Episode 36 - Hives, Lesions and Lumps; Urticaria in Horses

Episode Overview Join us as we venture into the stables to explore urticaria in horses - those mysterious swellings that appear seemingly out of nowhere and may disappear just as suddenly. Expert guest Dr. Valerie Fadok shares her extensive experience as both a veterinary dermatologist and immunologist to help us understand what causes these puzzling conditions, how to differentiate them from other lumps, and when to investigate further rather than automatically reaching for steroids. Featured Guest Dr. Valerie Fadok - A dual specialist bringing unique expertise as both a veterinary dermatologist and immunologist. With experience across three veterinary schools, private practice, and as a field specialist with Zoetis, Val brings a wealth of practical knowledge from working with veterinarians and horse owners around the world. Episode Breakdown Introduction to Urticaria in Horses Val discusses how horses are the most commonly affected species with urticaria among the animals veterinarians treat, and how this condition can drive both horses and their owners to distraction. The disease presents unique challenges, with sudden onset cases that sometimes resolve on their own, and chronic cases where horses experience repeated outbreaks over time. Clinical Presentation and Diagnosis What Urticaria Looks Like: Val emphasizes the importance of palpation—urticarial lesions tend to be soft compared to nodular diseases like eosinophilic granulomasIndividual lesions wax and wane, even if the horse has hives every dayLesions can take fascinating shapes: round, linear, or ring-like configurations (serpiginous patterns)Not all horses with urticaria are particularly itchy Papular Urticaria: Papular (miliary) lesions are commonly associated with insect bitesVal shares examples of horses moving from northern US states to Florida developing papular urticaria in their first year due to high insect pressure from mosquitoes and CulicoidesThese cases often resolve after the first yearSue confirms similar patterns in the UK with Culicoides Immunological vs Non-Immunological Reactions The Role of Mast Cells: Urticaria involves mast cells in the skinImmunological urticaria occurs when allergens bind to IgE on mast cells, triggering the reactionNon-immunological causes involve "twitchy" mast cells that react to physical triggers Physical Urticaria: Pressure urticaria and dermatographism—where a handprint appears on the horse's flank after touchingCold-induced urticariaHeat-induced urticariaExercise-induced urticariaSome horses have both immunological and physical components, making diagnosis particularly challenging History is Key: Observant owners can provide crucial information (e.g., "hives appeared after training session" or "outline of saddle appeared after removal")Owner observations are often the best way to differentiate between causes Acute vs Chronic Urticaria Acute Urticaria Management: Most acute urticaria in horses is drug-related (antibiotics, pain medications) or from blood transfusionsVal's approach: Don't do an intense workup immediatelyTreat with antihistamines (Val prefers hydroxyzine) for a few months to let mast cells settleIf it recurs after stopping medication, then investigate furtherSue agrees: not chronic unless present for 8+ weeks or recurring annually When to Investigate: Sue and Val agree: 8-12 weeks or recurrent episodes warrant deeper investigationBoth emphasize the value of owners who keep detailed calendars noting when hives appear50% of urticaria in people remains idiopathic—same often true for horsesCompetition horses present particular challenges due to medication restrictions Investigation and Testing Seasonal Cases: For seasonal urticaria, Val recommends intradermal or serum allergy testingHorses with urticaria respond well to allergen immunotherapy compared to other speciesMost horse owners are comfortable giving injections Non-Seasonal Cases: Consider dietary factors and whether feed changes throughout the yearHorse owners are surprisingly open to food trialsVal has only proven a handful of food-related urticaria cases (alfalfa and grains)Diet trials are difficult in horses, though owners are willing Environmental Allergens: House dust mites and storage mites are the most commonly identified allergens across all speciesMolds are important triggers, especially in humid environmentsVal notes regional differences: Florida has unusual pollens and insects, Texas is drier with mainly pollens, Pacific Northwest sees more mold allergiesSue observes autumn cases in UK when horses start wearing rugs, potentially related to house dust mites, temperature, dampness, or molds Allergen-Specific Immunotherapy Val's Approach: Uses traditional step-up procedure for injection immunotherapyConsults pollen charts (from Greer allergy company, pollen.com, Google searches)Selects major allergens relevant to the horse's region and historyDoesn't include everything that tests positive—focuses on major ...