Equine Corneal Ulceration

Take Home Message  

Equine corneal ulceration is very common in horses and is a sight threatening disease requiring  early clinical diagnosis, laboratory confirmation, and appropriate medical and surgical therapy.  

Discussion  

Ulcers can range from simple, superficial breaks (Fig. 1) or abrasions in the corneal epithelium,  to melting of the corneal collagen (Figs. 2, 3), to full-thickness corneal perforations with iris  prolapse (Fig. 4).  
 

The prominent eye of the horse may predispose to traumatic corneal injury.  

Both bacterial and fungal keratitis in horses may present with a mild, early clinical course, but  require prompt therapy if serious ocular complications are to be avoided.  

Corneal ulcers in horses should be aggressively treated no matter how small or superficial they  may be. Corneal infection and iridocyclitis are always major concerns for even the slightest  corneal ulcerations. Iridocyclitis or uveitis is present in all types of corneal ulcers and must be  treated in order to preserve vision.  

Globe rupture, phthisis bulbi, and blindness are possible sequelae to corneal ulceration in horses.  Proteinases in the Tear Film  

Tear film proteinases normally provide a surveillance and repair function to detect and remove  damaged cells or collagen caused by regular wear and tear of the cornea. These enzymes exist in  a balance with inhibitory factors to prevent excessive degradation of normal tissue.  

Two major families of proteinases that may affect the cornea include the matrix  metalloproteinases (MMP) and the serine proteinases. MMPs predominate in the horse.  

Bacterial and fungal pathogens induce corneal epithelial cells, corneal stromal fibroblasts, and  leukocytes (PMN) in the tear film to upregulate cytokines (IL-1, IL-6 and IL-8) that induce  MMP production and elicit inflammatory and degradative processes.  

Proteinases that may contribute to corneal ulceration in the early stages of infection could be of  bacterial or corneal cell origin. In the later stages as PMNs accumulate, PMN-derived  proteinases predominate as the main factor in corneal tissue destruction.  

In pathologic processes such as ulcerative keratitis, excessive levels of these proteinases can lead  to rapid degeneration of collagen and other components of the stroma, potentially inducing  keratomalacia or corneal "melting" (Figs. 2, 3).  

Corneal Sensitivity in Foals and Adult Horses  

Corneal sensation is important for corneal healing. The cornea of the adult horse is very sensitive  compared to other animals.  

Corneal touch threshold analysis revealed the corneas of sick or hospitalized foals were  significantly less sensitive than those of adult horses or normal foals. The incidence of corneal  disease is also much higher in sick neonates than in healthy foals of similar age.  

Ulcerative keratitis in the equine neonate often differs from adult horses in clinical signs and  disease course. Foals may not show characteristic epiphora, blepharospasm, or conjunctivitis,  and the ulcers may be missed without daily fluorescein staining. This decreased sensitivity may  partially explain the lack of clinical signs often seen in sick neonates with corneal ulcers.  [...]