hives can last for days or months but if they persist for several months to years and are then referred to as chronic urticaria. These hives can be one or many bumps, and vary greatly in size and shape. Although they more commonly appear on the legs and trunk, they can appear anywhere on the body as a result of the body’s reaction to factors such as certain foods, pollen, animal dander, drugs, insect bites, cold, heat, light or emotional stress. The release of histamine from mast cells in the skin causes the hives to form. Hives are generally not an emergency, but when they appear suddenly, spread rapidly and are accompanied by a swollen throat, tightness of the chest, wheezing or difficulty breathing, this indicates a severe allergic reaction that requires immediate medical attention

What Is Happening In My Horse’s Body When I See These Signs?

An allergy is an adverse immune reaction to a protein (called an allergen) in your horse’s environment. This allergen is normally harmless to the non-allergic horse.

Think of your horse’s body as a weapon. The immune system is a machine of war, ready to take on invaders. The immune system has its own internal affairs department: it can identify cells which are infected with viruses and which are in the process of becoming tumours. But like all armed forces, your immune system makes mistakes. In real life they call it friendly fire, an oxymoron like negative growth or military intelligence. In your horse's body it's called an allergic reaction. In its allergic mode, the horse’s immune system misidentifies an otherwise benign protein substance as harmful, and attacks the protein. This can cause any of the symptoms described above, or it can lead to the complete failure of the organ, the immune system is supposed to protect.

It takes around eight days for your horse's body to become primed with IgE antibodies, the antibodies that react to intrusion. After that, if the allergen is encountered, it triggers a destructive domino effect within the immune system known as an allergic cascade. So whether it's a pollen particle that's been inhaled or venom of a honeybee, it results in the same sequence of events.

Here’s what happens next:

  1. IgE antibodies recognise the allergen. They actually identify it by its protein;
  2. The IgE antibodies attach themselves to the protein
  3. The binding process sends an alert to a group of special proteins called the complement complex that circulate in your horse’s bloodstream;
  4. After the IgE antibody, binds to an allergen, the first complement protein attaches itself to the site
  5. This alerts the next protein in the sequence, which in turn alerts the next one, and so on
  6. When this process is completed, the allergenic cell is destroyed. In a healthy immune system this is fine. But in when the body has misidentified the cell it triggers what's called allergic episode. When this happens, the cells involved are mast cells;
  7. When mast cells are destroyed, their stores of histamine are released into the surrounding tissue and bloodstream
  8. This triggers a dilation of surface blood vessels and a drop in blood pressure. The spaces between the surrounding cells then fill with fluid
  9. Depending on the part of the body or the allergen involved, this heralds the onset of various allergy symptoms, some of the most common being: a. Itching of the body, eyes and nose; b. Hives; c. Sneezing; d. Wheezing; e. Diarrhoea;


How Does An Allergy Develop?

If your horse is atopic, his immune system will react immediately to the allergen because he has inherited a genetic sensitivity to certain proteins. He will also show an immediate allergic response to skin tests and as a result he will have high levels of the antibody known as IgE. If he is not genetically predisposed he is known as non-atopic and an allergy will develop after repeated exposure to the causative allergen. If he is atopic, your horse will be made sensitive the first time he is exposed to the protein, (allergen). If he is non atopic, sensitisation develops over a longer time; no adverse reaction appears to occur during this sensitisation. Some time later after repeated allergen exposure, the full-blown allergic reaction will occur. It is thought that non atopic allergies involve another antibody known as IgG as well as (but not always) IgE.

Mast cells play a vital role in allergies because they produce histamine, a key weapon in the body’s flight against infection. The problem is that when histamine is released in too great a quantity or at the wrong time it creates trouble.

The immune system produces five kinds of antibodies, each with its own function. Allergic reactions involve the IgE antibody. Allergic horses form IgE antibodies against substances that are harmless in most other horses such as certain types of food and pollen, called antigens or allergens. Once the IgE antibodies are formed against a specific allergen, they bind to special mast cells which are found in the skin, lungs, nose and intestines and are also known as basophils which circulate in the blood. This process is called allergic sensitisation. When the horse is continuously exposed to that specific allergen, the IgE antigens attached to mast cells bind to the allergen, causing a reaction that causes the mast cell to burst. When the mast cell bursts, it releases chemicals such as histamine. These chemicals make the blood vessels dilate and smooth muscles contract. For example, the smooth muscles of the bronchi contract and the air passage narrows, which may lead to Respiratory Airway Obstruction, (RAO). If the allergic reaction is severe, so much histamine is released that the horse's blood pressure drops, blood circulation slows down and the bronchi contract, causing respiratory distress. Such a severe allergic reaction is called anaphylactic shock and may cause death. Fortunately this is rare. Remember that sensitisation may last from several days to a few decades depending upon whether the allergy is inherited or not. The repeated exposure to an allergen triggers the immune system to form the antibodies that cause the allergic reaction to the specific allergen.

Is Intolerance The Same As An Allergy?

No! Intolerance reactions are "Non-allergic" hypersensitivity reactions which are reproducible adverse reactions BUT they do not involve the immune system. Diarrhoea might be a good example of a non-allergic hypersensitivity as long as sudden changes in diet or stress have been ruled out.

There are different types of non-immune reactions to food. The most obvious example is simple toxicity. Contaminants (substances which get into food during the growth, harvesting, processing, packaging or storage of food) are occasionally harmful. In severe cases these contaminants can make food poisonous and any horses eating them will suffer ill effects. ALL reputable feed manufacturers test cereals and forages for mycotoxins before using them; check that yours does!

