The most common cause of conjunctivitis in cats is due to cat flu. There are a number of pathogens associated with cat flu, with the most common being feline herpes virus, feline calicivirus and a chlamydial (bacterial) infection. Herpes viruses re-emerge when cats are stressed or immuno-compromised (eg. have feline AIDS) so often there is an under-lying illness or sometimes a behavioural issue (eg. an anxious cat having a new cat introduced into the household).
Cat flu is transferred in infected discharge from the eyes and nose. It is very contagious and can be contracted by either direct contact between cats or via infected food bowls or bedding. Pure breed and shelter cats are more likely to experience cat flu as they have a higher likelihood of exposure.
Conjunctivitis can also be seen when cats have reactions to various allergens such as plant pollens, fleas and foods. Foreign bodies, such as grass seeds as well as cat scratches to the surface of the eye, can lead to corneal ulcers which then results in conjunctivitis. The loss of the supporting fat pad behind the eye in cats that lose a lot of weight due to illness, can cause the eyeball to sink into the eye socket and the eyelids to roll under. This causes the fur to rub on the surface of the eye causing irritation which can lead to conjunctivitis.