According to the Merck Manual for Pet Health, a dog eye sty is an infection of a gland near the eye. This is considered a short-term, rather than a long-term, medical condition.
British veterinarian site Vetstream reports that a sty looks like a small, raised, red bump on the eyelid. It is painful if touched. There is sometimes a discharge from the eye. A sty is a bacterial infection caused by Staphylococcus aureus. To diagnose a sty, a vet conducts an examination and takes a bacterial culture. Vetstream indicates that treatment for a sty is usually warm compresses and antibiotics; however, a sty sometimes resembles a more serious infection, so it needs evaluation by a veterinarian.