Equine Corneal Disease
The equine cornea is a common site for injury and disease, and as such, is
cause for many calls for the equine practitioner.
Ulceration is probably the most common of all corneal afflictions in the horse.
The cause of corneal ulceration is nearly always trauma, although a specific
episode of injury is rarely noted. Nevertheless, one should assume that trauma
is the cause unless there are compelling signs otherwise. A routine superficial
ulcer will often heal within a week. Appropriate therapy includes a broad
spectrum topical antibiotic, such as neomycin-polymyxin-bacitracin, along with
an oral non-steroidal antiinflammatory. Iridocycloplegia (dilating the pupil and
paralyzing the ciliary body) should be done cautiously. Atropine has long been
used to achieve iridocycloplegia. However, atropine can have effects which last
weeks, and rarely iridocycloplegia will cause an increase in intraocular
pressure. A simple way of avoiding this situation would be to use tropicamide
instead. Tropicamide has the same actions as atropine except that it has a much
shorter half life. If intraocular pressure were to rise as a result of
iridocycloplegia, the effects of tropicamide would wear off much sooner than
atropine.
An important part of examining an ulcerated eye is determining whether pupillary
light responses are present. Even if the pupil in the affected eye is miotic or
not visible due to opacities, a consensual response to the opposite eye can be
evaluated. One shines a strong light into the diseased eye, and an assistant
observes to see if the opposite pupil constricts. Absence of this response is a
grave sign and warrants a poor prognosis for restoration of vision. Lack of a
consensual light response implies that there is disease of the deeper ocular
structures, such as the retina and optic nerve. These can be further evaluated
by ultrasound.
An ulcer which has not healed within a week should prompt a more critical
examination of the eye, and perhaps referral. Depth of the ulcer, change in
color or consistency of the ulcer, and intraocular changes should be
ascertained. Signs of bacterial infection include pain (manifested as
blepharospasm), severe uveitis, and a yellow or gray color to the ulcer. Often
the pain is out of proportion to what one might expect from the appearance of
the cornea. One might also notice that the ulcer bed appears soft; this is
likely a prelude to "melting", which is characteristic of a Pseudomonas
infection. A fungal keratitis will appear very similar to a bacterial keratitis,
though the specific features of the ulcer appearance may vary. Fungal keratitis
is less common than bacterial keratitis, and is often associated with previous
topical steroid use. Either type of infection can cause rapid loss of vision or
the globe. Immediate aggressive diagnostics and treatment are critical. Corneal
scraping for cytology and culture, and possibly corneal biopsy, are required for
precise diagnosis. Topical treatment may need to be done 6-8 times per day, and
a subpalpebral lavage system may need to be placed to facilitate treatment in a
painful. Frequent recheck examinations with a slit lamp biomicroscope are also
important. If medical therapy fails, the infected cornea may need to be removed
by surgical keratectomy followed by placement of a conjunctival flap.
Other causes for failure to heal include viral infection, undetected foreign
body, underlying ocular disease, and epithelial dystrophy. While rarely
reported, equine herpesvirus has been documented to cause keratitis. Slit lamp
biomicroscopy will assist with detection of small foreign bodies, or possibly
even ectopic cilia (this is rare in the horse but has been documented). Nearly
any intraocular disease, such as uveitis and glaucoma, has detrimental effects
on the cornea and can cause an ulcer to become refractory. Lastly, epithelial
dystrophy will cause an ulcer to be refractory. This type of ulcer often has a
rim of nonadherent epithelium at its periphery, and the eye may be relatively
comfortable. Electron microscopy has shown this condition to be due to basement
membrane abnormalities in humans and dogs; the same probably holds true in other
species. This condition requires surgical intervention. A multiple punctate or
grid keratotomy can be done as a standing procedure, and is about 80%
successful. The success of surgical keratectomy approaches 100%, and is used in
the event that simpler procedures fail.
Corneal neoplasia is known to occur in the horse. While a number of neoplasms of
the cornea and conjunctiva have been documented, squamous cell carcinoma is by
far most common. Appaloosas and Draft horses are predisposed. The appearance is
that of a raised, pink, highly vascular mass involving the eyelids, conjunctiva,
third eyelid, cornea, or all of these structures. This type of neoplasm is quite
amenable to surgical excision followed by irradiation.
While Onchocerca is know to have corneal manifestations, this appears to be
uncommon in our area.