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Allergic skin disease & allergy testing

Apr 20 2021, 15:45
Stephanie O’Reilly - BVMedSci(Hons) BVM BVS(Hons) MRCVS

Allergic skin disease

In the UK allergic skin disease is estimated to affect approximately 12% of the equine population, so is a common concern seen in practice.

The clinical signs we see in horses experiencing allergic skin disease include itching, hives, scabs, hair loss and scaly skin. Some horses also experience secondary problems such as bacterial skin disease where the skin barrier has been broken. These can be seen anywhere on the body but are commonly first identified at the base of the mane and the top of the tail, but this varies with the allergen.

An allergic reaction occurs when a horse comes into contact with a normally harmless substance, however the horse’s immune system has an inappropriate reaction to the substance causing the signs we see as an allergic response.

There are a number of common causes of allergies, broadly split into environmental (dusts, pollens, moulds), food and insect hypersensitivities.

Understanding allergies

Hypersensitivities are a very challenging disease process to manage and a huge welfare concern. We see many horses presenting with ‘Sweet itch’ which is the clinical presentation of an insect bite hypersensitivity associated with Culicoides species of midges. However, we now understand that allergies are often a bit more complicated than a single cause and although we may see Culicoides as the primary cause, there is in fact something called the allergic threshold. These different allergies each contribute to something called an allergic threshold.

The allergic threshold is a concept that allows us to think of each horse having a maximum amount of allergens it can tolerate. Once the allergic burden exceeds this point, we then see clinical signs. This is why it is important to consider all possible causes that may be contributing to the allergic disease process. Doing this allows us to optimise our treatment plan to keep individual horses below their allergic threshold.
Our aims of controlling allergies is keeping exposure to a minimum, therefore allowing the horse to stay below the threshold and not showing clinical signs, improving welfare.

Here is an example of a horse who has previously had allergic skin disease. We identified he had environmental allergies and an insect hypersensitivity. A management and treatment plan focusing on reducing his exposure levels to these keeps him below his allergic threshold meaning he doesn’t have any clinical signs at the moment.

What we can do about allergies?

After ruling out other potential causes of skin disease and your vet feels that allergies are the most likely cause, we can consider testing to identify the things your horse is allergic to.

This can be done in one of two ways; intradermal skin testing or a blood test.

The blood test for allergies is a quick simple method of identifying what might be causing your horse’s allergies which will then aid in the management of your horse’s condition.

How does the test work?

  1. We take a blood sample. This should be done when your horse’s symptoms are at their worst and when no treatment is in place. This gives us the best chance of detecting the possible causes.
  2. The sample is sent away to the lab where they test for antibodies against common causes of allergies.
  3. Your vet is sent a report with a breakdown of all the allergens your horse is sensitive to. This covers common causes including foods, insects and environmental factors. These are then considered alongside your horse’s history to facilitate creating the best treatment plan possible.
  4. The treatment plan- based on the results this could involve avoiding the allergens, immunotherapy, systemic medication, dietary changes or topical therapies.

Identifying the individual allergens of a horse is the best method of control and also allows us to consider immunotherapy where we desensitise your horse to the allergens.

Allergy treatment:

The aims of allergy treatment in the long term are based around keeping your horse below their allergic threshold. Methods of treatment focus on:

  • Allergen avoidance
  • Maintain skin and coat hygiene
  • Controlling the primary disease
  • Repairing the skin barrier
  • Controlling flare-ups

By identifying the causes the majority of treatment plans will involve avoiding those particular allergens but by knowing what your horse is allergic to does also open up the opportunity to consider immunotherapy.

Allergy- specific immunotherapy is a tailor made to each horse method of treatment for allergies which involves injecting small amounts of the allergens to stimulate the immune system with the aim of building up a tolerance to the allergen. This method of treatment has been shown to be around 50-80% effective and can be used as a long-term treatment. Each vial can contain up to 8 allergens and is administered as a subcutaneous injection (under the skin) with a loading regime over 12 weeks followed by monthly maintenance doses.

Please note: If you have any concerns regarding your horse and feel their allergies are not being controlled, please do contact us for advice, skin disease can be a huge welfare concern if poorly controlled.