Conjunctivitis in Cats

By Courtney Barnes, BSc, DVM; Tammy Hunter, DVM; Ernest Ward, DVM

What is conjunctivitis?

Conjunctivitis means inflammation of the conjunctiva. The conjunctiva is a mucous membrane like the lining of the mouth and nose. This membrane covers the white part of the eyeball and lines the inner eyelids.

Cats have a third eyelid, or nictitating membrane, in the inner corner of the eye, which is also covered by conjunctiva. In healthy cats, the conjunctiva of the eyelids is not readily visible and has a pale pink color. When conjunctivitis occurs, the conjunctival membranes become red and swollen. Conjunctivitis can affect one or both eyes.

What are the clinical signs of conjunctivitis?

If you see excessive tearing or watering from one or both eyes, abnormal discharge (cloudy, yellow, or greenish), or reddened conjunctival membranes, your cat may have conjunctivitis. Your cat may also squint or keep their eyes closed because of either discomfort or reluctance to be in bright light (photophobia). In severe cases, the conjunctival tissue or the third eyelid may be so swollen that it may partially or fully cover the eye. If your cat exhibits any of these signs, they should be examined by your veterinarian immediately.

What are some causes of conjunctivitis?

The most common causes of conjunctivitis can be roughly divided into two categories: infectious diseases and non-infectious conditions. Conjunctivitis may also be a secondary symptom of another eye disease.

Infectious Causes of Conjunctivitis: Infectious agents such as bacteria, viruses, and fungi are the most common causes of conjunctivitis in cats. In many cases, viruses such as feline viral rhinotracheitis (FVR), also known as feline herpesvirus or feline calicivirus, are the initial cause of inflammation. Primary viral infections are often complicated by secondary infections with a variety of bacteria including Streptococci and Staphylococci. Two other bacteria, Chlamydophila felis and Mycoplasma, are also capable of initiating primary conjunctivitis. These infectious diseases can lead to inflammation of the cornea (keratitis) and ulceration.

Non-infectious Causes of Conjunctivitis: Persians, Himalayans, and other long-haired breeds may be born with eyelids that turn in, a condition called entropion. Entropion causes corneal irritation when the eyelashes constantly rub against the eyeball. Foreign material, such as dirt, sand, and plant material, may become trapped inside the eyelids.

Exposure to irritant chemicals may also initiate conjunctivitis. Allergies are believed to be a common cause of conjunctivitis, but the specific allergens can sometimes be difficult to identify or avoid. Conjunctivitis is also a symptom of eye and eyelid tumors. Other eye conditions that can cause conjunctivitis include corneal ulcers or injuries to the eyeball, dry eye (keratoconjunctivitis sicca or “KCS”, which is a lack of normal tear production), glaucoma (high pressure inside the eye), and uveitis (inflammation inside the eye).

How is conjunctivitis diagnosed?

Your veterinarian will obtain a medical history and thoroughly examine your cat’s eyes and surrounding tissues, often using a device called an ophthalmoscope. They will look for conditions such as foreign material in the eye, a corneal ulcer or other injury to the eye, entropion, and tumors.

Specific tests may be performed based on the examination and history.

  • Your veterinarian may measure the tear production and pressure inside the eyes (intraocular pressure).
  • The cornea may be stained with fluorescein dye to look for underlying corneal injuries or ulcers.
  • Conjunctival scrapings or biopsies may be obtained and sent to a diagnostic laboratory for specialized testing.
  • The nasolacrimal or tear ducts may be flushed to ensure proper drainage.
  • Blood tests may be performed to determine if the conjunctivitis is related to a systemic condition.

If the examination and testing does not show another cause, then treatment is usually started based on the tentative diagnosis of non-specific infectious conjunctivitis. Most bacterial and viral infections will resolve within 5 to 14 days. In cases that are not improving or where other pets are at risk of infection, further testing will be performed to reach a definitive diagnosis.

How is conjunctivitis treated?

The general approach to non-specific infectious conjunctivitis is to use eye medications containing a combination of broad-spectrum antibiotics to control the primary or secondary bacterial infection, and anti-inflammatory drugs to reduce the inflammation. These preparations come as either drops or ointment for instilling into the eyes. Local treatment may need to be supplemented with injections and/or pills.

If a specific diagnosis has been reached, one of the following treatment regimens may be used:

1. Herpesvirus conjunctivitis

  • Although these infections are usually mild and self-limiting, infected cats remain carriers of the virus and may have intermittent relapses.
  • Treatment may not be required for mild cases.
  • Antibiotics are often used for secondary bacterial infection.
  • Antiviral medications are used in severe or poorly responsive cases.
  • There is anecdotal evidence that L-lysine may accelerate healing and reduce recurrences, though studies have not shown much benefit. 
  • Interferon-alpha may be used as an immune stimulant, though this medication has been increasingly hard to get and may not be available in many countries.

2. Chlamydophila or Mycoplasma conjunctivitis

  • Tetracycline ophthalmic ointment.
  • Azithromycin oral antibiotic.

3.  Eosinophilic or allergic conjunctivitis

  • Topical corticosteroid ointment or drops.
  • Topical medications to stop or decrease the allergic reaction.

How do I administer eye medications?

Regular and frequent treatment is essential in successfully treating conjunctivitis. Most ophthalmic drops need to be administered 3 to 6 times a day at the start of treatment. Ointments may require less frequent administration but may be more difficult to administer.

Two people may be necessary to administer medication, at least until the discomfort and sensitivity of the eyes has decreased: one person to hold the cat and the other to administer the eye medication.

Apply 1/4 to 1/2 inch (0.6 to 1.25 cm) of ointment to each eye and then close the lids to smear the ointment across the eyeball. Liquid preparations can be applied directly onto the surface of the eye; one or two drops per eye are usually sufficient. If you have any doubts as to how to give your cat's medication, ask your veterinarian to demonstrate the proper procedure for you (see handouts "Applying Eye Drops to Cats" and "Applying Eye Ointments to Cats" for further information).

When should I expect a response?

Normally you will see a rapid improvement within a few days. However, 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.

What is the prognosis for a cat diagnosed with conjunctivitis?

The prognosis depends on the specific diagnosis. With some non-infectious cases, the conjunctivitis will recur if the underlying cause is not removed. Some of the viruses that cause infectious conjunctivitis are incurable and may persist in a hidden form (called a carrier state), with flare-ups from time to time, especially during periods of stress or illness. The therapeutic goal for these patients is to minimize the frequency and severity of recurrences through optimum nutrition, appropriate vaccination against preventable causes of disease, and medical management when indicated.

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