Understanding Pink Eye in Horses: Signs, Care and Prevention

Pink eye in horses, which your vet will call conjunctivitis, is the most common disease affecting the equine eye, and according to Horse & Hound's veterinary advice on conjunctivitis it always makes the eye weep. The conjunctiva is the thin pink membrane lining the eyelids and covering the white of the eye. When it becomes inflamed the result is a red, puffy, watering eye that plainly bothers the horse. The difficulty for an owner is that this same red, weepy appearance is the eye's answer to almost everything that can go wrong with it, from a speck of arena dust to a corneal ulcer that could cost the horse its sight.

This guide is written for the person standing in the field at seven in the morning looking at a shut, streaming eye. It covers what you are actually seeing, what to do in the first few minutes, which signs mean you stop reading and pick up the phone, and how to keep flies and dust off your horse's face through a British or South African summer. It also works through the causes one by one, because what is actually irritating the eye decides everything the treatment has to do.

One study should shape how seriously you take a weepy eye. Malalana and colleagues at the University of Liverpool examined 327 horses aged fifteen and over and found abnormal ocular findings in 287 of them, yet the owners of those same horses reported eye disease in only 3.3 per cent of cases (Malalana F, McGowan TW, Ireland JL, Pinchbeck GL, McGowan CM. Equine Veterinary Journal 2019;51(2):212-217, PMID 30080275). Agreement between what owners noticed and what the vet found was poor across every category the team looked at. Eye problems in horses are commonplace and routinely missed, so the instinct to wait and see is the wrong one.

What Is Pink Eye in Horses?

Conjunctivitis means inflammation of the conjunctiva. When that tissue is irritated or infected its blood vessels dilate, the normally pale pink lining turns an angry red, and the eye produces extra tears and mucus. Swelling of the conjunctival tissue around the cornea has its own name, chemosis, and the MSD Veterinary Manual's chapter on disorders of the conjunctiva in horses, reviewed by Kirk N. Gelatt VMD DACVO, notes that it occurs to some degree with every case of inflammation but is most dramatic with trauma, allergic reactions and insect bites.

Why "Pink Eye" Is a Description, Not a Diagnosis

This is the single most useful thing to understand. In cattle, pinkeye names a specific infectious disease. In horses it does not. Equine pink eye is a sign, and the conjunctiva reacts the same way whether the underlying problem is a grass seed under the third eyelid, a fly-borne bacterial infection, a pollen allergy or the early stage of equine recurrent uveitis. The MSD manual is explicit that the appearance of the conjunctiva is usually not enough on its own for a vet to reach a diagnosis from a physical examination.

That is why this article will keep sending you back to your vet rather than to a tube of ointment. Treating the redness without knowing what is causing it is how a treatable scratch becomes a permanent scar.

Signs of Pink Eye in Horses: What You Will Actually See

Most owners spot the discharge before they spot the redness, because the tear staining running down the face is visible from the gate while the conjunctiva itself needs a proper look under the lid.

The Early Signs

• Redness of the lid linings. Pull the lower lid down gently and compare it with the other eye. A healthy conjunctiva is pale pink, not scarlet.
• Puffy or swollen eyelids, sometimes with the tissue around the cornea visibly bulging.
• Discharge and tear staining, running down the face from the inner corner.
• Blinking, squinting or a partly shut eye. A horse holding an eye closed is telling you it hurts.
• Light sensitivity. The horse seeks shade, turns away from the sun or works with the eye half closed.
• Rubbing. The horse scrapes its face on a fence post, a stable wall or its own foreleg, which reliably makes the damage worse.

Reading the Discharge: Watery, White or Yellow

The character of the discharge is a genuine clue, although not a reliable enough one to treat on. Thin, clear, watery discharge tends to go with irritants, allergies and viral inflammation. Thick discharge that turns white, cream, yellow or green suggests pus and points towards a bacterial component. The MSD manual notes that where a mucus and pus-filled discharge is present a vet may prescribe a topical antibiotic, but adds the important caveat that the antibiotic alone may not bring about healing if other factors are involved.

