Texas Equine Veterinary Association

www.texasequineva.com • 13 THE PRURITIC HORSE—WHERE TO BEGIN? GENERAL CONCEPTS OF PRURITUS Pruritic Threshold and Summation of Effect Phenomenon An important concept to remember when faced with any itchy horse is that each individual can tolerate a certain level of pruritic stimuli, but once the threshold is exceeded, itch prevails. Because the level of pruritus experienced by the patient is the sum of all itch stimuli, it is imperative to identify and control/manage all contributing flare factors. A good rule of thumb is to always exclude parasites and infections first before leaping to "allergy". Indeed, therapeutic success is directly correlated with the successful elimination of all instigating prurituc flare factors. Many of these factors can be identified through the patient's history (e.g., lack of appropriate fly control measures or deworming suggests insect-related pruritus or habronemiasis and/ or onchoceriasis, respectively; multiple horses affected is suggestive of an infectious or parasitic etiology; seasonal pruritus suggests problem with insects and/or atopy) and a physical examination (e.g., pruritic "pattern"; type of lesions). Distribution Patterns and Differential Diagnoses for Pruritus Pruritus is manifested as rubbing, licking, chewing, kicking, head shaking, feet stomping, and self-inflicted wounds. Many of the visible cutaneous changes are the result of pruritus including excoriations and alopecia, followed by changes in skin pigmentation and thickness. However, papular eruptions and wheals are also common lesions seen with ectoparasitism, infections (folliculitis), and allergic skin disease. Interestingly though, pruritus may not be a feature of allergic urticarial. The distribution of pruritus and corresponding lesions can be suggestive of the inciting cause as seen below and in Table 1. • Face, ears, mane/neck: fly bites, psoroptic mange, trombiculiasis, forage mites, onchocerciasis, insect hypersensitivity (Culicoides), atopic dermatitis, cutaneous adverse food reaction • Dorsum/trunk: pediculosis, psoroptic mange, insect hypersensitivity • Ventral midline: fly bites (black, horn, horse, and stable flies), poultry mites, Pelodera dermatitis, onchocerciasis, insect hypersensitivity • Lower limbs: fly bites, chorioptic mange, trombiculiasis, forage mites, poultry mites, insect hypersensitivity, atopic dermatitis, cutaneous adverse food reaction, photodermatitis (lightly pigmented skin) • Croup/rump/perineum: pediculosis, chorioptic mange, psoroptic mange, oxyuriasis, Malassezia dermatitis (intermammary fold/groin), insect hypersensitivity, atopic dermatitis, cutaneous adverse food reaction, herpes coital exanthema, vice/behavioral • Sparsely haired body regions: allergic contact dermatitis Fly Control Measures The best therapy is to reduce fly exposure through carefully constructed management practices and insecticides, since allergen-specific immunotherapy is generally considered ineffective for controlling insect- related allergy. Below are some general tips to better control flies on the horse and premises. Management Practices on the Premises • Since many flies feed at dusk and dawn, stable horses during these 'peak' times of fly feeding. • Routinely remove soiled bedding, manure, and decaying vegetation from stalls as often as possible to prevent the accumulation of organic debris in which flies lay their eggs. Likewise, do not dump this material right outside ABOUT THE AUTHOR Dr. Patterson is a Clinical Associate Professor Chief of Dermatology—Small & Large Animal College of Veterinary Medicine & Biomedical Sciences Texas A&M University, College Station, Texas by Adam P. Patterson, DVM, DACVD Dermatology

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