The bottom line first: discharge is not a diagnosis, it is a sign that the eye is being irritated. Before the colour or the amount, look at one thing โ is your cat holding the eye open comfortably?
Dog owners and cat owners tend to ask slightly different questions at the clinic. From dog owners it is usually "why are the eyes watering so much"; from cat owners it is "there is always gunk in the corner of the eye." There is a reason for that difference. By far the most common cause of feline conjunctivitis is feline herpesvirus type 1 (FHV-1), and once a cat has it, the virus never entirely leaves the body.
According to VCA Animal Hospitals, roughly 80% of infected cats become lifelong carriers. That does not mean all of them stay unwell. The same source notes that fewer than 45% of cats with latent infection go on to develop recurrent eye disease; the rest carry the virus without much trouble. This is why a cat that had a rough bout of sneezing and snuffles as a kitten can be fine for years and then, one day, turn up with a swollen, weeping eye.
The trigger for a flare-up is usually stress. Moving house, a new person or a new cat in the home, a hospital stay, surgery such as neutering, or an illness elsewhere in the body โ anything that loads the system can reactivate a dormant virus. The structure is much like a cold sore in a run-down human, and Cornell's Feline Health Center makes exactly that comparison, describing recurrences as appearing "just as cold sores appear from time to time in humans." Which is why managing feline conjunctivitis is as much about settling the environment as it is about medication.
Herpesvirus is not the only cause, of course. Cornell lists calicivirus, Chlamydia felis and mycoplasma alongside herpesvirus as the most frequent infectious causes, and makes an important point: it is the cat's own inflammatory immune response to these organisms that produces the great majority of the signs, rather than the organisms directly. Add to that non-infectious causes such as dust, airborne chemicals and allergic reactions to outdoor plants. Cats infected with feline leukaemia (FeLV) or immunodeficiency (FIV) virus have weakened immune systems and are more susceptible.
The colour and consistency of the discharge is the first thing an owner can actually observe. But colour alone cannot confirm a cause. The same herpesvirus case may start as clear watery tearing and turn yellow once a secondary bacterial infection takes hold. Use colour to gauge which stage you are at, and keep pain as the deciding factor.
| What you see | What it often means | What to do |
|---|---|---|
| A little dried brown or reddish crust, mornings only | Normal discharge that has oxidised. Small amounts in both eyes are usually fine | Wipe with a warm damp cloth and watch |
| Clear, watery tearing that keeps running | Early irritation, early viral infection, allergy, or a foreign body | Watch a day or two; see a vet if it does not settle |
| Whitish, sticky, mucoid | Conjunctival inflammation already under way | Book a visit soon |
| Thick yellow or green, pus-like | Likely secondary bacterial infection | Same-day visit |
| Crust so heavy the lids are stuck shut | Severe conjunctivitis; risk of the lids adhering | Same-day visit |
| Blood-tinged or frankly red discharge | Trauma or severe inflammation | Emergency visit now |
The row that matters most is lids stuck shut. VCA notes that in severe cases the eyes may seal closed with discharge. Forcing the lids apart at that point can tear already fragile conjunctiva or cornea, so lay a warm damp piece of gauze over the eye for a moment to soften the crust, wipe gently, and get to a clinic that day.
One more thing worth recognising, especially in kittens: conjunctiva that swells up like jelly. This is chemosis, and the European Advisory Board on Cat Diseases (ABCD) describes it as a hallmark of Chlamydia felis infection. The same guideline notes that chlamydia is mostly a problem in cats under one year old and is rare over the age of five. If you have recently taken in a kitten and one eye has become markedly swollen and gummy, it is worth raising that possibility at the clinic.
Each cause has its own typical age group, mode of onset and accompanying signs. The table below is organised the way it is useful to organise your own observations before a visit. Treat it as a checklist so nothing gets forgotten, not as a diagnostic tool.
| Cause | Typical age / setting | Characteristic picture |
|---|---|---|
| Herpesvirus (FHV-1) | Any age; recurs after stress | Sneezing and nasal discharge alongside, a repeating history, sometimes progressing to corneal ulcers |
| Chlamydia felis | Usually under 1 year, multi-cat settings | Marked chemosis; starts in one eye and becomes bilateral |
| Calicivirus | Kittens, shelter backgrounds | Often with mouth ulcers and drooling |
| Mycoplasma | Young cats, layered on other infections | More often part of a mixed infection than alone |
| Corneal ulcer, foreign body, trauma | Any age; sudden onset | Almost always one eye, held tightly shut, pawed at, clearly painful |
| Allergy and environmental irritants | Any age; tracks the season or setting | Usually both eyes, itch rather than pain |
The one that races the clock is the corneal ulcer. The cornea is the clear window at the front of the eye, and damage to it is intensely painful and moves fast. PetMD warns that untreated ulcers are at risk of rapidly worsening and can "melt" away the affected part of the cornea. Recovery time varies enormously with depth โ the same source gives a range of one to eight weeks after treatment begins. Most ulcers start with trauma of some kind; in cats that often means a claw from a scuffle, or damage done by the cat rubbing its own face. Herpesvirus can also attack the cornea and cause ulceration directly.
