Conjunctivitis in Horses

What Is Conjunctivitis in Horses?

Conjunctivitis in horses causes swelling of the conjunctiva, or thin mucus membranes that protect the eye by covering the whites of the eye and the inside of the eyelids. The swelling can be caused by an infection or have other non-infectious causes.

Conjunctivitis is most seen in young horses due to their weaker immune systems, but it can occur at any age.  

It’s important to note that any abnormality with a horse's eye should be considered a medical emergency and a veterinarian should be called immediately. Eye issues in horse eyes can become serious quickly. 

Symptoms of Conjunctivitis in Horses

A horse with equine conjunctivitis may experience the following symptoms: 

• Eye discharge/drainage 

• Squinting  

• Excessive itching or rubbing of the eye 

• Head shaking 

• Redness of eye or around the eye 

• Swelling of any part of the eye or around the eye 

What Causes Conjunctivitis in Horses?

Conjunctivitis can either be infectious or non-infectious. Common causes for each type include: 

• Infectious 

  • Bacterial 

  • Parasitic 

  • Viral 

  • Fungal 

  • Protozoal infections (single cell organisms)

• Non-infectious 

  • Uveitis

  • Trauma (cuts to the face)

  • Allergic reaction/irritants (dust, pollen)

  • Nasolacrimal duct obstruction (part responsible for draining tears through a very small passage that travels from the inside corner of the eye to the nostril)

  • Cancer  

Most commonly, horses get pink eye due to trauma or through an infection.

Young horses may get conjunctivitis if they have direct contact with a fly that has been in direct contact with an infected horse.

How Veterinarians Diagnose Conjunctivitis in Horses

To diagnose equine conjunctivitis, a veterinarian will take a thorough history and perform physical and eye exams. They may also recommend diagnostic tests, including blood work.  

Veterinarians may ask questions such as:  

• "How long have you noticed the new eye symptoms?” 

• "Has the horse been traveling recently or been around any new horses?" 

• "Is this the first time the horse has had any eye issues?" 

• "Have you noticed your horse showing any other abnormal behaviors lately?" 

The veterinarian will also want to know the horse's vaccine and deworming history.  

Physical Exam

Your veterinarian will complete a physical exam by:

• Looking at the mucus membranes/gums

• Listening to the heart, lungs, and gastrointestinal tract

• Checking body temperature,

• Looking the horse over for any abnormal swellings or other physical abnormalities

Finally, your veterinarian may also ask you to hand-walk the horse to make sure there is no lameness or neurologic signs.  

Ophthalmic (Eye) Exam

Once the physical exam is complete, the veterinarian will perform an ophthalmic exam on the eye and the surrounding area surrounding.

First, they will look for any visual abnormalities, checking both eyes for comparison.

After getting a good look at the structures in both eyes, the veterinarian will likely want to stain the eye (using a fluorescein eye stain) to look for any scratches or breaks in the cornea. Fluorescein eye stain will adhere to any damaged cells on the cornea and be easily visible with a (usually) green tint.

If there is any stain uptake, the veterinarian may diagnose the horse with a corneal ulcer and create a plan from there.  

Based on the eye exam findings, your veterinarian may advise further diagnostics such as:  

• Culture and sensitivity testing to look for bacterial causes 

• Conjunctival scraping or biopsy 

• Viral testing 

• Blood work such as a complete blood count 

• Chemistry looking for systemic disease 

• Specific testing for bacterial disease(s) 

Treatment of Conjunctivitis in Horses

Treatment for a horse eye infection depends on the underlying cause and severity.

Commonly, horses are put on a combination of medications, including both topical and oral medications to treat the symptoms and the underlying cause.

Topical broad-spectrum antibiotics such as Neo-Poly-Bac (BNP) are one of the most common medications used in the treatment of equine conjunctivitis if there is any ulceration present; if not, a BNP ointment containing a steroid may be used to help with inflammation.  

Conjunctivitis can be very painful due to the swelling and irritation of the eye. Therefore, oral systemic anti-inflammatories are commonly prescribed, such as Banamine® (flunixin meglumine).  

