Feed Related Allergies
Food intolerances and allergies are becoming increasingly common in the human population and there are numerous articles in the press regarding what you should and shouldn’t eat! Food intolerance appears to cause more irritating long-term symptoms whereas some food allergies are extremely dangerous. Severe food allergies can cause anaphylactic shock and people suffering from them have to be extremely careful about what they eat. Luckily this type of catastrophic food allergy does not seem to exist in the horse; in fact, researchers have found that true feed related allergies in horses are relatively rare compared to those in the human population.
Different types of allergies that occur in horses include:
Respiratory – inhaled allergen – COPD (chronic obstructive pulmonary disease e.g. fungal spores), PIPD (pasture induced pulmonary disease e.g. pollen)
Pharmaceutical – injected/ingested drug reaction
Contact – skin contact with allergen
Feed – ingested allergen
This article is aimed at the true food related allergies.
In practice, though allergic reactions to foods in horses and ponies are fairly common. These reactions can make horses quite subdued and depressed and horses may be generally off-color for several weeks. These reactions to food do not generally need immediate veterinary attention and should not be confused with inhaled acute respiratory allergies or pharmaceutical drug reactions particularly of rapid onset that do need immediate veterinary treatment.
Many horses and ponies will have the odd allergic reaction to food but occasionally some horses will develop regular recurrences of the condition and these can become quite problematical.
Symptoms of feed allergies are extremely inconsistent, varying from the soft, typical urticarial type swellings (hives) just under the skin to small hard bumps or papules. The swellings or lumps may be very itchy (known as pruritis) or not itchy at all. When horses are really itchy they may rub themselves raw. Alopecia or bald patches may also follow resulting in significant unsightly hair loss. Fluid from these soft lumps may also eventually drain downwards underneath the skin resulting in large swellings between the front legs or under the belly, which may take several days to clear. Allergies may be localized or cover the whole body or significant parts of it. Most horses suffering from allergies do not develop a temperature, but it is important to note that occasionally some viral infections may also result in similar lumps appearing.
To confuse the matter further many of these symptoms may be caused by other skin conditions such as mites, fungal infections, rain scald, bacterial inflammation of the hair follicles, allergic reactions to insect bites, or contact allergies.
Loose droppings may also imply a feed allergy with gastrointestinal allergic symptoms such as gastroenteritis although this is less common.
Allergies or hypersensitivities are an outcome of the horse ending up being sensitized to a minimum of one antigen. To cause a true feed allergy, the antigen must actually pass through the barrier of the digestive tract i.e. from the ingested food so that the white blood cells of the horses’ immune system are exposed to it. The majority of the time the gastrointestinal system works well to prevent this from occurring. Current research study recommends that compounds called glycoproteins present in numerous fresh and ready supplements and feeds might function as a base for these antigens. However, researchers do not know which part of the glycoprotein or even which glycoprotein is causing the problem! Allergic hives are often called protein bumps, but it is not the amount of protein causing the problem, rather it is a single protein (glycoprotein) or allergen, which causes the hypersensitivity reaction in the horse. These proteins are typically discovered within cereals especially barley or wheat however might be discovered in other foods.
Wheat and wheat by-products
Oats and oat by-products
Barley and barley by-products
Supplements on cereal carriers
Some pasture plants and therefore hay/haylage, chaff
Once the allergen is recognized by the horses’ bodies’ immune system, a chain or cascade of events occurs at the cellular level. The allergic reaction that follows is an inappropriate response by the immune system to the perceived threat of the allergen and the immune system becomes hypersensitized. The chain of events results in mast cells within the skin releasing histamine and this produces the typical symptoms seen.
Once the horses body has been primed with antibodies which have been produced to fight the perceived threat of the allergen, then whenever this is next encountered, the body will react in the same way, i.e. the immune system will be activated and the allergic cascade begins.
