Foggy Vision: Conditions That can Cloud the Equine Cornea

Last Updated on February 25, 2022 by Allison Price

10 Conditions that can cloud the horse’s cornea

The eye of your horse is both incredibly strong and frustratingly fragile. Although it can withstand a lot of things, including exposure to ultraviolet light and dust, and microbes, the eye is still susceptible to injury or disease. A sudden, opaque cloudiness of the cornea is one sign that there’s something wrong.

Your veterinarian can help you. There are many things that can cause cloudy eyes, some benign and others more serious. These issues can affect any horse, whether it is a senior horse with an eye scar from an old corneal ulcer or a Paint with squamous cells carcinoma.

Anatomy of the Eye Surface

Ann Dwyer, DVM of Genesee Valley Equine Clinic in Scottsville, New York, says that having some knowledge about the anatomy and cornea of the eye can help us understand some eye problems.

The cornea, the clear outer covering of the eye, is a metabolic tissue that is made up of many layers. The entire transparent tissue layer is approximately 1 mm in size. You can see:

  • The epithelium is approximately 0.1 mm thick, and less than 15% of cornea’s thickness.
  • The stroma is the most important part of the cornea’s thickness.
  • The membrane of Descemet, which is very thin and elastic;
  • The endothelium is just one layer thick.

Dwyer compares the epithelium’s cellular structure to a stone wall, with thicker stones/cells at the bottom and thinner ones towards the top.

She explains that beneath the epithelial cells, there is a basement membrane. This could be compared to Saran Wrap.

The stroma is made up mainly of long, thin fibers. They are organized in layers with a few cells called keratocytes. Dwyer says that with so few cells it can be difficult to detect a problem. The body requires cells to heal injury.

Foggy Vision

The Descemet’s membrane underneath the stroma is like another piece Saran Wrap, with the endothelium following suit. Dwyer says that “Unfortunately (the endothelium’s) cells don’t reproduce well and thus don’t regenerate very well.” “The epithelium is capable of regenerating completely and it does this continuously, just like your skin. Healthy corneas will replace all their cells within a week. The endothelium is unable to regenerate and their density decreases with age. The endothelium plays a crucial role in maintaining transparency. If it is damaged, the eye can be in serious trouble.

Because these layers, particularly the stroma are relatively dry, the cornea is transparent. Dwyer says that fluid is pulled out of the stroma by the endothelium, which is the innermost layer. Dwyer says that if the endothelium (innermost layer) is old or damaged, the cornea will’steam up’ in the affected region, becoming cloudy.

Illuminating the Issue

If your horse develops a cloudy eye, don’t panic. You can determine if your horse is in pain or comfortable (e.g., signs such as squinting and tearing, swelling of the eyelids, and sudden behavior changes). Next, look at the eye under different lighting to see the problem and communicate it with your veterinarian.

Richard McMullen (DrMedVet), Dipl. “Sometimes, you’ll see things when the horse is outside that won’t be as obvious in a barn or in dim lighting,” McMullen says. Associate professor of equine ophthalmology, Auburn University, Alabama, CertEO (Germany), ACVO, ECVO. This is similar to the way that fingerprints and dust on your windshield at sunrise and sunset are more visible, but disappear when you turn away.

To really see the cloudiness, take a flashlight back to the barn. McMullen says, “Move your flashlight left to right and observe if the opacity changes. Note the location.”

It is important to get your veterinarian involved as soon as possible, no matter what the issue may be. McMullen says, “Even if the issue is minor, a veterinary exam can confirm it, and you will have peace of mind.” “Good news isn’t always a bad thing!” I’d rather see a horse fifteen times for nothing than once a week.

The earlier you begin treatment, the greater your chances of seeing a positive outcome.

Heather Smith Thomas

Causes of Corneal Cloudiness

Corneal disease may be either infectious or immune-mediated. The latter has longer-term complications.

“Immune-mediated corneal disease is the most common condition associated with an opaque, cloudy eye. This is not because horses are more susceptible to them, but because they aren’t quite as painful as other conditions. So the eyes are open so you can see the cloudiness,” Richard McMullen DrMedVet Dipl. Associate professor of equine ophthalmology, Auburn University, Alabama. “By contrast, if you have more pain due to an eye problem, the first thing you will see is squinting or tearing.”

Eye cloudiness can be caused by many conditions. Your veterinarian should make a diagnosis immediately and start treatment. Here are 10 things that can lead to eye cloudiness.

Traumatic injury

McMullen says that edema could be caused by a blunt force, a puncture wound from an acorn (or any other injury to the cornea), or both.

