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The vet will look at the eye and socket for any kind of foreign bodies (such as a foxtail). The vet may also apply a special dye called fluorescein which reveals any damage to the surface of the eye. To see the eye in greater detail, the vet will use an ophthalmoscope which has a series of lens to magnify both the surface and the deeper chamber of the eye. To perform a thorough eye exam, the vet may need to dilate your horse's eyes. Once your vet has diagnosed your horse, a medication (like an antibiotic) will probably be prescribed. This will probably be an in the form of drops or ointment. Drops may be helpful for several days because they can relieve pain, but your horse will need to be in a shaded area or have a fly mask since drops can dilate the pupils. Understand how to correctly apply these medications to your horse's eye:  Apply an ointment 2 to 6 times a day, as recommended by the vet. Run the ointment over the top of the eyelid to make sure it cleans the whole eye. Apply eye drops (like atropine) by pulling up on the skin above the eye to widen the opening. Prepare the ointment by squeezing a small amount to the end of the tube. Approach the horse's eye from behind so that he doesn't see it coming towards him. Use a finger and thumb on one hand to open the eyelid and spot the drop against the inside of the lower lid. You can also drop it onto the surface of the cornea. Let the horse close his eye, which will naturally spread the ointment over the surface. If the vet diagnoses your horse with a corneal ulceration, the vet will probably numb the surface of the eye using local anesthetic drops. The vet will carefully rub the surface with a sterile cotton swab to remove any dead tissue that's stuck to the eye. Removing the tissue will speed up healing. The eye will be rinsed with sterile saline and the vet will probably prescribe antibiotic eye drops. You'll need to apply them 2 to 6 times a day (depending on the medication) for around 7 to 10 days. A corneal ulceration is often caused by grit or a branch rubbing against the surface of the eye. This creates a divot on the surface of the eye that the vet can see during the examination. If not treated, corneal ulcerations may wear down into the eye. The vet may diagnose your horse with ERU or "Moon Blindness" which causes the pupil to spasm and contract. While there's no cure for ERU, treatment will focus on handling the symptoms. The vet may prescribe antibiotic drops to put in once or twice a day to dilate the pupil and stop the painful spasms. You'll probably want to keep your horse in a shaded area and avoid riding him while the pupil is dilated, since this can interfere with his vision.  Steroid drops may also be prescribed to reduce inflammation. Pain medication may also be prescribed. ERU is an inflammation in the uvea within the eye, which is made up of the iris, the fibers that hold the lens in place (ciliary body), and the lining of the eye. It's a painful condition which prevents the eye from functioning properly. This keeps the horse from focusing and can make it difficult for him to see. The vet will treat this bacterial infection by keeping the eye clean. Disposable cotton pads are soaked in saline and wiped over the eye. The vet will prescribe an antibiotic ointment to be applied once or twice a day for 7 to 10 days. Conjunctivitis is a bacterial infection of the surface parts of the eye. This can be hot, itchy, and painful for the horse, creating a lot of discharge from the eye. This discharge can gum the eyes shut, making it more attractive for flies to lay their eggs there. To prevent fly infestation, it's important to keep the eyes clean.

Summary:
Get your horse examined by a veterinarian. Learn how to apply common types of medications. Treat corneal ulceration. Treat Equine Recurrent Uveitis (ERU). Treat conjunctivitis.