Eye infections in horses are common but require prompt diagnosis and treatment to prevent permanent injury or blindness. Eye infections are caused by various bacterial, viral, fungal, and allergen irritants. The two most common infections are conjunctivitis and equine recurrent uveitis, which cause uncomfortable symptoms in the eye. A vet can diagnose the cause of your horse's eye infection through a physical examination, culturing, and sometimes an MRI or CT scan. If you catch and treat an eye infection early, the prognosis is typically optimistic but dependent on the stage of the infection and its cause.
What Are Eye Infections?
An eye infection is a disease of the eye caused by harmful bacteria, fungi, or viruses. Horses can develop eye infections like conjunctivitis, and equine recurrent uveitis (ERU), sometimes caused by injury to the eye or exposure to allergens like pollen and dust. Conjunctivitis causes the mucus membrane around the eye to swell and become red. ERU results in recurrent episodes of eye inflammation and is the most common cause of blindness in horses.
Symptoms of Eye Infections in Horses
There are a variety of symptoms that point to eye infections in horses. An untreated eye infection is very painful for your horse and can lead to complications. Visit your vet right away if you suspect something is wrong with your horse's eye.
Eyelid swelling can happen for a variety of reasons. In addition to blunt head injury and allergies, an eye infection is a likely cause of swollen eyelids in a horse. Visit your vet if you're not sure what's causing your horse's eye to swell.
While some eye discharge is a normal function of a healthy eye, the type of discharge your horse is producing can indicate infection. Some tearing shouldn't be cause for worry, but if the discharge is thick, yellow or white, and pussy, an infection is probably to blame.
If your horse's eye is tearing excessively, it may be a sign of ERU. Again, some tearing is normal, but pay attention to the volume of your horse's tears.
Hazy Appearance of the Eye
Early in infection, your horse's eye may appear hazy or cloudy. A whiteish film over the eye may indicate ERU. There are many reasons a horse's eye may look cloudy, so pay a visit to your vet for a definitive diagnosis.
Redness and Inflammation
Redness and inflammation is the most common indicator of any eye infection, and usually points to conjunctivitis or ERU. It can also indicate eye injury or allergies.
Causes of Eye Infections
There are several causes of eye infections in horses of varying severity. Visit your vet for a definitive diagnosis and a subsequent treatment plan.
- Bacterial and viral infection: Bacteria and viruses can get into your horses eye in a number of ways. Bacteria can enter the eye through insects and dirt, and viruses can be transported similarly.
- Fungus: It's normal to have some fungus living on the surface of a horse's eye, but when there is a wound that allows the fungus to enter inside of the cornea, infection can occur. Fungal infection in a horse's eye can lead to abscesses and blindness.
- Allergens: Allergens like pollen and dust can irritate the eye and lead to conjunctivitis. Conjunctivitis isn't always accompanied by pain and is especially prevalent in spring and summer months.
- Parasites: Infection with the parasite Thelazia, also known as the eye worm, is also a cause of eye infections in horses. Horses are infected with this parasite through flies, and they appear as thin worms in the eyes.
- Injury: Horses can develop an infection after having a foreign object lodged within the eye. If the eye's surface is scratched, environmental bacteria can quickly cause an infection. This could make the horse's eye appear cloudy and red. The horse will likely squint and tear profusely and may be reluctant to let you take a close look.
Diagnosing Eye Infections in Horses
A vet can usually diagnose an eye infection quickly through a physical examination of the eye, but the diagnosis doesn't always reveal a cause. Your horse may need to be sedated during this process. If the examination isn't conclusive, your vet may need to swab and test your horse's eye. Your vet will check for damage to the cornea as well as anything that may be lodged in the eye, such as splinters, awns from grasses, or grit. Developments in the microscopic corneal analysis have helped vets to identify funguses and bacteria in horses' eyes. A tool called an in vivo corneal confocal microscope is used to perform this type of exam.
Swift action following diagnosis is key to successfully treating a horse's eye infection. Your vet may begin treatment with an ointment or gel to apply to the eye multiple times a day. If the infection is bacterial, your vet may prescribe antibiotic eyedrops. A horse with a fungal eye infection is likely to be prescribed a topical anti-fungal and may need surgery if ulcers are present. Treatment for ERU centers on managing inflammation and reducing discomfort. Topical inflammatories and corticosteroids can help but won't prevent future flareups. In severe cases, injections into the eye or surgery may be necessary.
Just because you may see improvement within a few days doesn't mean you should stop the medication before the entire course is up. Stopping treatment before the infection or injury is completely healed can result in the infection flaring up again and possibly causing more damage. In some cases, the cause of the infection (for example, bacteria) may become resistant to the medication if the full course is not administered.
When treating a horse with an eye problem, be aware that the infection may affect its vision, making it jumpier than usual. Talk gently, so you don't surprise your horse if you walk up on a blind-side.
Prognosis for Horses With Eye Infections
When treated swiftly, the prognosis for horses with mild eye infections is good. However, an untreated eye infection can lead to blindness and secondary infections.
The long-term prognosis for horses with ERU isn’t as optimistic. More than half of horses with ERU won’t be able to return to their normal levels of work and may become blind. Treatment will slow progression but is not curative.
How to Prevent Eye Infections
Keeping your horse's environment as dust-free as possible can help stave off infection. If hay is dusty or bedding is very dry, you can dampen it or use other material or bedding. Make sure that sharp edges on water troughs, metal buildings, fences, or other obstacles are covered or inaccessible to prevent your horse from wounding its eye. Pound in or pull any old nails protruding from fences or other structures. Eye and face nets may help keep flies away from your horse's eyes. If you outfit your horse with a fly mask or veil, ensure it isn't irritating the eye.
Check your horse's eyes daily to catch the infection early and prevent it from worsening.
Types of Eye Infections in Horses
- Conjunctivitis: Conjunctivitis is the swelling of the conjunctiva, the mucus membrane in the eye, sometimes caused by infection, allergen irritants, or foreign objects. The conjunctiva lines the inside of the eye and extends to the cornea. Conjunctivitis will cause discharge, redness, and discomfort, and if untreated, it can lead to complications like blindness.
- ERU: Equine Recurrent Uveitis, or ERU, causes recurring bouts of inflammation in the eye and is the leading cause of blindness in horses. The blindness is often caused by equine glaucoma secondary to ERU. The development of ERU is complex and involves many factors, including injury, immune system issues, and viral infection.
Do flies cause eye infections in horses?
Flies can carry harmful parasites transmittable to your horse's eye through contact. Take preventative measures such as fitting your horse with a fly mask to protect its eyes from flies.
Is it normal for my horse's eye to tear?
Some tearing signifies a healthy, functioning tear gland, but your horse's eye may be infected if the tearing is excessive. Watch for colored discharge from your horse's eye as well.
Will my horse become blind from an eye infection?
If treated swiftly, your horse will probably not become blind from an eye infection. If you leave an infection like ERU untreated, your horse may suffer from permanent vision damage.
Overview of Equine Recurrent Uveitis. Merck Veterinary Manual.
Eye Worms in Horses. Kentucky Equine Research.
Cornell Clinicians Develop Safer, Faster Way to Diagnose Horse Eye Problems. Cornell University College of Veterinary Medicine.
Eye Injuries. Kentucky Equine Research.
- Equine Recurrent Uveitis. UC Davis Center for Equine Medicine.