In practice that caveat is the whole story. A yellow discharge on a horse with a grass awn still lodged under the lid will keep coming back until somebody finds the grass awn.

One Eye or Both? What the Pattern Tells You

Whether one or both eyes are affected genuinely narrows things down, and it is worth telling your vet on the phone. The MSD Veterinary Manual sets out the pattern plainly: conjunctivitis in only one eye may result from a foreign object, inflammation of the tear sac (dacryocystitis) or dry eye, while conjunctivitis occurring in both eyes is commonly caused by infection with a virus or bacteria. Environmental irritants and allergens are common causes of either.

Add your own yard knowledge to that. Two horses in the same field with streaming eyes in August is a very different picture from one horse with one shut eye after a hack through a thorn hedge.

What Causes Pink Eye in Horses?

Horse & Hound's veterinary advice lists the common causes as bacterial infections, irritants such as wind, dust, flies and chemicals, allergens such as pollen, trauma, foreign bodies, eyelid deformities where a wound has left a gap in the lid, parasites such as habronema, and tumours. The practical split for an owner is between the things you can do something about and the things that need a vet's kit.

Irritants and Fly Pressure

This is the biggest group and the most preventable. Dust off a dry arena, sand, wind, pollen and smoke all abrade the ocular surface. Flies are worse than irritants on their own because they feed on the tear film at the corner of the eye and travel from one horse's face to the next, which is how a local infection becomes a yard-wide one. Horse & Hound notes that habronema sores are common in sunny summers, and the same seasonality shows in the search data: UK interest in pink eye in horses rises several times over from the spring trough to the August and September peak, which is exactly fly season.

Bacterial and Viral Causes

Moraxella is the genus most often named in connection with equine conjunctivitis, and a recent Italian case report is a useful corrective to how confidently it usually gets cited. Grassi and colleagues isolated Moraxella osloensis and Moraxella equi from swabs taken from two symptomatic horses with conjunctivitis and rhinitis, identified by MALDI-TOF, with both isolates sensitive to all the antimicrobials tested and both horses recovering without relapse (Grassi L, De Zan G, Deotto S et al. Veterinaria Italiana 2026;62(3), PMID 42661563). The authors are careful to flag the difficulty of assessing the pathogenic role of a commensal organism such as Moraxella, which is a reminder that growing a bacterium from a swab is not the same as proving it caused the disease.

Viruses matter too. Equine herpesvirus can produce conjunctivitis, as the MSD manual notes, and viral cases usually arrive with company: nasal discharge, a cough, a dull horse. If the eyes are red and the horse is also snotty and off colour, say so when you ring the vet, because that changes the picture from an eye problem to a whole-horse one.

Which herpesvirus is a fair question, and the honest answer is less settled than it is usually made to sound. Equine herpesvirus type 2 is the one implicated in recurrent or persistent conjunctivitis, particularly in young horses, though its precise role as a pathogen remains uncertain. EHV-5 is often named alongside it and should not be: Hollingsworth and colleagues found EHV-5 DNA just as commonly in ophthalmically normal horses as in horses with idiopathic keratoconjunctivitis, and concluded that PCR was not useful for establishing a cause (Hollingsworth SR, Pusterla N, Kass PH et al. Veterinary Ophthalmology 2015;18(5):416-421, PMID 25594353). Equine influenza, equine viral arteritis and adenovirus can all inflame the conjunctiva as part of a wider respiratory illness, usually with both eyes involved plus a temperature and nasal discharge, and strangles can set off conjunctivitis through direct irritation.

