Keratoconjunctivitis Sicca in Horses

Keratoconjunctivitis sicca, often called KCS or dry eye, is a disorder where the eye does not produce enough of the watery part of the tear film. Tears do much more than make the eye look moist. They lubricate the cornea, wash away debris, support healing, and help protect against infection. When tear production drops, the cornea and conjunctiva become inflamed and vulnerable.

In horses, KCS is considered rare, but it is still important because equine eyes are already prone to trauma and surface disease. A horse with poor tear production may develop a dull or dry-looking cornea, stringy mucus, recurrent conjunctivitis, chronic keratitis, or corneal ulceration. Some horses have obvious low tear production on testing, while others may have a more qualitative tear film problem where the eye still looks dry despite tear values that are not dramatically low.

For pet parents, the main takeaway is that a persistently irritated eye is never something to watch for days on end. Eye discomfort can worsen quickly in horses, and early treatment often gives your vet more options to protect comfort and vision.

KCS develops when the tear film is not adequate to keep the eye surface healthy. In horses, this may happen because the lacrimal gland is not producing enough tears, because the nerves that stimulate tear production are not working normally, or because the tear film quality is poor even if measured tear volume is not dramatically low.

Reported and suspected contributors in horses include facial nerve (cranial nerve VII) dysfunction, trauma around the eye, neurologic disease, chronic ocular surface inflammation, and some cases of chronic ulcerative keratitis or keratoconjunctivitis. Equine ophthalmology references also note that dry eye can occur secondary to conditions such as guttural pouch disease, stylohyoid osteopathy, West Nile virus, or direct trauma affecting facial nerve function.

In some horses, KCS may be part of a broader inflammatory eye problem rather than a stand-alone disease. That is one reason your vet will usually look carefully for corneal ulcers, recurrent uveitis, foreign material, eyelid problems, blocked tear drainage, and infectious causes before settling on a treatment plan. The exact cause is not always obvious on day one, and follow-up exams are often part of good care.

Diagnosis starts with a full eye exam. Your vet will look at both eyes, assess comfort, check the corneal surface, and look for clues that point toward ulceration, uveitis, trauma, eyelid disease, or infection. Because many equine eye problems can look alike early on, this step matters as much as the tear test itself.

A key test is the Schirmer tear test (STT), which measures aqueous tear production. In horses, ophthalmology references note that KCS is rare, but the STT is especially useful in chronic ulcerative keratitis, chronic conjunctivitis, or eyes that appear dry. The strip should be placed before eye medications or stains are applied so the result is meaningful.

Your vet will often also use fluorescein stain to check for corneal ulcers, because ulceration is common in horses and can be sight-threatening. Depending on the exam findings, additional steps may include tear film assessment, corneal cytology or culture, nasolacrimal duct evaluation, neurologic assessment, or referral to a veterinary ophthalmologist. If the case is chronic, painful, or not responding as expected, referral can be very helpful.

Not every case of KCS can be prevented, especially when nerve dysfunction or chronic inflammatory eye disease is involved. Still, good eye care can lower the risk of surface damage and help your vet catch problems earlier. Keep your horse's environment as low-dust as practical, reduce exposure to irritating bedding or hay particles, and use well-fitted fly protection when recommended.

Prompt attention to any eye abnormality is one of the most important preventive steps. Horses are prone to trauma, ulcers, and recurrent inflammation, and those conditions can worsen the tear film and corneal surface. If your horse has repeated discharge, squinting, or a dull-looking eye, schedule an exam rather than trying over-the-counter products on your own.

If your horse already has a history of eye disease, ask your vet whether routine rechecks, shade management, or a medication plan during flare-ups makes sense. Early treatment does not guarantee prevention of every complication, but it often helps preserve comfort and gives your vet more options before the cornea becomes badly damaged.