An example of non-allergic food intolerance in humans is the reaction caused by naturally occurring chemicals in food, or by food additives (chemical reaction). Food additives include a variety of substances, such as preservatives, flavouring agents, colouring agents, etc. Well-known examples are tartrazine, monosodium glutamate (MSG), sulphur dioxide and benzoates. Chemical reactions to food or food additives are not true allergic reactions because they do not involve the immune system. It is worth noting that Dodson & Horrell Horse feed does not contain any artificial colourings, preservatives or flavourings.

Is True Food Allergy Common?

True food allergy is less common than popularly believed. In humans, it is estimated that only between 1% and 4% of the general population suffers from a definite food allergy. Professor Derek Knottenbelt at the 3rd International Conference on Feeding Horses, (Northants, 2000) noted that 'allergies due to feed are fortunately rare; diarrhoea suggests that the allergic responses also affect the intestine and therefore we generally assume that the allergen is in the food'

What Are The Symptoms Of Food Allergy?

The symptoms of food allergy may be confined to the digestive system only (the most common sort), or involve other parts of the body as well. Symptoms of the upper part of the digestive system, which come on rapidly, include swelling of the lips, and itching and redness around the mouth. In people, the mouth may become intensely itchy and the uvula (the small hanging fold of flesh at the back of the throat) may become swollen. Foods which most often cause such reactions in humans are eggs, nuts, shellfish, citrus fruits and berries.

In people reactions of the lower part of the digestive tract may take a little longer to develop such as nausea and vomiting; other common symptoms are stomach cramps (colic) and diarrhoea. Horses would develop diarrhoea. Other systems which may be involved in food allergy are the skin; 'the horse is probably the most susceptible animal to the development of urticaria’ (Professor Derek Knottenbelt) or the respiratory (breathing) organs.

Are There Tests For Allergies?

As a number of conditions resemble allergies but do not involve the immune system, it is not always easy to diagnose an allergy. In the case of a mild allergy, tests may not be necessary and the vet can base their diagnosis on your horse’s clinical history and a thorough physical examination.

However, certain laboratory tests are suggested when the allergy is severe. A clinical immunologist usually performs these tests. The following laboratory tests may be done to verify the cause of an allergy:

Complete blood cell count – although not always routinely used for allergy testing, this blood test is performed to detect an increase in the eosinophil white blood cell count. If the test results show a higher than normal number of these white blood cells, it is an indication that the body is trying to fight off a foreign invader such as an allergen.

Nasal smear – a sample of mucus obtained from the nose is examined for an increase in eosinophil white blood cells. Radio-allergosorbent (Rast) test – the amount of IgE antibodies in the bloodstream is measured in serum that recognises allergens inhaled by the patient. It is a reliable test for inhalant and food allergens. If the test results show a high amount of IgE antibodies against a certain allergen, it is highly probable that your horse has an allergy to that allergen. However do be careful that the tests have been standardised for equine allergens. If a result comes back that the horse is allergic to a grass or even chocolate, it is probable that the test is human orientated and the allergen is likely to be the grass pollen rather than the grass itself. Skin test – if it is not clear what your horse is allergic to, a skin prick test may be done. In this test, a solution of a suspected allergen is injected into the skin to determine whether there is IgE immunologic hypersensitivity. If your horse is allergic, a small hive will appear within 15 to 20 minutes on the skin area where the test was done. This test yields the best results when testing for inhalant allergies, insect sting allergies and drug allergies. It can also help to determine whether your horse has food allergies.

These tests are not completely reliable; if too much of the substance is injected, it may cause a reaction in a non-allergic horse...

Would An Allergy Cause Excitable Behaviour In My Horse?

In people, the effects of food allergy on the central nervous system (CNS) is a controversial area and one which tends to attract a lot of media attention. CNS disorders which have been linked to food allergy include migraine, the allergic tension-fatigue syndrome and hyperactivity. Several foods have been shown to trigger migraine: chocolate, red wine, yeast extracts, hard cheeses, milk and eggs, all foodstuffs which are linked with having an effect on serotonin or its precursors and subsequently effecting bloodflow.

Children with the allergic tension-fatigue syndrome have pale faces with dark rings under their eyes, giving them a tired look. They may be difficult to rouse in the mornings and tend to concentrate poorly at school, especially in the mornings. These children tend to be irritable and to sleep badly at night. It has been found that there is a link between this syndrome and an excessive intake of milk, cool drinks and chocolates. Elimination of these foods from the diet may improve the symptoms of these children quite dramatically.

The research literature is still in debate as to whether migraine is a true allergenic response or whether an intolerance to certain foods which effect blood flow accumulates over time together with other confounding factors such as stress or changes in hormones.

Normally an increase in excitability in horses is a response to excess calories in their diet. The calories are usually provided by fast releasing energy sources. Reducing the number of calories the horse gets reduces the excitable behaviour. If your horse had a true allergic reaction to the food it would show some of the symptoms described above.

What Should I Do If I Think My Horse Has An Allergy To Its Feed?

Talk to your vet, who will consult a nutritionist and an elimination programme will be put together. This involves removing all the potential dietary allergens and then introducing them one at a time over a period of time. Remember that hay, grass, hedgerows as well as cereals contain proteins which might be potential allergens.

Professor Knottenbelt, (3rd International Conf Feeding Horses) notes that bran, wheat, barley and oats have been implicated in food sensitivities in the horse and it is the proteins in these cereals which are responsible for the reactions.

Remember too, that fat, (oil) and carbohydrates, (starch, sugars, cellulose) are NOT allergens and CANNOT under any circumstances cause an allergic response in your horse. It would be very dangerous if a vital nutrient such as glucose which is needed for the brain was to illicit an allergic reaction!