( ADVICE FROM THE VET ) Flies & Allergies A
llergy occurs when the horse’s immune system becomes overly sensitive to a particular substance and produces an exaggerated response to that substance,
causing an allergic reaction. Allergens can be common things in the environment or feed, and are usually harmless to unaffected individuals, but for horses that suffer from allergies they can be hugely uncomfortable.
Pathophysiology:
In all species, the immune system functions by producing antibodies to pathogens. These antibodies attach themselves to the harmful substances, flagging them up to other elements of the immune system, which then destroy them. In an allergic reaction, the immune system misidentifies a harmless substance as a threat. Antibodies attach and cause a cascade of responses that try to destroy the allergen, resulting in the outward clinical signs that we see. Once the body reacts to an allergen, the condition is lifelong and requires constant management.
Causes:
A few common causes of allergy include: l Fly and midge bites l Bee/wasp stings l Contact allergies i.e. a substance that touches the skin, such as bedding, washing detergent, clipper oil, etc…
l Food allergy l Respiratory allergy to dust or pollen l Medication, such as penicillin.
Clinical signs vary depending on the allergen and how it comes into contact with the horse, but generally they can be split into skin, respiratory, and gastrointestinal signs.
Clinical Signs:
Skin signs: l Itching, l Oozing, l Thickening, l Urticaria or wheals, l Rubbed mane/tail, l Sweating, lAbnormal swelling.
Respiratory signs: l Increased respiratory rate, l Wheezing or a heave line, l Coughing, l Nasal discharge, l Exercise intolerance.
Gastrointestinal signs: l Diarrhoea, l Chronic colic, l Weight loss, l Bloating
Urticaria
respiratory distress, swelling, sweating, and colic. Anaphylaxis is an emergency and if you suspect your horse may be affected you should call your vet immediately.
T reatment:
In the first instance, your vet will usually advise a course of treatment which may include anti-inflammatory medication, such as corticosteroids or anti-histamines, and antibiotics if a secondary infection is present.
The next challenge is to try to identify the allergen. We can make sensible assumptions about the type of allergen from the clinical signs and their seasonality, however oſten it is difficult to pinpoint a precise cause. We are now able to test for a variety of common allergies in horses in the form of a blood sample.
The most straightforward way to manage a horse with an allergy is to remove it from contact with the allergen entirely. This may involve avoiding certain foods or environments, and sometimes a change of yard can help. It is important to make a note of any management changes that have occurred, for example a new hay bale or bag of shavings.
Allergen-Specific Immunotherapy (“ASIT”):
If avoidance of the allergen is not possible, then ASIT is now available for use in horses. With most allergies, there is a threshold amount of allergen, above which the body gives an abnormal response. ASIT involves injecting small amounts of allergen into the skin over a period of ten months which oſten causes a reduction in clinical signs. Most horses require a monthly maintenance dose aſter this to keep the allergy under control.
Fly bite reactions
Chronic skin thickening, patchy hair loss and pruritis affecting the entire body: This horse was found to have an allergy to dust and storage mites.
Anaphylaxis:
Anaphylaxis in horses is rare, and occurs most commonly with exposure to high amounts of allergen. Signs include
22 MAY/JUNE 2019
by Emily Douglas-Osborn BVSc MRCVS Assistant Veterinary Surgeon
Avonvale Equine Practice, Ratley Lodge, Ratley, Banbury OX15 6DT
www.avonvaleequine.co.uk T
el: 01295 670501
For the latest news visit
www.centralhorsenews.co.uk
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