Conjunctivitis is the most common reason for an otherwise healthy dog's eye to become swollen shut, according to WebMD. Conjunctivitis is characterized by swollen, puffy eyes, a discharge of mucus and pus, and thick secretions that cover and cake onto the eyelids. If there is no discharge from the eye, allergies, a systemic disease such as distemper, or a foreign body in the eye can cause swelling.
When a dog's eyes are infected with conjunctivitis, it is important to flush them up to four times a day using an over-the-counter saline eyewash to flush the bacteria out of the eyes, according to WebMD. While this treatment is effective in most cases, some serious cases require prescription topical antibiotics or corticosteroids to cure.
It is important to treat eye swelling or any other symptoms relating to eye health quickly, as a delay in treatment can lead to blindness, says WebMD. If there are no symptoms of conjunctivitis, visit a veterinarian promptly to determine the cause of the dog's eye swelling. Other serious problems such as keratitis, uveitis or glaucoma should also be discussed when a dog is squinting and shutting his eyes, with or without swelling, as these are symptoms of these diseases.