Episode 36 - Hives, Lesions and Lumps; Urticaria in Horses (Valerie Fadok)

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 granulomas Individual lesions wax and wane, even if the horse has hives every day Lesions 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 bites Val 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 Culicoides These cases often resolve after the first year Sue confirms similar patterns in the UK with Culicoides Immunological vs Non-Immunological Reactions The Role of Mast Cells: Urticaria involves mast cells in the skin Immunological urticaria occurs when allergens bind to IgE on mast cells, triggering the reaction Non-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 touching Cold-induced urticaria Heat-induced urticaria Exercise-induced urticaria Some 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 transfusions Val's approach: Don't do an intense workup immediately Treat with antihistamines (Val prefers hydroxyzine) for a few months to let mast cells settle If it recurs after stopping medication, then investigate further Sue 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 investigation Both emphasize the value of owners who keep detailed calendars noting when hives appear 50% of urticaria in people remains idiopathic—same often true for horses Competition horses present particular challenges due to medication restrictions Investigation and Testing Seasonal Cases: For seasonal urticaria, Val recommends intradermal or serum allergy testing Horses with urticaria respond well to allergen immunotherapy compared to other species Most horse owners are comfortable giving injections Non-Seasonal Cases: Consider dietary factors and whether feed changes throughout the year Horse owners are surprisingly open to food trials Val 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 species Molds are important triggers, especially in humid environments Val notes regional differences: Florida has unusual pollens and insects, Texas is drier with mainly pollens, Pacific Northwest sees more mold allergies Sue 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 immunotherapy Consults pollen charts (from Greer allergy company, pollen.com, Google searches) Selects major allergens relevant to the horse's region and history Doesn't include everything that tests positive—focuses on major allergens that fit the history Builds up from 2-3 injections per week to maintenance (once weekly to once monthly, depending on the horse) Customization is Key: Frequency depends on individual horse response Traveling horses present challenges (Val shares experience with a Budweiser Clydesdale that traveled nationwide) For traveling horses, select major allergens common across regions (cedar trees, ragweed, common grasses) Seasonal Management: Val prefers to wait until the season is over before starting immunotherapy Aims for at least 6 months of treatment before the next allergy season Backs off frequency during off-season (e.g., monthly injections) Increases frequency during active season (weekly if needed) Never stops completely during off-season to avoid starting over Sometimes "less is more"—half a milliliter every two weeks may work better than full dose every four weeks Success with Horses: Horses respond particularly well to immunotherapy compared to other species Dedicated horse owners are excellent at fine-tuning treatment based on their horse's patterns Flexibility is key: can adjust dose and frequency as needed Treatment Options Antihistamines: Val's preference: hydroxyzine (though colleague Stephen White prefers doxepin) First-line treatment when possible Corticosteroids: Most US equine vets prefer dexamethasone (less expensive) Val prefers prednisolone (learned from equine mentor at Texas A&M) Alternate-day prednisolone is useful approach Long-term dexamethasone is concerning—if needed, aim for every 3-4 days For competition horses, medication restrictions are a major consideration Off-Label Options: Apoquel has helped some difficult cases when antihistamines and steroids aren't sufficient Very expensive and off-license (requires justification) Not on horse competition drug registers (as of recording) Can be useful short-term, such as before shows Not a long-term solution Long-Term Outlook Realistic Expectations: Flares will likely be part of life even with successful immunotherapy Stress can trigger urticarial eruptions (similar to people) Hope is to avoid year-round medication, but some horses require continuous treatment for comfort Some owners relocate horses from high-allergen areas (e.g., Florida/Southeast) to northern states Education Needs: Val sees room for growth in equine veterinary use of immunotherapy Cautions against testing too early (not after just one outbreak) Healthy animals can make IgE without it being clinically relevant Need for education on proper use of testing and setting realistic expectations Horse Owner Compliance Both Val and Sue emphasize how remarkably compliant and dedicated horse owners are: Horse owners will food trial willingly Will shampoo horses twice weekly in freezing weather Keep detailed records and calendars Are observant about patterns and triggers Are open to considering food allergies Follow through consistently with immunotherapy protocols The bond between pleasure horse owners and their horses makes treatment particularly rewarding Key Takeaways Palpation matters - Soft lesions that wax and wane suggest urticaria over other nodular diseases Don't over-investigate acute cases - Wait 8-12 weeks or for recurrence before extensive workup History is everything - Detailed owner observations are invaluable for diagnosis Horses respond well to immunotherapy - Better success rates than many other species Flexibility in treatment - Adjust immunotherapy frequency and dose based on individual response 50% remain idiopathic - Many cases resolve without identifying the cause Horse owners are exceptional - Compliance and dedication make management possible