Squamous Cell Carcinoma of the Horse Eye: Recognition and Treatment Options
Understanding the most common equine ocular neoplasia
When examining a horse with a red or pink ocular mass, many veterinarians immediately suspect squamous cell carcinoma (SCC). While this assumption is often correct, Dr. Rachel Allbaugh, DACVO, emphasizes an important diagnostic caveat: "not all red and pink ocular masses are going to be squamous cell carcinoma."¹
The Diagnostic Challenge
Squamous cell carcinoma represents the most common ocular surface neoplasia in horses, but differential diagnoses must be considered.¹ Other pink, fleshy lesions include mast cell tumors and hemangiosarcomas affecting the palpebral conjunctiva and scleral surface.¹ Dr. Allbaugh recommends that if a lesion "does not look classic by all means feel free to take a biopsy."¹
SCC lesions commonly present as pink fleshy masses affecting the eyelids, third eyelid, or ocular surface.² The lateral and medial limbus are frequent starting points, with lesions potentially growing into the cornea.²
Risk Factors and Breed Predisposition
UV light exposure combined with poor periocular pigmentation creates the primary risk environment.² Genetic predisposition plays a significant role, with Belgians and Haflingers showing particularly high susceptibility.² Appaloosas and Paint horses also demonstrate increased risk.²
Dr. Allbaugh recommends enhanced surveillance for high-risk breeds: "for your Belgians and your Haflingers…you would add a fifth thing…retropulsing the globe because when we talk about third eyelid masses…you’re not going to see the third eyelid usually if it’s nice and normal."² This examination technique involves gentle retropulsion of the eyeball to visualize the third eyelid, allowing early detection of small masses.
Clinical Presentation Patterns
Squamous cell carcinomas present as classic pink, fleshy masses or more subtle erosive ulcerative marginal lesions.² Dr. Allbaugh notes that “erosive ulcerative marginal lesions” can be particularly confusing to practitioners, but these too represent squamous cell carcinoma requiring prompt attention.²
Location significantly impacts prognosis and treatment success. Ocular surface lesions offer better outcomes compared to extensive eyelid involvement.² Surface lesions provide better capacity for complete mass dissection and adjunctive therapy, while extensive eyelid masses have higher recurrence rates.²
Treatment Approach and Options
The treatment goal is destroying the tumor while preserving ocular function and cosmesis.¹² This requires surgical excision of all visible tumor tissue combined with adjunctive therapy to address microscopic disease.¹²
Surgical and Adjunctive Therapies
Treatment begins with mass removal via keratectomy and conjunctivectomy, followed by adjunctive options.¹ Cryotherapy is a well-established adjunctive treatment, applied to the tissue bed post-operatively.¹
Carbon dioxide laser therapy offers another effective option. “Following a keratectomy and conjunctivectomy to remove the mass, we’ll just paint the tissue bed with the carbon dioxide laser,” Dr. Allbaugh describes.¹
Specialized treatments include strontium-90 applications, topical compounded mitomycin C, and photodynamic therapy.¹ The mitomycin C protocol involves four times daily application for one week, followed by a week off, repeating for four total cycles over eight weeks.¹
Photodynamic therapy involves mass removal followed by tissue bed infiltration with a special dye, then activation with a specialized laser.¹
Post-Treatment Considerations
Understanding the difference between granulation tissue and tumor regrowth is crucial for post-operative management. Rapid granulation tissue can occur within weeks of laser treatment.¹ If a “smooth rounded mass” appears after SCC removal and laser treatment, this typically represents granulation tissue rather than tumor regrowth.¹ This responds well to topical steroid therapy such as neopolydex applied three to four times daily.¹
Prognosis and Long-term Management
Recurrence remains a significant concern, with higher rates for eyelid and adnexal lesions compared to ocular surface tumors.¹ The “adnexal ones are the higher recurrence rate,” though ocular surface lesions can also recur or develop in adjacent areas.¹
Long-term management requires vigilant monitoring. Dr. Allbaugh recommends “having these horses reevaluated every six months lifelong” due to predilection for recurrence or new growth development.¹
Prevention Strategies
UV light protection plays a crucial role in preventing recurrences and new tumor formation. Normal fly masks provide approximately 70% UV reduction, while specialized UV-blocking masks achieve 90% protection.¹ These enhanced masks are optimal for proactive prevention.¹
For horses with genetic predisposition or family history of SCC, Dr. Allbaugh recommends implementing UV protection "from the outset before they even form squamous cell carcinoma."¹
Clinical Recommendations
Early detection is paramount for optimal outcomes. Veterinarians should incorporate systematic third eyelid examination into routine exams for high-risk breeds, particularly Belgians and Haflingers.² When SCC is suspected, prompt biopsy provides definitive diagnosis and guides treatment planning.²
The most effective approach combines complete surgical excision with appropriate adjunctive therapy, followed by diligent long-term monitoring and UV protection. The fundamental principles of early detection, complete excision, and prevention-focused aftercare remain the cornerstones of successful SCC management.
This content is based on veterinary continuing education presentations by Dr. Rachel Allbaugh, DACVO. For more advanced equine ophthalmology topics and treatment techniques, visit VetOnIt CE On-Demand.
References
- Advanced Equine Ophthalmology, Dr. Rachel Allbaugh, DACVO, VetOnIt CE
- Introduction to Equine Ophthalmology, Dr. Rachel Allbaugh, DACVO, VetOnIt CE
Tags: Ophthalmology, Equine, Large Animal, Oncology