Bacterial conjunctivitis

Owners notice squinting, frequent blinking and discharge collecting at the inner corner of the eye. The conjunctiva is red and swollen, and the discharge ranges from watery to thick and dark or purulent; mild discomfort, tearing and light sensitivity are common. In cats the disease may begin in one eye and spread to both, which suggests an infectious cause, and chemosis can be marked. Chlamydial conjunctivitis in cats tends to be chronic.

Bacterial conjunctivitis is inflammation of the conjunctiva, the membrane lining the eyelids and covering the front of the eyeball, caused by bacterial infection. In dogs it is usually secondary: trauma, dry eye, inflamed tear sac, eyelid deformities or, in brachycephalic breeds, incomplete lid closure and poor tear spread let resident or opportunistic bacteria multiply, most often Staphylococcus pseudintermedius, Escherichia coli and Corynebacterium. In cats the main bacterial agents are Chlamydia felis, which replicates inside conjunctival epithelial cells, and Mycoplasma; bacteria also invade secondarily after feline herpesvirus damages the surface. Inflammation dilates conjunctival vessels, swells the tissue (chemosis) and increases discharge, which turns mucopurulent when bacteria are involved.

In dogs, risk rises with poor immune status, living environment, standard of care and humidity; brachycephalic breeds and long-haired breeds whose hair or lashes irritate the eye are predisposed. In cats, infectious conjunctivitis spreads readily in multi-cat households and shelters by direct contact with eye discharge and contaminated equipment. Chlamydia felis mainly affects cats under one year old; it has been found in 12-20% of cats with eye or upper respiratory signs against about 3% of healthy cats, and recovered cats can remain carriers after discharge stops. Newborn kittens may develop conjunctivitis with thick discharge.

Viral conjunctivitis (feline herpesvirus, calicivirus in cats), allergic conjunctivitis, environmental irritants, foreign bodies, dry eye, tear sac inflammation, eyelid and lash abnormalities and parasites all produce a red, discharging eye. One-eyed disease suggests a local cause such as a foreign body or dacryocystitis, whereas spread to both eyes suggests infection. Appearance alone is not diagnostic: history, Schirmer tear test, conjunctival scrapings, bacterial culture with susceptibility testing and occasionally biopsy separate these causes.

Conjunctiva, eyelids, tear film and nasolacrimal apparatus (tear sac), cornea

Usually favourable: many feline cases resolve even without medication and most respond to topical antibiotics. Antibiotics alone may fail if an underlying cause persists; herpesvirus carriers relapse, and neglected cases can progress to blindness.