Other medications that may be added include:  

• Atropine ophthalmic drops 

• Silver sulfadiazine cream solution 

• Antifungal topical medications 

• Topical steroids (such as dexamethasone) are only prescribed if there is no corneal ulcer present, as steroids can delay or hinder the healing of the corneal layers

• Serum (clear parts of blood that the veterinarian can collect from the horse) 

It’s important to follow your veterinarian’s instructions, including the amount of medication to give, the number of times to give it per day, and how long to wait between medication administration if you’re instructed to give more than one at a time. Often, it’s important to wait at least two to five minutes in between medications.  

Some medications, such as atropine, are given to help with pain but will also cause the eye to dilate and the horse to be extremely sensitive to light. During treatment, it may be important to keep your horse in a barn out of the sunlight during the day, or at least have your horse wear a fly mask.  

Nasolacrimal Duct Obstruction Treatment

The nasolacrimal duct is a channel that passes from the inside corner of the eye to a small opening just inside the nostril.

If the veterinarian has diagnosed the horse with an obstruction in the nasolacrimal duct that’s causing the conjunctivitis, the horse will likely need to be sedated because this procedure can sometimes be slightly uncomfortable to relieve the blockage.

A small tube will be placed just inside the nasolacrimal duct opening in the nose, and sterile solution will be carefully pushed through the duct to clear away any obstruction. It’s not uncommon for this duct to become plugged with debris, and it’s generally an easy fix.  

There are over-the-counter eye wash solutions, such as Vetericyn® eye wash, that may be helpful to clear away discharge from the eye.

Before using any of these methods, ask your veterinarian if this would be appropriate to for your horse.

How To Apply Eye Drops to a Horse's Eye 

1. Gently and carefully clear away any discharge with a wet tissue, cotton ball, or eye wash solution.  

2. Place the topical eye medication tube or bottle in one hand between your pointer finger and thumb with the tip down. If possible, rest your wrist against the horse so that if the horse throws his/her head, the eye medication bottle will not accidentally poke the eye and cause trauma to the eye.  

3. Use your opposite hand to gently pull the lower lid of the eye down to show a little pocket in the inside of the lid.  

4. Place the instructed amount of topical eye medication into the lower lid, being careful not to touch the tip of the medication tube to the eye. This prevents any bacteria or debris from getting on the medication which is then transmitted at next usage, or accidentally irritating or damaging the sensitive eye tissue.

5. Allow the eyelid to resume normal position. Your horse will likely blink a few times, allowing the eye medication to be distributed across the eye.  

6. If a horse is on multiple medications or is difficult while applying medication, your veterinarian may recommend a subpalpebral lavage system (SPL) in certain cases. This includes having a small plastic “plate” under the eyelid with a tube running back through the mane, ending with an opening that can be used to apply medication from a safe and easier distance.

Recovery and Management of Conjunctivitis in Horses

Conjunctivitis in horses can heal in as short as five to seven days, or it could take weeks depending on the underlying cause, severity, and how quickly treatment is started.

If left untreated, equine conjunctivitis will likely get worse and may cause permanent damage to the eye as well as impairment of vision for the horse.  

Prevention of Conjunctivitis in Horses

In general, conjunctivitis can be prevented in several ways, including: 

• Fly control in barns and pastures: If you live in an area where flies are abundant, using a fly mask is a great way to help keep the eyes clean and fly free. Flies not only can pass infectious agents from horse to horse, but can also cause irritation and trauma to the eye. There are many fly control products for horses you can use to keep these pests away.

• Establish a preventive health program: It’s important to keep horses up to date on vaccinations and deworming. Ask your primary veterinarian to help you develop the best protocol for your horse. 

• Reduce environmental allergens: If horses are stalled, watering down dusty shavings can be helpful. 

• Treat any underlying eye or systemic disease that could make the horse more prone to conjunctivitis.  

• Limit sun exposure in lightly pigmented horses: This may decrease the incident of squamous cell carcinoma (SCC) of the eye and eyelid seen in horses. SCC is the most common form of tumor to affect the eye and eyelid in horses and second most common cancer seen in horses.