Although edema caused by blunt force may not be painful, it could indicate a more serious problem such as glaucoma, which is dangerously elevated ocular pressure. After glaucoma is diagnosed, the vet may recommend treatment that includes anti-inflammatory and immuno-suppressive (such steroid) topical as well as systemic medications to reduce intraocular inflammation.

McMullen says that sometimes, surgery is required to fix corneal injuries or lacerations. Penetrating injuries require aggressive antimicrobial treatment to prevent secondary infections. If left untreated, it could lead to the need to have the eye removed.

Corneal Fibrosis

Corneal Fibrosis refers to an old scar caused by previous damage, such as injury, surgery, or ulcers. Scarring can occur if there was enough damage to the stroma. Dwyer says that the scarring is a white area and the size of the wounds varies depending on the severity of the injury. “When fiber layers (repair themselves),… those repairs aren’t laid down with the same collagen that nature began with. The repair may be irregular and lose the symmetrical geometry that allows transparency. A spot of cloudiness or blemish is visible on the eye.

As long as the horse has not suffered any eye damage, these blemishes aren’t usually painful. Dwyer says that this would be like an eyeglasses spot with a little grease. It’s blurred, but you can still see around it.

The horse’s vision and performance are not affected by this except if it is extremely large or covers a large area. Dwyer says that scars will shrink over time and become smaller. You’ll see what you end up with after six to twelve months. Sometimes, a large scar shrinks to a small area of fibrosis after it starts.

You should inform your horse’s new owner about the history of his eyes so they can determine if the scarring is something to be concerned about.


Squamous cell carcinoma (SCC ) can cause focal areas of cloudiness or look similar to . McMullen says that although they can be seen initially as a non-transparent or cloudy area on the cornea, they will grow larger and become fleshier over time. “Initially however, you will see a leading edge or cloudiness in your cornea. This is an indicator that something is happening through the cornea.

McMullen says that surgery to remove the tumor followed by treatment to prevent it from regrowing is the best approach to SCC. He says that the prognosis is good for vision preservation, globe maintenance, and non-recurrence if the treatment is done by an experienced ophthalmologist.


Keratomycosis can cause corneal opacification from one of many types of infections. McMullen says that keratomycosis is also known as superficial fungal diseases. “Keratomycosis can often be described as corneal opacities with a punctate-to-linear pattern and lesions placed in an irregularly placed manner.” This almost looks like some sand on top of a glass table with Saran Wrap.

These small spots push the epithelium upwards, creating a bumpy, lumpy appearance rather than smooth surfaces. McMullen says that you would need to magnify the surface under specific lighting in order to see it clearly. They often look like small areas of corneal cloudiness to horse owners.

To diagnose these conditions, the veterinarian may take a cytology sample using a cotton swab. This allows the veterinarian to examine the cells under the microscope. McMullen says that epithelial cells can easily rub off if you have an infectious disease such as fungal disease.

Stromal Abscess

stromal stromal abscess can cause corneal cloudiness and is very serious. The affected horses will have a smaller pupil and keep their eyes closed. Dwyer says that the hallmark sign of this condition is yellow-tan to yellowish in color, with intense pain, inflammation and sometimes blood vessel growth. It is usually an infection but it can be difficult to diagnose. Although it may look like a pimple, the veterinarian can’t drain it.

Deep stromal Abscess may cause severe vision problems and can be costly to treat. Severe cases require corneal transplants.

Dwyer says that about 3/4 of these are fungal. However, this is difficult to prove as you can’t culture them. Dwyer says that a smaller number of these are bacterial and not associated with microorganisms. A new imaging technique, confocal Microscopy (see, can be used to identify the source of stromal abscesses.

Eric Ledbetter (DVM, Dipl.) from Cornell University was the one who invented ACVO. ACVO is a noninvasive imaging technique that’s similar to putting a microscope on an eye. The examiner places the light beam close to the eyes of the horse, and then adjusts the scanning device to view the cornea layer by layer. Dwyer says that this method allows us to quickly determine if the abscess has fungal.


Endotheliitis is a condition in which a damaged or dysfunctional area of the endothelium causes a portion or all of the cornea’s to swell with excess water, resulting in it becoming opaque. Dwyer says that while we don’t know much about this condition, there are no good treatments. He also said that ongoing research is underway. The affected cornea section looks bluish due to watery fluid that has entered the tissues and disrupted the transparency. We can see the cornea failing in any area that is bluish. We try various treatments to fix it.

This condition can sometimes be accompanied by glaucoma. She suggests that veterinarians measure horse’s intraocular pressure in order to rule out this condition. The entire cornea can become cloudy if the endothelium stops working as a whole.

