Immune-mediated keratitis (IMMK) is a common non-infectious eye disease in horses, resulting in chronic corneal opacity without corneal ulceration or significant uveitis. There are four specific types of IMMK, each characterized by the depth of the corneal lesion. These include epithelial, superficial stromal, midstromal, and endothelial. Superficial stromal keratitis is the most common type of IMMK affecting horses.
- Epithelial IMMK: Considered to be the most superficial form of IMMK, and usually occurs at the point where the horse's eyelids touch, resulting in eyelid inflammation (blepharitis), conjunctivitis (pinkeye), and swelling of the conjunctiva (chemosis).
- Chronic superficial stromal IMMK: Usually found under the horse's upper lid and is characterized by prominent subepithelial arborizing vascularity from the limbus. he conjunctiva is hyperemic (i.e., showing increased blood flow).
- Chronic deep stromal IMMK: This form of IMMK occurs when the horse's blood vessels rupture. This releases plasma into the stroma, causing the cornea to present as a greenish-yellow color.
- Endotheliitis: Considered to be the most dangerous and damaging type of IMMK in horses. It affects the innermost layer of cells in the horse's cornea (endothelium). Affected horses often develop a vertical corneal edema which is a deep blue color. Endotheliitis can cause significant pain in both eyes and does not respond to medication.
IMMK is caused by a dramatic corneal immune response to a foreign protein, microbial antigen or a self-antigen. IMMK usually occurs unilaterally, affecting just one eye. It is important to distinguish IMMK from other causes of keratitis, particularly infectious keratitis which can be of bacterial or fungal origin.