Eyeworm disease in horses is caused by the eyeworm (Thelazia spp.) parasite, which is transmitted by face flies. Thelazia lacrymalis is usually the parasite associated with most occurrences. The symptoms associated with eyeworm disease are sometimes absent or can vary from mild to severe conjunctivitis (pink eye) or inflammation of other areas of the eye including the cornea, tear ducts, tear sacs and eyelid.