In summer the causes tend to overlap. Windows are open, so dust and pollen get in; a cat sleeping in the path of an air conditioner ends up with a drying ocular surface. Add the stress of heat and visitors reactivating a latent herpesvirus, and an allergic irritation and a viral conjunctivitis can arrive in the same week, which makes the picture much harder for an owner to read. That is why simply noting when it started, which eye, and what changed just beforehand improves the quality of the consultation.
Everything here is general information, and an accurate diagnosis has to come from a veterinarian. The eye is an organ where a few days can leave permanent damage, and two problems that look alike from the outside can be entirely different inside.
Not every bit of discharge is an emergency. But if any of the following apply, that day is better than tomorrow morning. These are the items emergency veterinary sources consistently agree on.
By contrast, a small amount in both eyes, clear or pale brown, eyes opening comfortably, eating normally can reasonably be watched for a day or two. Even then, if it is unchanged after three days or the amount increases, book a visit. And if the eye signs come with a loss of appetite, this may not be only about the eye. Setting a baseline with the cat food calculator means you can tell the vet how much less than usual your cat is eating as a number rather than an impression.
There is not a great deal to do at home before the visit, but a few things help. Keep the cat somewhere quiet and dimly lit to reduce strain, then dampen clean gauze with lukewarm water and wipe once from the inner corner outwards, discarding that section of cloth afterwards. Never press on a painful eye, and if the cat keeps rubbing, use a protective collar to prevent further damage. Call the clinic before you set off so they can prepare. If your cat dreads the carrier, the acclimatisation tips in how to choose a cat carrier are worth having in place before an urgent trip.
Knowing the sequence makes the explanation much easier to follow. Most visits run roughly like this.
Treatment then depends on the cause. For ordinary conjunctivitis Cornell describes antibiotic drops or ointment three or four times a day for two to three weeks, with a topical antiviral added alongside the antibiotic where herpesvirus is suspected. This is a common point of confusion: the antibiotic is not there to kill a virus, it is there to stop bacteria colonising an eye the virus has weakened. VCA lists tobramycin, fusidic acid and oxytetracycline as topical antibiotics, idoxuridine and trifluridine as topical antivirals, and notes that oral famciclovir penetrates the tear film effectively.
Confirmed chlamydia is handled differently. The ABCD guideline specifies doxycycline at 10 mg/kg orally, maintained for four weeks to ensure elimination, with some clinicians continuing for two weeks beyond the resolution of clinical signs, and treating in-contact cats to prevent re-infection. The mistake owners make most often here is stopping because the eye looks clear. The point at which signs disappear and the point at which the organism is gone are not the same. Finishing the full course is the single most reliable way to avoid a relapse.
One place where sources genuinely diverge is worth stating plainly. Cornell notes that most cases of conjunctivitis resolve on their own without medication, while emergency-practice sources urge owners not to wait when there is thick discharge or pain. These are not really in conflict โ they are describing mild, painless cases and painful or severe ones respectively. The difficulty is that an owner cannot reliably tell the two apart by appearance, which is why pain is the safer trigger to act on.
If you live with a herpesvirus carrier, the goal is not a cure but longer gaps between episodes. There is no way to clear the virus from the body, but reducing the triggers genuinely changes how often signs appear.
The biggest variable is stress. Make sure there are enough litter trays, bowls and hiding places, and give plenty of lead time before big changes such as a move or introducing another cat. In multi-cat homes, competition over resources is itself a chronic stressor, so the usual advice โ one more litter tray than there are cats, and food and water stations placed out of sight of each other โ helps here too. While one cat is unwell, separate bowls and hand-washing between cats will do a great deal to limit spread.
The environment can be adjusted as well. Cut airborne dust, and if you are using a litter that throws up a lot of it, try a low-dust product. Keep air fresheners and spray cleaners out of the rooms the cats live in, and make sure air conditioning does not blow directly onto a favourite sleeping spot. Cigarette smoke is a particularly strong irritant.
