React. Equine Allergy Clinic

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What are allergies?
Allergies are an individual's immune system over reacting to harmless foreign matter (known as allergens). Instead of quietly removing this material, the immune system attacks it with a range of cells and chemicals as if it where an invading organism. The chemicals released produce inflammation and the ensuing symptoms are similar to those caused by any other type of inflammation such as an infection from a bacteria or virus. 

Why do horses get allergies?
At the molecular level, it is poorly understood why any animal develops an allergy and undoubtedly there are many different factors that may lead to the start of an allergic reaction ranging from genetic susceptibility to infection with a virus. For example Icelandic ponies are particularly susceptible to Sweet Itch and it is well recognised that after influenza infection the chances of developing a lung allergy are increased. 

Another way of looking at the problem is to ask why we should be surprised that our horses commonly develop allergies when we consider that generally they are kept in alien conditions from those in which they evolved. We should not be surprised. In evolutionary terms, the horse has only recently been domesticated and taken from being a roaming herd animal living outdoors on extensive grasslands or steppes. Now we take them indoors, bed them on dusty dry straw, give them dusty dried grass (hay) and a cornucopia of other exotic grains to eat. Do you really think the horse is adapted to eat large amounts of soya and peas, for instance? And yet these are very commonly in compound feedstuffs. Quite simply the horse has not evolved to cope with these living conditions and therefore its immune system which has evolved over millions of years, occasionally fails to handle the new challenges of today's environment. 

Treatment of Allergies
The majority of allergies are treated principally with cortisone (otherwise known as corticosteroids), which tempers the immune reaction and reduces the inflammation. Cortisone is very effective at this, but it has unwanted side effects such as laminitis, diabetes, and immune suppression, which make the animal more susceptible to infections. However, it only lessens the symptoms and does not address the cause. Therefore, in most cases the allergy returns as soon as the cortisone wears off. 

Incremental desensitisation has been used for many years and was the first technique devised to modulate the immune system's response to specific allergens, or, in other words, curing the allergy, as opposed to just damping down the effects of the allergic reaction. The technique relies on identifying what materials (allergens) set off the allergic reaction and then injecting large and increasing amounts of those allergens. However, two major problems occur with this. Firstly, acute and very severe reactions (anaphylaxis) can easily be provoked and, secondly, the success rate is not very high. There are commercial treatments using incremental desensitisation available, but they depend on identifying the causative allergens from blood samples (by measuring antibody titres of IgE to specific antigens). This technique is not considered very reliable and the gold standard means of identifying the cause is by skin testing (see below). 

Allergy Neutralisation is a newer technique for modifying the immune system so that it does not over react to harmless materials and therefore prevents an allergic reaction. It depends upon intradermal skin testing to identify the causative allergens but unlike incremental desensitisation, a concentration of the allergen is found which actually switches off the immune system's reaction. Then the patient is injected only with this concentration of allergen. It is a relatively low dose and is not increased throughout the treatment, which means that it is very safe and the incidence of anaphylactic reaction is unknown in horses. The treatment consists of a very small volume easy to give injection (0.1ml through a human insulin syringe). This is given daily until symptoms resolve, which takes a variable time, from 6 weeks to 6months depending on the disease being treated and its severity. 

The other big advantage is that allergy neutralisation is effective. At the REACT Equine Allergy Clinic it has been used very successfully in the treatment of COPD (chronic lung allergy akin to asthma) and allergic skin disease (urticaria and atopy). It has also been shown to benefit a proportion of Headshakers. 

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