Parasites: Thelazia Eyeworms and Habronema

Eyeworms are far more common than most owners realise, and they are a classic cause of a conjunctivitis that will not clear up on antibiotics. Giangaspero and colleagues examined both eyes of 128 slaughtered native horses in the Abruzzo region of Italy and found 50 of them, just over 39 per cent, parasitised by Thelazia lacrymalis; gross examination showed conjunctivitis, most often follicular conjunctivitis, in 58 per cent of the infected horses (Giangaspero A, Tieri E, Otranto D, Battistini ML. Parasite 2000;7(1):51-53, PMID 10743649). An earlier survey by the same group in Apulia examined 409 horses and found 14.7 per cent parasitised, with a mean burden of six worms per head and a range of one to twenty (Giangaspero A, Lia R, Vovlas N, Otranto D. Parassitologia 1999;41(4):545-548, PMID 10870558).

Both surveys were carried out in Italy, so the figures describe Italian horses rather than British or South African ones. The point they make still travels: if an eye keeps relapsing despite sensible treatment, worms in the tear ducts belong on the list of possibilities, and they are found by examination rather than guessed at. Habronema larvae, deposited around the eye by flies, cause the stubborn, weepy, granular lesions known as equine summer sores, which appear and worsen through the fly months.

None of that is an argument for putting the horse on a worming calendar, and this is where owners most often go wrong after reading about eyeworms. BEVA's guidance on a sustainable approach to equine wormers says to avoid blanket use, that any wormer must be justified by risk assessment and testing, that typically four out of five adult horses do not need regular treatment, and that of the commonly used wormers ivermectin appears to be the most toxic to the environment. The British Horse Society is blunter still: dosing at regular intervals drives resistance and has not been recommended for many years, and there is now resistance to every one of the five classes of deworming drug in the UK. Treat a diagnosed eyeworm or Habronema lesion on your vet's direction, and leave the calendar out of it.

Allergies and Blocked Tear Ducts

Some horses react to pollen, dust or storage mites in bedding with streaming, itchy eyes in the same way others react with a runny nose. If the eyes flare on the same few weeks every year and settle when the horse comes in, allergy is worth investigating, and our guide to managing hay fever in horses covers the management side.

A blocked nasolacrimal duct, the narrow channel that drains tears from the eye down to the nostril, produces a permanently wet eye and a wet face without much redness. It is easily missed and easily fixed: the vet flushes the duct through and the overflow stops.

Lumps, Growths and the Things That Are Not Conjunctivitis at All

Horse & Hound lists tumours among the causes, and the eyelids and the tissue around the eye are a favoured site for several of them. Sarcoids commonly sit on the eyelid, where even a small one rubs the ocular surface with every blink; our reference entry on equine sarcoids covers the types. In grey horses, a firm black nodule near the eye or under the jaw raises the question of melanoma in horses, which is common enough in older greys to be worth knowing about before you find one. On the eyelid margin and the third eyelid the tumour to know is squamous cell carcinoma, which favours pink, unpigmented skin around the eye and can start as nothing more than a rough, weeping patch that looks like a conjunctivitis refusing to clear.

Swelling can also come from the bone rather than the eye. A kick, a fall or an ill-fitting headcollar can injure the facial crest that runs from below the eye towards the ear, and the resulting zygomatic arch injury or periostitis produces a swollen, sore, weeping eye region that no eye ointment will touch. If the swelling feels firm and bony rather than soft and puffy, mention it.

What To Do When You First Find a Red Eye

You will not diagnose the cause in the field, and you should not try. What you can do is gather the information your vet needs and stop the horse making it worse in the meantime.

1. Look at both eyes in good light and compare them. The sound eye is your reference for what normal looks like on this horse.
2. Note what you see before you touch anything. Which eye, how much it is open, the colour and thickness of the discharge, whether the clear front of the eye looks clear or cloudy or blue-tinged.
3. Take a photograph on your phone. Eyes change through the day, and a picture taken now is genuinely useful to the vet, especially for an out-of-hours call.
4. Clean the discharge away gently with a clean, damp cloth or cotton wool, wiping from the inner corner outwards. Use a fresh piece for each wipe and a separate cloth for each eye.
5. Stop the rubbing. Fit a soft, clean fly mask or bring the horse in, away from the fence posts and the flies.
6. Phone the vet and describe what you found. Do not wait to see how it looks tomorrow morning.

