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Sweet itch, hay dust allergy & co.

Each summer there comes a time when many horses scratch their tails and manes due to various allergies. However, it is not only sweet itch, where horses are allergic to the saliva of the Culicoides mosquito, that is a major problem, but also allergies triggered by factors found all year round, such as mould in hay (“hay dust allergy”) or pollen.

The mentioned allergies are known as type I allergies, meaning at some point there was initial contact, which was still asymptomatic, and the body then forms what are called IgE antibodies. These antibodies then always recognize the penetration of these allergens into the body, activating mast cells and basophilic granulocytes. During this reaction, various chemical messengers such as histamine are released, which promote an inflammatory reaction and subsequently cause itching, teary eyes, coughing, or urticaria, i.e. the typical symptoms of an allergy.

In contrast to horses suffering from pollen allergy, for whom the grazing season is the more difficult time of year, horses with an allergy to moulds from hay are usually symptom-free in the grazing season and suffer more in winter. However, not all horses that cough are necessarily allergic.

RAO/COPD is often not an allergic condition

Horses with RAO (Recurrent Airway Obstruction) or COPD (Chronic Obstructive Pulmonary Disease), as the disease is also known, are often referred to as hay dust allergy sufferers. These horses react to the dust in the hay with irritated airways, coughing, and sometimes respiratory distress. Cases have been described in the literature in which IgE antibodies against mites have been detected; allergies to mould spores have also been shown, but not to dust. So there is no such thing as a “hay dust allergy”; horses can only be allergic to mites or moulds contained in hay. However, hay dust in itself can trigger a (non-allergic) dry cough in horses with inflamed airways.

Several factors can ultimately lead to COPD, ranging from chronic suppuration of the paranasal sinuses and inadequate husbandry practices (such as closed stables with poor air quality) to feeding errors such as providing mouldy hay. COPD can therefore be triggered by allergies (such as moulds, mites, pollen, and animal hair), but it is not necessarily the case. The first step is to identify the specific cause of the symptoms so that targeted treatment can be provided to help the horse. A diary can help to pinpoint the possible causes more precisely by comparing the occurrence and severity of the symptoms with factors such as the time of year, pollen calendar, and changes in husbandry or feed quality.

Nose nets and customised feeding

For pollen allergy sufferers who often react with headshaking, a scratchy nose, or nasal discharge, nose nets have often proved effective. Free access to the stable during the day is also advantageous so that the horses can retreat. They are often exposed to less pollen in the stable than in the turnout or pasture.

Mould allergy sufferers are quite different: they are more likely to benefit from grazing in summer and avoiding exposure to mould, for example, by using shavings bedding and warm air-dried hay. Any bale of hay that is even slightly mouldy and still tolerated by healthy horses can trigger the next allergy attack, ranging from coughing to severe respiratory episodes.

A sweet itch rug is usually recommended for horses suffering from sweet itch, unless they live in coastal regions or at altitudes where there are no allergy-causing mosquitoes. In the case of sweet itch, however, it is also essential to clarify the presence of KPU and adjust the feed accordingly. The more sugar-rich the diet of these horses, the more attractive they are to mosquitoes. Feeding can therefore have a significant influence on the occurrence and severity of the allergy.

Horse with sweet itch rug
© Adobe Stock / Petra Eckerl

Congenital allergies in horses

In Icelandic horses, in particular, there is often discussion about whether their allergy to the saliva of the Culicoides mosquito could also be congenital, suggesting a certain genetic predisposition within bloodlines or ancestry. Unfortunately, to date, no clear “sweet itch gene” has been identified.

In addition, environmental factors play just as significant a role in allergies as genetics.

As far as we know today from studies on various animal species, several genes are usually involved in the development of allergies, making identification not so easy, and affected animals cannot simply be excluded from breeding. In addition, as mentioned above, epigenetics also play a very important role in allergies because a certain mutation in a gene, for example, does not always lead to clinical symptoms.

So even if there should be a “genetic test” for sweet itch one day, this does not guarantee 100% whether the horse will also develop the disease. This is because such genes initially only indicate a predisposition, meaning a genetic tendency to a certain disease or sensitivity. Whether this disease then manifests and becomes clinically relevant or not depends, among other things, on living conditions such as feeding, husbandry, medication, stress levels, etc. This offers little relief to the allergy-stricken horse. Knowing the exact cause is therefore much more helpful, as it enables you to identify which allergens to avoid.

Allergy tests for horses

There are now several allergy tests available for horses. IgE antibodies are detected in serum using the so-called ELISAs (Enzyme-Linked Immunosorbent Assays), which are most commonly used. However, these tests are not yet sensitive enough to provide a reliable statement. This can lead to both false negative and false positive results. This means that substances can be recognized as problematic allergens without the horse showing symptoms (false-positive), or substances are not recognized as allergens even though the horse reacts to them with allergic symptoms (false-negative).

Blood sampling from a horse
© Adobe Stock / Charlymorlock

Assays that measure the leukotriene content of basophil granulocytes (CAST – Cellular Antigen Stimulation Test) or the histamine content (FIT – Functional In Vitro Test) are more suitable. However, the greatest difficulty here is always to identify the exact allergen.

If successful in identifying the triggering factors, desensitization via immunotherapy in horses would also be more successful. At present, however, the success of desensitization in horses is extremely limited. Good management of the allergic horse usually has a greater impact on quality of life than such treatments.

Immunotherapy in horses

Currently, the greatest hope in the treatment of allergies in horses is placed in immunotherapy. However, research in this area is still in its infancy. Most publications report an improvement in symptoms in the first two years, which then subsides. More reliable tests that identify the exact antigen and a targeted immunotherapy based on this would be desirable, but are still dreams of the future. Until then, the only course of action is to manage the horse through appropriate husbandry and feeding measures so that it can cope as well as possible.

Allergy prevention and management

The less stress a horse is under and the fewer feeding mistakes are made, the lower the risk of developing an allergy in the first place. If the intestines are not functioning properly, the susceptibility to allergies increases significantly, as around 70% of the horse’s immune system is localized in the intestines. Inflammation of the intestinal mucosa and the associated “leaky gut syndrome” overload the immune system located in the intestinal wall. Even if the horse does not (yet) show any symptoms of an allergy, you can preventively avoid feed that promotes the development of an allergy, such as haylage.

However, stress can also lead to chronic inflammatory reactions in the gut, but this is often very difficult to recognize in horses. This can be triggered by chronic pain or unsuitable housing conditions, for example. Not every horse is suitable for group housing; especially old or low-ranking horses are often under constant stress in large groups or in herds with frequent changes. Chronic pain, for example, due to osteoarthritis, subclinical laminitis, or because the horses have to stand on hard ground in the turnout all day, can also be problematic. In the long term, they not only cause stomach ulcers but also inflammation of the intestinal mucosa, which in turn increases susceptibility to allergies.

Of course, all these factors also apply if the horse already suffers from an allergy. By reducing stress, avoiding feeding errors, or stabilizing the intestines, the allergic disposition can usually be greatly reduced, providing significant relief for the horse.