What is Conjunctivitis?

Conjunctivitis is inflammation of the pink membrane part of the eye, which lines the sclera (white part) and the inner eyelid.  The condition commonly also affects the third eyelid—the membrane positioned in the inner corner of a cat’s eye, between the lower eyelid and the eyeball.

The conjunctiva can become quite red, swollen and sore.  Symptoms may be present in one or both eyes and may involve constant or intermittent squinting.  Conjunctivitis can occur on and off for months to years and may occur with other eye problems such as corneal ulceration (painful open sore on the cornea), keratitis (corneal inflammation), and/or uveitis (intra-ocular inflammation).

Signs of Conjunctivitis 

If you see any of the signs below, your cat may have conjunctivitis  and should be seen by one of our veterinarians as soon as possible.

These signs may be seen in one or both eyes:

  • Swollen and red conjunctival membranes. The third eyelid may also be red and swollen
  • Squinting or frequent blinking (your cat may be reluctant to face light – photophobia)
  • Discharge or excessive tearing from one or both eyes. The discharge may range from colorless and watery to thick green, yellow or dark coloured
  • Your cat may rub their eye/s
  • Sometimes cats will show signs of an upper respiratory tract infection, and may be sneezing (Occasionally this is seen prior to the onset of conjunctivitis)

What causes conjunctivitis?

Causes of conjunctivitis can be roughly divided into two categories: infections (bacteria, viruses or other infectious organisms) and non-infectious causes.

In the majority of cases, conjunctivitis in cats is caused by an infectious agent and may occur as part of “cat flu“.  The most common cause of conjunctivitis in cats is infection with Feline Herpesvirus (FHV-1), Feline Calicivirus, or one of two bacteria; Chlamydophila felis or Mycoplasma felis.  Bacteria such as Streptococci and Staphylococci can also move in and cause a secondary infection.  Conjunctivitis is sometimes seen in cats whose immune systems have been compromised by infection with the feline immunodeficiency virus (FIV) or the feline leukemia virus (FeLV).

Non-infectious causes of conjunctivitis are less common but are still important to consider. Environmental irritants, such as dust or sand may become trapped inside the eyelids and lead to irritation.  Exposure to irritant chemicals can cause conjunctivitis, as can exposure to certain outdoor plants.  Allergies are not an uncommon cause of conjunctivitis and unfortunately the allergens are often difficult to pinpoint and avoid.  Some breeds such as Persians may be born with a turning in of the eyelids called entropion. Entropion causes corneal irritation when the eyelashes constantly rub against the eyeball, which may lead to corneal ulceration– this requires corrective surgery.

How is conjunctivitis diagnosed?

Conjunctivitis can generally be diagnosed with a physical exam by your veterinarian but because there are many different underlying causes specific tests are commonly indicated. Initial diagnostic tests that your vet may performed at the time of examination include:

  • Examination using a direct ophthalmoscope, a Pan-ophthalmoscope and other forms of magnification.
  • Fluoroscein staining  – this involves application of a green dye called fluorescein onto the eye. The dye will adhere to any area of erosion or ulceration of the surface of the cornea. The dye fluoresces a bright green colour using a special magnifying cobalt blue light which makes it easier to see.
  • Schirmer tear testing – this involves placing a small strip of absorbent paper in the conjunctival sac to measure tear production. This is a painless procedure.
  • Intra-occular pressure measurement – this is performed using a device called “Tono-pen” after applying some local anesthetic drops to the eye.
  • After performing these in-house tests, if your vet suspects your cat has conjunctivitis they may recommend some pathology testing.

The most commonly requested laboratory test is a Conjunctivitis PCR panel (Polymerease Chain Reaction). This relatively new and sensitive test assists in the diagnosis of the most common specific pathogens including Chlamydophila felis, Feline Calicivirus, Feline Herpes Virus Type 1 and Mycoplasma felis by  detecting the organism’s DNA in the patient sample.

It is important to note that even with PCR swabs, Feline Herpes Virus Type 1 (FHV-1) infection can be tricky to diagnose because it is intermittently shed by infected cats, meaning that unless the cat is actively shedding virus at the time of sample collection, the swab may return a “false negative” result, despite the virus still being harboured in the body.  In such cases where there is a high index of suspicion, diagnosis of FHV-1 infection is made by careful evaluation of the medical history and clinical signs, and in some cases a treatment trial with an anti-herpes drug.


Conjunctivitis can be irritating and often quite painful so the immediate treatment involves medications to reduce pain and inflammation.  These may include; oral medications (pain relief medication), antibiotic eyedrops or lubricating eye ointments.  If a specific bacterial infection is identified via testing an appropriate course of antibiotic will be dispensed.

In suspected Herpes virus cases, an antiviral drug may be used.  In addition to specific antiviral therapy, alleviating stress is very important in cats affected with FHV-1 (Please refer to our Environmental needs for cats information sheet or book a behaviour consultation with one of our vets).

Supplements that support the immune system, such as L-Lysine, may also be of benefit to some cats.

Normally, with treatment, you will see a rapid improvement after a few days. But even if the conjunctivitis has resolved, do not stop treatment until the end of the prescribed period. Stopping your  cat’s medication early may allow a resurgence of the infection and make it harder to eliminate the next time.  Some of the viruses that cause infectious conjunctivitis (e.g. FHV-1)
may persist in a hidden form or as chronic infections and flare up from time to time, especially during periods of stress or illness (a bit like the cold-sore virus in humans).


All cats, regardless of breed or gender, are susceptible to conjunctivitis.  It can occur in older cats but is primarily a disease of younger cats and commonly occurs in multiple-cat environments.

The infectious agents that cause conjunctivitis are easily spread through direct contact between cats, through contaminated food bowls, on bedding or a person’s hands and by inhaling infected air. The clinical signs appear from 1 to 10 days after exposure and the disease usually persists for around 1 to 3 weeks, but in some cats this can be longer.

Prevention is based on the use of excellent standards of hygiene, ensuring routine healthcare (including vaccinations) is maintained and that the home environment is as cat-friendly and as stress-free as possible.

Do not delay in seeking treatment if you notice any signs of conjunctivitis.

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