What Not To Do

• Do not use leftover ointment from a previous case, on this horse or any other. The tube left over from last summer may be the wrong drug entirely, and it is certainly contaminated.
• Do not use human eye drops. Many contain steroids or vasoconstrictors that are actively harmful to an eye with a corneal ulcer.
• Do not force the eyelids apart on an eye the horse is clamping shut. That level of pain needs sedation and a vet, not a determined owner.
• Do not try to remove a foreign body you can see embedded in the surface of the eye. Pulling it out can take the cornea with it.
• Do not flush the eye with anything other than plain sterile saline, and stop if the horse resents it.

When to Call the Vet About a Horse's Eye

Some eye problems are genuine emergencies, and the honest position is that an owner cannot reliably tell those apart from the minor ones by looking. Treat the following as reasons to ring straight away rather than to monitor:

• The eye is held tightly shut, or the horse will not let you near it.
• The clear front of the eye looks cloudy, hazy, blue-tinged or white rather than transparent.
• There is thick coloured discharge, or the discharge is getting worse rather than better.
• The eyeball itself looks to be bulging or sitting differently from the other one.
• There is any visible wound to the eyelid or the surface of the eye, or you can see something stuck in it.
• The horse is bumping into things, hesitating at shadows or otherwise behaving as though it cannot see properly.
• The horse is also dull, off its feed or has nasal discharge, which shifts the suspicion towards a systemic illness.
• The eye is no better, or is worse, at the next check after treatment started.

Rather than working to a fixed number of hours, judge it by the direction of travel. An eye that is improving can reasonably be watched. An eye that is static or deteriorating gets a phone call, and one the horse is clamping shut gets one now. A cloudy or blue-tinged eye in particular can signal a problem inside the eye, and the Malalana study above is the evidence that owners systematically underestimate these things.

How Vets Diagnose Pink Eye in Horses

The vet's first job is not to confirm conjunctivitis but to rule out the conditions that mimic it and can blind the horse: corneal ulceration, uveitis and glaucoma. The MSD Veterinary Manual lists the workup as a medical history, a physical examination, tests on conjunctival scrapings, a Schirmer tear test and occasionally a biopsy.

• Fluorescein staining. A harmless orange dye is dropped onto the eye. It glows green wherever the corneal surface is broken, which identifies an ulcer that would change the treatment completely.
• Schirmer tear test. A calibrated paper strip measures tear production, distinguishing a genuinely dry eye from an overflowing one.
• Nasolacrimal duct flush. Saline is passed through the tear duct to check it drains to the nostril and to clear it if it does not.
• Swabs, scrapings and cytology. Sent to a laboratory to identify bacteria, fungi or parasites, and to guide antibiotic choice rather than guess at it.
• Auriculopalpebral nerve block. A small dose of local anaesthetic placed over the ear stops the horse blinking. On a painful eye it is often the only way to get the lids apart far enough to examine the surface properly, and it is worth knowing it is coming so it does not alarm you.
• Examination under the third eyelid. Usually after topical anaesthetic and often with sedation, because this is where grass seeds and awns hide.
• Intraocular pressure. Measured where glaucoma or uveitis is suspected.

The fluorescein test is the one that matters most to the owner waiting for an answer, because the result determines whether steroid-containing drops are safe. On an ulcerated cornea they are not.

Two terms are worth knowing before you hear them in the yard. When the inflammation involves the cornea as well as the conjunctiva, the condition is equine keratoconjunctivitis, a more serious presentation than conjunctivitis on its own. Inflammation of the cornea by itself is equine keratitis. Both involve the clear window the horse actually sees through, and both are a veterinary assessment rather than a tube of ointment.

Treatment for Pink Eye in Horses

Treatment follows the cause, which is why this section cannot tell you what your horse needs. What it can do is tell you what to expect, so the plan your vet gives you makes sense.

