The eye is constantly exposed to the outside world and is therefore vulnerable to a variety of infections and diseases. Parasitic eye infections are not common in America, and are very difficult to diagnose and treat. Acanthamoeba, for example, was a very rare parasite before the introduction of contact lenses. Today infections are often associated with contact lenses, which can harbour parasites and pass them into the eyes through tears in the ocular tissue. A number of parasites can infect the eyes, including nematodes, protozoa, trematodes and cestodes. Sanitation and hygiene, along with proper care of contact lenses, will reduce the risk of a parasitic eye infection.
Light sensitivity, or photophobia, is a common symptom of many parasitic eye infections. Contact lens wearers may notice heightened sensitivity when they are not wearing their contact lenses. Sensitivity can range from mild aggravation in bright lights to severe aversion to any light at all. It often causes migraine headaches as a result of squinting and straining the eyes.
Many parasitic eye infections cause impaired vision in patients. Ocular toxoplasmosis, for example, can cause permanent scarring of the eye tissue, which can result in impaired vision or even blindness in extreme cases. Acantamoeba, malaria and microsporidiosis can also cause impaired vision. Onchocerciasis, which is endemic in part of South Africa, South America and the Arabian peninsula, causes blindness in 5 per cent of the people it affects.
Although parasites are microscopic and invisible to the naked eye, some of the symptoms of eye parasites can be observed without a microscope. Strabismus, for example, is one symptom of toxoplasmosis, and causes the eyes to appeared crossed. Acanthamoeba can cause consistent redness of the eyes, or in rare cases can lead to the development of a white ulcer around the iris, or coloured part, of the eye. Loss of eyelashes or the regular appearance of white flakes on the upper lashes may indicate a demodex infection, which affects the eye follicules.