Clinical signs associated with allergy in horses

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Allergens normally considered harmless, can interact differently with the immune system of certain horses resulting in an allergic response. These allergens enter the body in several different ways and trigger a variety of clinical signs, which may occur singly or in combination. Because horses can exhibit such a variety of clinical signs, in response to the same allergen, it may take some serious detective work to identify the offending perpetrator(s)!

The first step in diagnosing equine allergies is making the link between the history and clinical signs. Obvious dermatological and respiratory signs can be clear to spot, and are most common, but sometimes signs may be less apparent such as a change in behaviour or drop in performance.

Dematological - skin

These cases are generally divided into horses with atopic dermatitis or insect bite hypersensitivity. As noted by Jensen-Jarolim et al. (2015) “atopic dermatitis in equines is becoming a more commonly recognised disease. It is interpreted in horses as an inherited predisposition to form specific antibodies to environmental allergens such as pollens of grasses, weeds, trees, but also to mould and dust.”
Insect bite hypersensitivity is a term used to describe the condition induced by an overreaction to biting insects. The most common example is sweet itch, where the irritation is believed to be caused by the saliva of the genus Culicoides (midges) and can lead to secondary infection and severe discomfort compromising the horse’s welfare at times.
Interestingly, the clinical signs for atopic dermatitis can be similar to those caused by insect bite hypersensitivity and it is has been recently noted that it is extremely common for horses to have both diseases.
It is also worth bearing in mind that adverse food reactions are in many cases also expressed as a skin issue. This is where there is an immune mediated hypersensitivity, or intolerance to a food constituent.
Clinical signs to look out for include:
• Pruritus (itchy skin)
• Urticaria (also known as hives)
• Alopecia (patchy hair loss)

Respiratory

The phrase ‘equine asthma’ is fairly new and is a helpful way to describe respiratory problems to owners. Previously, terms such as recurrent airway obstruction (RAO) and chronic obstructive pulmonary disease (COPD) have been used, with summer associated versions to contend with as well. All refer to issues with the respiratory system and can present as trouble breathing, excess nasal discharge and/or chronic sneezing and coughing. These debilitating issues can lead to secondary problems such as loss of performance and lethargy.

Gastrointestinal

Allergy may not be the obvious first thought for cases of recurrent low-grade colic, however where other possible causes have been ruled out it may be useful to consider a food reaction. Clinical signs such as diarrhoea and weight loss can be the result of hypersensitivities and in these cases an elimination diet may help resolve them.
Behavioural
Understandably allergies can cause horses to become uncomfortable and therefore sudden or unexplained changes in behaviour and temperament (including headshaking vices) may be a clue pointing to an underlying hypersensitivity.