Bacterial Conjunctivitis

Horse & Hound notes that with the common bacterial conjunctivitis, antibiotics are frequently prescribed as an ointment, gel or drop to be placed in the eye. Which product depends on the likely or confirmed organism, and in the UK your vet is prescribing from the range of licensed veterinary ophthalmic preparations listed in the NOAH Compendium of Data Sheets for Animal Medicines. Finish the full course your vet sets even if the eye looks better early, and go back for the recheck rather than stopping on the strength of how it looks.

There is one restriction owners are rarely told about, and it is worth understanding before you are standing in the yard waiting for a prescription. BEVA's ProtectME toolkit does not make chloramphenicol the default for a red eye: its example ocular policy lists cloxacillin first for conjunctivitis and reserves chloramphenicol for corneal ulceration. What the toolkit does flag is that chloramphenicol is not for use in food producing animals, and VMD guidance on the prescribing cascade is explicit that a horse which has not been signed out of the food chain in its passport must be treated as a food producing animal. Whether your horse is signed out therefore changes what your vet can legally put in its eye. Know which yours is.

Irritant and Allergic Cases

These improve when the trigger goes, and the management change usually does more than the medicine. Anti-inflammatory drops may be prescribed to settle the swelling, but only once an ulcer has been excluded. For insect-bite swelling specifically, the MSD manual notes that treatment is with topical corticosteroids and that these cases usually heal rapidly.

Foreign Bodies and Parasitic Causes

A foreign body is removed and the eye flushed, and the improvement is usually dramatic and immediate. Parasitic cases need the worms removed or targeted treatment rather than antibiotics, combined with serious fly control, or they simply come back. This is the group most often mistaken for treatment failure.

Pain Relief and Comfort

A painful eye is a welfare problem in its own right, and it also drives the rubbing that causes further damage. Your vet may prescribe a systemic anti-inflammatory, and in some cases atropine to relax the muscles inside the eye and ease the spasm. Keep the horse out of bright sun and away from dust while the eye recovers.

How to Apply Eye Medication to a Horse Safely

Medicating the eye of an animal that weighs half a tonne and does not want you near its face takes technique rather than force, and it is worth getting right because you will be doing it several times a day. Horse & Hound's advice is to rest your hand on the horse's cheek and gently turn out the lower eyelid so you can squeeze the medicine into the exposed conjunctival sac, adding that medication is often easiest to apply at the inner, medial corner of the eye.

1. Have a helper hold the horse on a headcollar and lead rope. Do not tie the horse up for this.
2. Clean any fresh discharge away first, wiping from the inner corner outwards.
3. Rest the heel of your hand on the cheekbone so your hand moves with the head instead of against it.
4. Turn the lower lid out gently to make a pocket, and lay the ointment or drop into that pocket.
5. Keep the applicator tip clear of the eye and the lashes. Touching it contaminates the tube for every subsequent dose.
6. Let the horse blink it in. Do not hold the lids shut.
7. Wash your hands afterwards, and between horses without exception.

If the horse will not tolerate repeated treatment, tell your vet early rather than skipping doses. A subpalpebral lavage system, a fine tube placed under the eyelid so medication can be delivered without going near the horse's face, exists precisely for this, and it is a far better answer than a half-treated ulcer. More general handling and horse care guidance is worth reading alongside this if you are new to nursing a horse through a course of treatment.

How to Prevent Pink Eye in Horses

Prevention is mostly about reducing the everyday assaults on the ocular surface, and it is genuinely effective. The seasonal search pattern makes the target obvious: the work is done in spring, before fly season arrives.

Fly Control Through the Summer

• A well-fitted fly mask is the single best tool, keeping flies, dust and sun off the face during turnout. Check the fit daily: a mask rubbing the eye is worse than no mask, and a dirty one carries the infection it was meant to prevent. Wash them, and never share one between horses.
• Attack the breeding sites, not just the flies. Muck heaps sited away from the fields, regular removal of droppings from paddocks and clean water troughs do more than any spray. Our guide to repelling horse flies covers the methods that hold up.
• Give horses somewhere to escape. Shade lets a horse take its own face out of the sun and the flies, and a run-in shed in the paddock does this better than a hedge because the horse can use it whenever it chooses without being brought in.
• Time turnout around fly activity where the yard allows it. Night turnout through the worst of the summer takes the pressure off completely.

