Genetics and Breed Predisposition

None known, although suspected in IMMK

Disease Forms/Subtypes

Clinical features of IMMK are based on the depth of the corneal lesion, and four distinct levels have been described. These include epithelial, superficial stromal (45% of cases) (Figure 1), midstromal (27% of cases), and endothelial (23% of cases). With endothelial IMMK, there is commonly focal or diffuse corneal edema and pigment deposition on or at the endothelial surface. Unilateral presentation of IMMK is most common (85%).

FIGURE 1 Superficial immune-mediated keratitis in a horse.

History, Chief Complaint

Chronic corneal opacities with mild to moderate cellular infiltrate and vascularization without secondary uveitis or severe ocular discomfort and not associated with infectious agents have been described as IMMK.

Physical Exam Findings

Etiology and Pathophysiology

The possible pathogenesis of IMMK in horses is that the immune system has recognized a self-antigen in the cornea (ie, molecular mimicry) or a foreign protein or organism antigen within the cornea. An underlying infectious agent may be either the inciting or perpetuating cause (or both) in many horses with IMMK. The microorganism may be directly inciting active inflammation or may have induced immunologic cross-reaction with self-antigen in the cornea. Immunologic cross-reaction with self-antigens has been well documented with leptospiral organisms or their DNA in the equine cornea. Further study is warranted to define the pathogenesis of IMMK.