🌞 Vet’s 2025 Guide to Equine Photosensitization

By Dr Duncan Houston BVSc

1. What Is Photosensitization?

Photosensitization, also known as photodermatitis, is an inflammatory skin reaction in horses when specific photodynamic compounds (from plants, chemicals, or liver metabolites) accumulate and react with sunlight—leading to severe sunburn-like lesions, especially on unpigmented (“white”) skin :contentReference[oaicite:3]{index=3}.

2. Types & Causes

• Primary (Type I): Ingestion or contact with phototoxic plants—e.g., St. John’s Wort, buckwheat, perennial ryegrass, clover, or exposure to certain chemicals & drugs (e.g., tetracycline, fly sprays) :contentReference[oaicite:4]{index=4}.
• Secondary (Type III / Hepatogenous): Caused by liver dysfunction. Phylloerythrin, a chlorophyll breakdown product, accumulates and reacts to sunlight :contentReference[oaicite:5]{index=5}.
• Type II & IV: Rare metabolic or undetermined origins :contentReference[oaicite:6]{index=6}.

3. Why It Matters

Affected horses experience pain, ulceration, infection risk, and possible long-term complications like squamous cell carcinoma. Liver-associated cases often indicate serious underlying disease :contentReference[oaicite:7]{index=7}.

4. Recognising Clinical Signs

• Redness, swelling, blistering, crusting on white skin areas—common on face, muzzle, pasterns :contentReference[oaicite:8]{index=8}.
• Hair loss, ulceration, and intense pain or pruritus may follow :contentReference[oaicite:9]{index=9}.
• Liver cases may show anorexia, jaundice, depression, and weight loss :contentReference[oaicite:10]{index=10}.

5. Diagnosis Process

• History: recent diet changes, toxic plant exposure, liver disease risk.
• Physical exam & dermatologic evaluation.
• Bloodwork: CBC, liver enzymes, bile acids, phylloerythrin levels :contentReference[oaicite:11]{index=11}.
• Skin biopsy to exclude other dermatoses :contentReference[oaicite:12]{index=12}.
• Liver ultrasound or biopsy if hepatogenous disease suspected :contentReference[oaicite:13]{index=13}.

6. Treatment & Supportive Care

6.1 Remove Phototoxins & Sunlight

• Eliminate toxic plants from pasture/feed; discontinue suspicious medications :contentReference[oaicite:14]{index=14}.
• Provide shade, stable with turnout at night, UV-blocking masks/sheets :contentReference[oaicite:15]{index=15}.

6.2 Skin Management

• Clean lesions gently; apply antibacterial or corticosteroid creams :contentReference[oaicite:16]{index=16}.
• Use fly control and prevent flystrike.
• Bandaging may be needed for ulcerated skin.

6.3 Systemic Therapies

• Short-term corticosteroids to reduce inflammation :contentReference[oaicite:17]{index=17}.
• NSAIDs for comfort.
• Antibiotics if secondary infection present.
• Liver support: antioxidants, milk thistle—though evidence varies :contentReference[oaicite:18]{index=18}.

7. Prognosis & Recovery

Primary photosensitization often resolves with treatment and sun avoidance, with good healing :contentReference[oaicite:19]{index=19}. Secondary/haptogenous cases often have a guarded prognosis as liver disease may be chronic :contentReference[oaicite:20]{index=20}.

8. Prevention Strategies

• Pasture management: remove toxic plants, rotate grazing, monitor sowing feed quality :contentReference[oaicite:21]{index=21}.
• Avoid medications known for phototoxicity.
• Genetic or liver-screen horses with elevated liver enzymes.
• Use UV protection: fly sheets, hats, sunscreen for white or scant-haired areas :contentReference[oaicite:22]{index=22}.

9. Case Study: “Star” the Face-Sensitive Mare

Star, a grey mare with large white blaze, developed muzzle ulceration after ingesting St. John’s Wort hay. She was stabled with a UV mask, given topical steroids and NSAIDs. Lesions resolved in 3 weeks with no scarring and improved pasture management prevented recurrence.

10. Ask A Vet Support 🩺

At Ask A Vet, we guide owners through diagnosis, feed analysis, liver testing, topical protocols, UV protection strategies, and follow-up monitoring. For horses with liver-related photosensitivity, we support ongoing liver-care plans and comfort management. Download the Ask A Vet app today for personalized telehealth support on equine skin health.

11. Quick Reference Table

12. FAQs

❓ Can sunscreen protect horses?

Yes—broad-spectrum (SPF 30–55) safe for equine use; reapply regularly :contentReference[oaicite:23]{index=23}.

❓ How long before management shows results?

Mild cases may heal in 1–2 weeks; severe ulcerative lesions may need 4+ weeks.

❓ Will liver cases resolve?

Depends on the liver disease—some improve, others need lifelong management.

13. Final Thoughts

Equine photosensitization is painful but manageable. Prompt removal of causes, UV protection, skin care, and liver support are critical. For guidance on identifying toxic feed, creating safe turnout environments, and ongoing treatment, trust Ask A Vet’s expert care. Download our app now to keep your horse safe and comfortable in 2025 and beyond! ❤️