An ulcer is an open, often deep sore on the skin's surface. Ulcers on the eyelid can affect both humans and animals, and they are caused by an array of ailments.
Eyelid Skin Cancer
One cause of eyelid ulcers is eyelid skin cancer, which according to the Midland Eye Website, is caused by "excessive exposure to sunlight." The most common type of eyelid cancer is basal cell carcinoma, which is also called a rodent ulcer.
Syphilis of the Eyelid
Syphilis of the eyelid, although rare, can cause eyelid ulcers. Syphilis is a sexually transmitted disease, and it can be passed to the eyelid by direct contact with an infected person.
Eyelid Ulcers in Dogs
Dogs are also susceptible to eyelid ulcers. Dogs most commonly develop eyelid ulcers through contact with sand flies carrying the leishmania parasite, which can then cause leishmaniasis in dogs.
Leishmaniasis manifests itself, according to the Go Pets America Website, as "red pimples at the lower or upper eyelid margins." This is most problematic in Europe, where the Center for Disease Control reports that up to 7 million dogs are at risk of leishmaniasis.
If untreated, leishmaniasis eventually causes blindness. To combat this, medicines are available to treat leishmaniasis, although since they are largely ineffective, a vaccine is also being developed.