Conjunctivitis is inflammation of the conjunctiva, a tissue that makes up part of the eye and eyelid. In people, the conjunctiva is the white part. Conjunctivitis is common in cats and can be a result of infection or injury.

Cat conjunctivitis

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The primary cause of most conjunctival infections in cats is usually feline herpes virus. The treatment for herpes-based conjunctivitis involves a combination of antibiotics and antivirals, both in eye drops and pills. Even cats who have had the herpes vaccine can get conjunctivitis because the vaccination does not prevent herpes, but allows a milder case.  

However, some conjunctival infections that are primarily viral also have a bacterial infection at the same time. The two bacteria species best known to cause conjunctivitis in cats are Chlamydia felis and Mycoplasma spp.

An infection caused by Chlamydia may cause redness, discharge, and excessive tears. Bacterial conjunctivitis caused by Chlamydia is treated with antibiotics. All cats in a household may need to be treated, as some cats who have it do not show any symptoms.

Mycoplasma is a natural inhabitant of every cat’s eye, so infection happens when there is an overgrowth of that bacteria. Treatment for the Mycoplasma infection is similar to that of Chlamydia: antibiotics. The goal in treating Mycoplasma conjunctivitis is to reduce the overgrown number of bacteria back down to a normal level because it is impossible to eliminate them entirely.

Who gets bacterial conjunctivitis?

Both cats and dogs can get conjunctivitis caused by bacteria or viruses. Conjunctivitis is the most common cat eye disease, and most cats will develop the infection once in their life. All breeds can develop infection, and juvenile and young adults are more likely to get it.

Clinical Signs of Either Form

• Red eyes
• Eyelid redness
• Eyelid swelling
• Excessive blinking
• Eye discharge
• Eye crusting
• Tearing
• Squinting
• Scratching the face to relieve irritation

 Diagnosis

A physical exam of the eye by your veterinarian is the first step, as this will rule out other causes of redness, such as trauma.  The easiest diagnosis is a cytology test, which means looking at scrapings of the irritated tissue under a microscope. If a lot of bacterial and inflammatory cells are seen, it is likely that the eye is infected. Checking scrapings is a quick test for infection that can be done at the clinic during your appointment. PCR is another test that can be used to detect the specific organism causing the infection, but typically needs to be sent out to a lab.

Treatment

Antibiotics are used to treat bacterial conjunctivitis, mainly doxycycline.  Mycoplasma and Chlamydia are the most common bacteria involved in eye infections, and doxycycline is effective against both.

Prognosis

Treatment should clear the signs and infection. However, if the eye still looks infected or just doesn’t look normal, contact your veterinarian. The ongoing bacterial infection may also be caused by an unknown factor that needs to be treated before the eye infection can be cured.

Prevention

There is a vaccine for Chlamydophila felis, but it is not required and is usually only suggested if your cat is at a higher risk for an infection or in a multi-cat household. There is no vaccine for a Mycoplasma spp. infection because it is normally in the eye anyway.

The key to preventing infection is separating sick cats away from the unaffected ones and bringing them to your veterinarian at the first sign of infection.