Dust, Bedding and Arena Management

• Water the arena surface before schooling, and think about where the wind carries the dust.
• Choose low-dust bedding and store it away from the stable. Soak or steam hay where dust is an issue.
• Muck out with the horse elsewhere rather than tied up in the middle of it.
• Walk the fields and check for protruding nails, broken rails, sharp edges and overgrown thorny hedges at head height.

Yard Hygiene When One Horse Has a Sore Eye

Until the cause is known, assume it could be infectious. Give the affected horse its own cloths, sponges, fly mask and headcollar, keep them physically separate rather than just mentally earmarked, and wash your hands between horses. It costs nothing and it is the difference between one sore eye and six. The British Horse Society's guidance on disease prevention sets out the general biosecurity principles that apply to any yard.

Look at the Eyes Every Day

Given how poorly owner observation matched veterinary findings in the Liverpool study, a deliberate daily look is the highest-value habit in this article. Build it into grooming: both eyes, in good light, lower lid turned down, comparing one side with the other. You are looking for a change from yesterday, not for a diagnosis, and change is something you can only see if you looked yesterday.

When It Turns Out to Be More Than Conjunctivitis

Sometimes the red eye is the first sign of something that will need managing for years. Equine recurrent uveitis, or moon blindness, is the leading cause of blindness in horses and often presents early as a painful, weeping, apparently ordinary sore eye. Cataracts are a separate problem, though the Liverpool study found increasing age associated with increased odds of both eye abnormalities generally and cataracts specifically, and Appaloosas at increased odds of cataract compared with other breeds.

Owners often dread the worst outcome more than the evidence warrants. Where an eye cannot be saved and has to be removed, a UK multicentre study is genuinely reassuring: Wright, Ireland and Rendle reviewed 170 enucleations across eight UK equine referral centres and surveyed 119 owners, finding 77.3 per cent of horses performing at an equivalent or higher level than before surgery, 90.8 per cent of owners pleased with the outcome, and 21.3 per cent of those wishing the procedure had been done sooner (Wright K, Ireland JL, Rendle DI. Equine Veterinary Journal 2018;50(2):186-191, PMID 28815702). That is not an argument for complacency about a sore eye. It is an argument against delaying a referral out of fear.

Referral is a reasonable thing to raise yourself when an eye will not resolve or keeps coming back. It is sometimes presented as a rule the RCVS or BEVA has laid down for eye cases, and neither body publishes anything of the kind. What the RCVS supporting guidance on referrals and second opinions actually says is general and applies to any condition: a vet should recognise when a case is outside their area of competence and be prepared to refer it to someone they are satisfied is competent. A stubborn or repeatedly recurring eye is often exactly that. So ask about it, as a question rather than as a requirement someone else has imposed.

The outlook is worth stating plainly, because this article has spent a long time on what can go wrong. Simple irritant and bacterial conjunctivitis, treated once the cornea has been checked, generally clears completely and leaves nothing behind. Allergic and viral cases tend to return with their season rather than to do lasting damage. It is the cases complicated by corneal ulceration, deep infection or recurrent uveitis that carry a genuinely guarded outlook, and the thing that most reliably decides which group a horse lands in is how quickly a vet saw the eye. You will notice this article gives you no number of days to expect. That is deliberate: the honest answer depends on the cause, and nobody can hand it to you from a page.

Final Thoughts

Pink eye in horses is common, it is usually treatable, and it is worth taking seriously every single time, because the cheap cause and the sight-threatening one look the same from the gate. Clean the eye, protect it, write down what you saw and phone the vet. Then spend the spring on fly control and daily eye checks, which is where the real prevention happens.

The one habit worth taking away is the comparison. Look at both eyes, every day, in good light. A horse cannot tell you its eye hurts until it is already holding it shut.

Frequently Asked Questions About Pink Eye in Horses