Dwyer says that the condition can lead to fluid from the outer epithelial layers bursting out into small blisters (bullae). This is known as bullous keratopathy. It manifests in a prominent globe with all or most corneas being robin-s-egg blue. Sometimes the eye is larger than others. This can sometimes be a sign of glaucoma. This is a rare condition that has no cure. Although specialists may try to do more drastic things, many horses will need to have their eyes removed.

Mineralization of the Cornea

Mineralization of the cornea refers to a condition where mineral (mostly calcium) is laid down. It looks almost like a residue from hard water in a glass of drinking water or barnacles on an oyster.

Dwyer says that horses may experience it when they have ERU ( horse recurrent uveitis), which is inflammation of a cellular layer of their eyes that contains blood vessels, ciliary bodies, and choroid. It could be due to the fact that horses receive a lot of steroids treatment.

Mineralization typically occurs in the epithelium, subepithelium, and upper stroma. Dwyer says that it is easy to diagnose, as a sample of cytology will show that the corneal tissue is not smooth but rather gritty. It is easy to see tiny crystals when you examine it under a microscope.

Although this problem is often recurring and chronic, it can usually be managed. It is important to get rid of as much mineral as you can. Dwyer says, “If the horse is able to live with the little bits of mineral that are left,”

To remove the mineral she uses a new method called diamond burr, which involves using a Dremel-tool-like instrument that gently “sands” the ocular surface in order to remove any loose or rough areas. She says that although it was originally developed for human ophthalmology (human ophthalmology), we have found it very useful in animals.

For long-term control she says she applies a topical preparation of 1% ethylenediaminetetraacetic acid. She says that it chelates the mineral and is well-tolerated in the eye. To help manage this condition, we sometimes make an ointment of it and apply it twice daily to the eyes.

Deposits in the Cornea

Older horses may develop deposits on the cornea’s inside when they have a form of posterior (inside the eye) uveitis. McMullen says that deposits can cause the cornea to become cloudy and incapable of keeping fluid out. This is a serious problem that can be chronic, but it presents quickly. These areas of edema are triangular-shaped and usually appear in the lower eye. They then move up to the top. They may also develop at the top of your cornea, but they are more common in the lower half.


Keratopathy is a condition that causes corneal lesions to not show signs of inflammation. McMullen says that these may also be accompanied by corneal opacities. McMullen says that viral keratopathies and keratitis are often diagnosed. However, herpesvirus corneal diseases may be overdiagnosed. These lesions could also be a sign of immune-mediated Keratitis.

Linear keratopathy refers to a single, narrow line of cloudiness that is approximately 1 to 2 millimeters in width and crosses the cornea as an arc. Dwyer says that it looks almost like a railroad track when you examine the picture closely. If you only see one line, it is likely that the Descemet’s layer was stretched because of trauma to the eye. This is similar in nature to Saran Wrap’s permanent stretching if pulled from both ends.

High intraocular pressure can cause multiple branching lines of cloudiness. Dwyer says, “If I see a horse that has only one line of opaque Keratopathy, I don’t worry too much. However, I do keep track of it and photograph it. And I measure the eye pressure.” Multiple branching lines can be a concern because they almost always accompany the presence of glaucoma.

Immune-Mediated Keratitis

It is an under-understood condition, but it seems to be increasing in prevalence. Dwyer says, “This is a grab-bag condition.” We know it involves the infiltration of cells that control the immune response in your cornea. Although there aren’t many studies or papers about it, we do see it every day. Immune-mediated keratitis, (IMMK), is a range of conditions I like to call different degrees of cloudy windshield.

She continues, “Sometimes horses have little dots in their corneas that look like a Chinese Checkers board.” Sometimes we see horses that have blotchy patches or cloudiness. There are many patients I have seen over the years with blotchiness all over the cornea. The infiltrating cells responsible for the opacity continue to move around.”

A diagnostic imaging of the affected corneas shows that there are no infectious elements involved. Instead, cells from the immune systems are. Dwyer says that it is possible for these cells to migrate into the stroma as a reaction to foreign antigens. Dwyer says that some horses with immune-mediated problems are known to have had an eye ulcer treated in the past. Although the original problem was simple and resolved without complications, we now see that there is an immune-mediated problem.

Many cases can be treated with immune-mediating drugs like cyclosporine, says She. The North Carolina State University research team developed an episcleral Cyclosporine surgical implant, which they have used with some success in treating certain IMMK cases.

Take-Home Message

Ocular cloudiness can be difficult to diagnose. Although it might seem uncomfortable, some horse owners may be reluctant to have their eyes examined. However, an exam can save you money than undergoing aggressive medical or surgical treatment due to the progressive damage that has occurred over time. It is important to determine if your horse has a serious eye condition and then take the necessary steps to correct it.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!