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EQUINE VETERINARY EDUCATION / AE / MARCH 2015


Review Article A diagnostic approach to the pruritic horse


S. D. White Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, USA. Corresponding author email: sdwhite@ucdavis.edu


Keywords: horse; pruritus; diagnostics; parasites; atopic dermatitis


Summary Pruritus is a common complaint in equine medicine. While pruritus in most horses is due to ectoparasites (midges, flies, mites) or environmental allergens (pollens, barn dust, moulds, etc.), other causes such as staphylococcal or fungal infections, vasculitis, and internal organ dysfunction should not be overlooked. In a rational approach to the pruritic horse, history and physical examination become very important as a guide to choosing diagnostic tests.


History


Most important historical information can be gleaned from the following 6 questions:


Does the horse scratch (itch, self-bite, rub or lick) excessively? This may seem self-evident if a horse is presented for pruritus – but in fact, there is a wide range of awareness among horse owners if their horse is actually pruritic. This is usually dependent on how much time during a week the owner spends with the horse. If the horse is only ridden twice weekly, or the horse was in training and just returned to the owner, any skin lesions on the horse may not be immediately recognised by the owner as due to pruritus.


What is the duration of the skin disease and is there any seasonality? Seasonal pruritus (due to insects or pollen allergies) is typically more pronounced in the warm weather, whereas allergy due to barn dust components may be more severe in the winter. Similarly, photo-aggravated vasculitis may be more severe in the summer. Of course, if the pruritus has been present for less than a year, this information may not be available.


Did the lesions look different from how the horse presents now (what is the progression of the lesions)? Did any generalised alopecia occur in small areas, then coalesce (such as would happen in a dermatophyte [ringworm] or pyoderma), or did the alopecia occur all at once, which is more suggestive of a drug reaction? If a rash (papules/erythema) is present, it is helpful to


determine if the rash preceded the pruritus or if the pruritus is only directed at the rash (i.e. is it a rash that itches [ectoparasites, bacterial- or dermatophyte-caused folliculitis] or an itch that rashes [allergies]). Not all horse owners will be able to answer this detailed a question, however.


What medications have already been used? Did any help (or worsen) the skin condition? Ask if the owner has either kept a list or still has all bottles, vials etc. of previously utilised drugs, including topical medications.


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Remember that a statement like ‘antibiotics (or corticosteroids, antihistamines, shampoos etc.) did not work’ means nothing without knowing what kind, what dosage, how long a duration etc. Response of pruritus to corticosteroids does not tell much, but failure of the pruritus to respond to corticosteroids is suspicious of pyoderma, dermatophytes or Chorioptes/Sarcoptes infestation. (Note: Sarcoptes infestation has not been reported in horses in the USA in almost 100 years.) Response to antibiotics usually typifies a pyoderma; however, some clients regard all pills as ‘antibiotics’. In addition, antibiotics are often given concurrently with corticosteroids and the owner will neglect to mention this fact.


What is the horse’s environment? This includes subsequent questions: Has the horse travelled out of the immediate area? Are other animals (or the owners/trainers) affected? Involvement of other animals or people obviously points towards contagious diseases (such as dermatophytes). However, lack of disease in other mammals does not necessarily rule this out.


What is the horse’s diet? While food allergy is very rare in horses (the author has only diagnosed this 4 times in the past 34 years), horses on a poor plane of nutrition may be more susceptible to infectious agents such as pyoderma, dermatophytes and lice. Also, the possibility of ingestion of plants containing potential liver toxins or photodynamic agents should be investigated. Note: the most pruritic horse the author has ever seen was one with end-stage liver disease (see Video S1).


Physical examination


When examining a horse for pruritus, the 2 main questions to answer are: What are the lesions? and Where are the lesions? Primary lesions are defined as those that arise spontaneously as the result of the disease process: examples are papules, pustules, wheals, tumours and nonpruritic alopecia. Secondary lesions are those that arise from primary lesions due to pruritus or movement of the horse. These include ulcers, alopecia secondary to pruritus, etc. Some lesions may be either primary or secondary, such as scale or crust.


Lesions Papules: are usually indicative of an allergic reaction and/or ectoparasite envenomisation (i.e. bites or stings).


Epidermal collarettes: These circular areas of scale are often over-looked, these are the ‘remains’ of a pustular disease, usually pyoderma, rarely a drug reaction or pemphigus foliaceus.


Urticaria (aka hives, wheals): is usually due to: 1) ectoparasites or a hypersensitivity to their bites or stings;


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