Fungal Eye Diseases
Only a few fungal species are regularly implicated in eye infections, and they most often manifest after eye injuries or surgery. Aspergillus, a genus of primarily asexual molds that are found ubiquitously in the environment, and Fusarium, a genus well known for plant pathogens, may both infect the outer layer of the eye during or immediately following superficial injuries. Species of Candida are increasingly recognized as causative agents of endophthalmitis, an inflammation of the interior of the eye, following corneal surgery. Common symptoms of fungal eye infections are similar to bacterial infection and include blurred vision, discharge, pain, redness, and light sensitivity. Treatment with topical antifungals as eye drops is effective for infections of the conjunctiva and outer layer of the eye. Natamycin is a common option for Aspergillus and Fusarium. Treatment of infections deeper in the eye requires oral or intravenous antifungals such as amphotericin or voriconazole. In rare cases, untreated or unresponsive infections may require surgery or eye removal to clear the infection.
The fungus Histoplasma capsulatum is commonly found in the soil of the Ohio and Mississippi River valleys of the central and eastern United States. It exists in two forms, a yeast and a filamentous form. It is the yeast that is capable of causing both systemic and ocular disease in human hosts. Primary infection occurs when the yeast spores are inhaled into the lungs, and ocular disease is thought to arise when the lung infection travels through the blood to the choroidal layer of the eye. The choroid is the ocular tissue layer underneath the retina. Within the choroid, infection results in inflammation and neovascularization (new blood vessel growth), which causes bleeding and decreases in visual acuity. Advanced disease results in permanent subretinal scarring and vision loss. Treatment involves early recognition, oral or periocular steroids, and laser photocoagulation to eliminate neovascularization.
Parasitic Eye Diseases
Demodex mites, which normally live harmlessly on the eyelid margin, contribute to blepharoconjunctivitis (inflammation of the eyelids and conjunctiva) in people, and the mechanism of this infection is not completely understood. Blepharoconjunctivitis is an irritation and inflammation of the eyelids (blepharo-) and conjunctiva. