Immune-mediated keratitis (IMMK) is a group of non-infectious corneal diseases that occur in horses. IMMK is characterized by chronic corneal opacity without corneal ulceration or significant uveitis. It is essential to distinguish IMMK from other causes of keratitis, especially involving infectious agents. Most cases of IMMK manifest in only one eye, although both eyes can be affected. The classic clinical findings include a lymphocytic-plasmacytic corneal cellular infiltrate, often accompanied by corneal neovascularization, edema, and fibrosis.

There are distinct subtypes of IMMK, characterized based on the location(i.e., depth) of the corneal lesion and type of cellular infiltrate. The United States has divided these up into five subtypes and the United Kingdom there are four subtypes.

• Superficial stromal IMMK: One of the most common subtypes in the USA (45% of cases), which manifests as a corneal opacity in the ventral or central aspect of the cornea. It is associated with superficial, branching corneal vascularization (growth of blood vessels in the cornea) and associated mild corneal edema.
• Mid-stromal IMMK: This is the second most common type of IMMK seen in horses in the USA (27% of cases). It is clinically similar to superficial IMMK except that the location of the vascularization and cellular infiltrate is in the center of the corneal stroma. The vascularization is usually less branching and more straight than what is seen with superficial stromal IMMK
• Endothelial IMMK: This is the third most common type of IMMK seen in horses in the USA (23% of cases). It is characterized by a chronic, slowly progressive, nonpainful, diffuse ventro-lateral or ventral full-thickness area of corneal edema. Horses will have normal intraocular pressure and no aqueous flare or miosis.
• Epithelial IMMK: This is one of the least common types of IMMK seen in horses living in the USA. It is characterized by multifocal punctate opacities in the ventral and ventral-paracentral corneal epithelium. There is usually no corneal vascularization (growth of blood vessels in the cornea) and horses typically show no signs of pain or discomfort.
• Eosinophilic keratitis (EK): Horses with this form of IMMK will present with moderate discomfort or eye pain. The clinical appearance of this condition is either a unilateral or bilateral white plaque on the surface of the cornea with surrounding corneal edema or a superficial stromal, perilimbal yellow infiltrate. The most common location is the cornea located under the third eyelid, followed by the ventral-medial and ventral-lateral cornea. Typically it involves multiple horses at a stable and occurs in the summer months.