Equine Sarcoid Arising From the Medial Caruncle With Invasion of the Third Eyelid in a Quarter Horse Gelding.

Equine sarcoid, a type of skin tumor linked to bovine papillomavirus, was identified originating from the medial caruncle of a Quarter Horse and invading its third eyelid, with surgical excision and cryotherapy used for treatment.

Background and Objective

• The study focuses on an unusual case of an equine sarcoid, a common skin tumor in horses.
• This sarcoid originated specifically from the medial caruncle, a small anatomical structure at the inner corner of the eye.
• Importantly, the tumor invaded the third eyelid, a protective structure within the horse’s eye.
• The objective was to describe clinical presentation, histologic features, and the treatment approach.

Case Description and Methods

• The subject was a 20-year-old Quarter Horse gelding presenting with a mass on the medial caruncle.
• An incisional biopsy was initially performed for diagnostic purposes.
• Histopathology of the biopsy suggested the tumor might be a sarcoid or another spindle cell neoplasm.
• Following the biopsy, the mass grew rapidly and spread to involve the third eyelid and adjacent conjunctival tissue.
• Definitive treatment involved the surgical removal of the entire third eyelid and medial caruncle along with the affected conjunctiva, combined with adjunctive cryotherapy (freezing treatment) to reduce recurrence risk.

Histopathological Findings

• Microscopic analysis revealed a densely packed population of neoplastic spindle cells arranged in interlacing bundles.
• These cells were embedded within a collagen-rich stroma, a typical feature of sarcoids.
• The overlying epithelium was hyperplastic (thickened) and extended downward into the tumor as thin rete ridges, a hallmark of sarcoid architecture.
• More than 90% of tumor cells showed strong molecular signals for bovine papillomavirus (BPV) types 1 and 2.
• This viral association supports the diagnosis of a sarcoid, as BPV infection is known to play a critical role in sarcoid pathogenesis.

Significance and Clinical Implications

• This case is the first reported instance of an equine sarcoid originating from the medial caruncle and spreading into the third eyelid.
• The findings suggest that when evaluating periocular masses in horses, veterinarians should include sarcoids in their differential diagnoses.
• The rapid growth of the sarcoid following incisional biopsy is consistent with known behavior of cutaneous sarcoids, implying caution in biopsy and treatment planning.
• Effective management combined surgical excision with cryotherapy to prevent recurrence, highlighting a potential treatment protocol for similar cases.

Conclusion

• This report expands the knowledge of sarcoid behavior in horses by documenting an unusual anatomical location and invasive growth pattern.
• The combined surgical and cryotherapeutic approach was successful in addressing the tumor.
• Veterinarians should be aware of sarcoids as a possible cause of masses involving the medial caruncle and third eyelid, especially given the potential for rapid growth after biopsy.