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

1. Palpation matters - Soft lesions that wax and wane suggest urticaria over other nodular diseases
2. Don't over-investigate acute cases - Wait 8-12 weeks or for recurrence before extensive workup
3. History is everything - Detailed owner observations are invaluable for diagnosis
4. Horses respond well to immunotherapy - Better success rates than many other species
5. Flexibility in treatment - Adjust immunotherapy frequency and dose based on individual response
6. 50% remain idiopathic - Many cases resolve without identifying the cause
7. Horse owners are exceptional - Compliance and dedication make management possible