Horses may be genetically primed i.e. inherit a sensitivity such as seen with sweet itch. This is known as atopic or it may be acquired over a period of time and this is called non-atopic.
Following a suspected feed causing an allergic reaction in the horse, it is important to take a full history of everything the horse has eaten and note whether there have been any recent changes to the diet. Has the horse recently been vaccinated or wormed? Has the source of forage been changed? Is the horse being treated with pharmaceutical drugs? Also, it is important to check it is not a contact allergy. A typical example of this occurs where horses are only showing lumps or bumps in the areas of contact for example outside of the rug. This is often seen with contact allergies to bedding or where horses have lay down on pasture containing plants which may cause allergies in sensitized horses. This gives a clue as to the cause is it feed related or a contact allergy?
Horses with repeated allergic reactions may benefit from intradermal (into the skin) allergen testing although this has variable results with false positives and negatives. In severe cases, though it could be an option.
Hair analysis does not appear reliable and the RAST radioallergosorbent test appears more reliable for humans present. These tests for horses should be standardized for equines, not humans. You don’t; want to know that your horse is allergic to chocolate and egg whites for example!
There is a likewise a brand-new equine ELIZA blood test (Enzyme Linked Immunosorbent Assay). Once the allergen/s is identified horses may be offered immunotherapy treatment or desensitization. Some horses react really rapidly to this treatment whereas others might take numerous months in all cases horses frequently need more injections throughout their life as this does not cure the problem; it simply reduces the level of sensitization.
Horses with reoccurring feed allergic reaction issues must be put on a removal diet plan to figure out the source and attempt of the issue. This is not easy particularly for horses in hard work, but often it this is the only option left.
Feed grass hay or alfalfa mix to appetite within reason for 4 weeks. A single grass hay such as timothy or ryegrass is preferred
If out, pasture alone but again this could be the cause, but if so you will soon find out
Maintain pure salt supply – forage is often low in sodium therefore salt should be supplemented
After 1 month introduce one straight cereal such as oats, just a small amount for 2 weeks, and see if there is a reaction
After 2 weeks add a small amount of barley and so on until you find the culprit feed.
Introduce a multivitamin/mineral on a limestone or liquid base i.e. not containing a cereal carrier.
Many horses and ponies in light work are fed cool mixes or cubes but these more often than not contain barley or other cereal products, again ask the manufacturer. Grass pellets and unmolassed beet pulp are often a good choice being a single ingredient, however, occasionally they may be the cause! If the horse is allergic to a specific feed ingredient this elimination diet should result in improvement of symptoms within four weeks. Of course, it is important to note that horses may be and are often allergic to more than one allergen!
Once the offending ingredient has been found the horse may be returned to other compound feeds not containing it. Feed companies and supplement manufacturers should supply a full list of ingredients enabling the correct choice of feed to be made, this is really important. If they will not supply this information you should look elsewhere.
Immediate veterinary treatment usually consists of antihistamines and glucocorticoid therapy although these are not useful in the long term and the elimination diet is probably the best route even though it is time consuming.
These are common in people but not in horses, probably because the number of different foods available to horses is extremely small compared to our average diet!
Food intolerances are sensitivities to certain foods and are not allergic responses in that they do not affect the immune system. Behavioral problems in children commonly associated with food additives are also not relevant to horses. Toxicity from mycotoxins in feed, however, may cause a kind of food intolerance in horses. Mycotoxins are invisible poisons which can persist in feed and hay even when the molds that produced them are no longer present. Most good feed manufacturers will screen for these in feed. Certainly, moldy feed/hay should be avoided at all times. Mycotoxins can be harmful to horses producing minor symptoms such as loss of appetite to severe symptoms such as paralysis or death. There is currently a huge recall of feed in the USA, which is thought to be contaminated with the very harmful mycotoxin called Aflatoxin. This is thought to have been found in a single ingredient used to make up the feed. This may be classified as intolerance to feeding but again it is not classified as an allergic reaction.