Cause can vary from infectious to environmental irritants. Commonly seen in summer months, the cause can often be from insect bites - specifically flies. The eye will appear red, swollen - sometimes almost swollen shut - and will display a discharge. This usually resolves rapidly with the application of a topical ointment prescribed by your veterinarian. If this doesn't cure it, the cause may be due to some foreign body, environmental irritant, or parasite and should be diagnosed by your veterinarian.