Vaccination is the other half of it. Herpesvirus and calicivirus are both included in the core vaccine. It will not prevent infection outright, but it does reduce the severity of disease, and VCA notes that intranasal herpes and calicivirus vaccines may stimulate local immunity. The chlamydia vaccine exists only within multivalent products and is a non-core vaccine, which ABCD recommends specifically for multi-cat households with a history of infection. If you have lost track of where your cat is in the schedule, the cat vaccine calculator will work out the next date.
Finally, daily observation. Simply glancing at the eyes each morning catches changes far earlier. Three questions are enough: is there more discharge than usual, is it one eye or both, and is the cat opening the eyes the way it normally does. In an older cat, watch water intake and body weight alongside the eye signs โ eye changes sometimes arrive together with the early signals of a systemic problem rather than on their own.
Sources disagree sharply here. L-lysine has been sold as a feline herpesvirus supplement for years and some places still recommend it, but a 2015 systematic review in BMC Veterinary Research reached the opposite conclusion. After examining seven studies on feline herpesvirus and ten on human herpesvirus, the authors recommended an immediate stop to lysine supplementation because of the complete lack of scientific evidence for its efficacy. More striking, several clinical trials in cats reported a higher infection frequency and greater disease severity in the cats receiving lysine. The original theory was that lysine lowers arginine and so blocks viral replication, but the review found that in cats extra dietary lysine does not lower plasma arginine at all. On top of that, cats cannot synthesise arginine themselves, and arginine deficiency causes hyperammonaemia, which can be fatal. In other words the mechanism does not hold and the benefit has not been shown. Opinion in practice is still divided, so if you are already giving it, discuss it with your own vet โ and above all do not let a supplement become a reason to delay eye drops or a proper examination.
Please don't. The most dangerous case is any drop containing a steroid. If the cornea is scratched or ulcerated, a steroid slows healing and can let the ulcer deepen, and where herpesvirus is the cause it can help the virus replicate and make things worse. The problem is that you cannot tell from the outside whether the cornea is damaged. That is exactly what the fluorescein stain test is for, and it is why the choice of drop should come after the test, not before. Human redness-relief drops carry the same issue: the vasoconstrictor in them takes the red out for a while, but the cause is untouched and an ulcer can progress behind an eye that looks better. Even a bottle left over from a previous bout of conjunctivitis is a gamble, because there is no guarantee this episode has the same cause. At home, clean gauze dampened with lukewarm water or sterile saline is about the limit; everything else should wait for the examination.
One-sided signs deserve more attention, not less. Environmental irritants such as dust or allergens usually affect both eyes fairly evenly, so a change confined to one eye often means something is happening in that eye specifically. A claw scratch, a grass seed or grit, a corneal ulcer, or a problem on the inner surface of the eyelid all tend to start on one side. Chlamydia infection is also described as typically starting in one eye and becoming bilateral a few days later, so a one-sided phase may simply be early. If the cat also struggles to open the eye, blinks constantly, or paws at its face, that is pain, and a same-day visit beats waiting until tomorrow. By contrast, a small amount of brownish crust in both inner corners each morning, with the eyes opening comfortably, is usually normal discharge.
Between cats, very much so. Herpesvirus, calicivirus, chlamydia and mycoplasma all spread through direct contact with tears, nasal discharge and saliva, through mutual grooming, and via shared bowls and human hands. That is why conjunctivitis is so common in multi-cat homes and shelters. While one cat is unwell, separate the food and water bowls, wash your hands after handling, and if you can, keep the affected cat apart during the worst of it. If chlamydia is diagnosed, many sources suggest treating in-contact cats too so the infection is not simply passed back and forth โ worth raising with your vet. Transmission to people is rare. Chlamydia felis has been documented causing chronic follicular conjunctivitis in humans, so wash your hands after touching a sick cat's eyes and avoid rubbing your own. Feline herpesvirus is a different virus from human herpes and does not infect people.
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Written and reviewed by the Mongnyang Lab editor ยท Last updated 2 August 2026
This article was compiled by cross-checking publicly available veterinary and pet nutrition sources. Where sources disagree, that disagreement is stated rather than smoothed over. The Mongnyang Lab editor is not a veterinarian, and this article does not replace diagnosis or prescription. If your animal seems different from usual, going to a clinic is always better than reading further.
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This content is general information and does not replace veterinary diagnosis. If your cat shows signs of eye pain, contact a veterinary clinic immediately.