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  • Conjunctivitis is inflammation of the conjunctiva (the tissues lining the inner eyelids and the white portion of the eye).
  • Diagnosis is based on physical examination findings, but fluorescein staining and other tests may be recommended to determine the extent of the problem.  
  • Treatment usually involves applying medication to the eyes; follow-up examinations and diagnostic testing may be recommended.

What Is Conjunctivitis?

Conjunctivitis is the medical term used to describe inflammation of the conjunctiva—the soft tissues lining the inside of the eyelids and the white portion of the eye.  

What Causes Conjunctivitis?

Conjunctivitis can occur as part of an upper respiratory tract infection, a condition that resembles a common cold. It can also be associated with a localized problem that causes trauma to or irritation of the eyes, such as certain viral, bacterial, and fungal infections. Other causes include:

  • Airborne irritants, such as cigarette smoke, dust, and perfumes
  • Systemic illnesses (illnesses that affect the whole body), such as feline herpesvirus, feline immunodeficiency virus (FIV), canine distemper, and bartonellosis (infection with the bacteria that cause “cat scratch disease” in humans)
  • Dry eye (a medical condition characterized by inadequate tear production)
  • Entropion (a malformation of the eyelids that causes the edge of the lids to roll inward; the hairs on the eyelids scrape against the eye and cause irritation)
  • Trauma to the eye, such as a blow

What Are the Clinical Signs of Conjunctivitis?

The clinical signs of conjunctivitis vary depending on the severity of the inflammation. Signs include:

  • Discharge from the eyes (can be pus, watery, or thick, like mucus)
  • Swollen eyelids
  • Red, “bloodshot” eyes
  • Squinting
  • Rubbing the eyes with a paw or against other objects, such as furniture or the floor

If the conjunctivitis is severe, permanent damage to the cornea (the clear covering on the surface of the eye) can occur. This may result in blindness or require surgery to remove the eye to prevent further pain, inflammation, and infection.

How Is Conjunctivitis Diagnosed?

The medical history and physical examination findings can provide valuable information for your veterinarian. The medical history may include trying to determine how long the conjunctivitis has been going on and whether any other signs of illness have been observed. Physical examination findings may reveal evidence of underlying illness. For example, a cat with an upper respiratory tract infection may have a runny nose, sneezing, and a fever in addition to conjunctivitis.

Diagnosis of conjunctivitis is usually based on physical examination findings. If the pet is squinting because the eyes are painful, your veterinarian may begin the examination by applying a drop of liquid topical anesthetic directly to the eye. This is not painful, and after a few minutes, it makes the surface of the eye numb so the examination can proceed. During the examination, your veterinarian will likely look for foreign material, wounds, or other causes of conjunctivitis. Entropion can also be diagnosed during the physical examination.

While examining your pet’s eyes, your veterinarian may apply fluorescein stain to the eye. Fluorescein is a green-tinted dye that fluoresces (glows) under blue light. If the surface of the cornea is intact, the fluorescein dye will not stick to the eye. However, if there is a scratch, ulcer, or wound on the cornea, the dye adheres to the defect and can show your veterinarian where and how serious the injury is. Fluorescein staining is not painful and can provide valuable information about the condition of your pet’s eye.

If your veterinarian suspects that the conjunctivitis may be caused by dry eye, he or she may recommend a test to determine if tear production is adequate. Similarly, if a systemic illness (such as FIV) is suspected, blood testing or other diagnostic tests may be recommended.   

How Is Conjunctivitis Treated?

Most cases of conjunctivitis are treated with drops or ointments applied directly to the eyes. If the conjunctivitis is associated with another illness, like an upper respiratory infection, antibiotics or other medication given by mouth may also be recommended.  In many cases, the eye starts looking better after only a few treatments. However, all medications should be given as directed for the full course of treatment.

If the conjunctivitis is associated with entropion, surgery may be recommended to stop the problem from recurring. Similarly, if a pet has dry eye, long-term management may be recommended to control the condition.

Your veterinarian may recommend recheck exams during the course of treatment to monitor how well the condition is responding to therapy. Notify your veterinarian right away if your pet’s eye begins to look worse or you see other signs of illness.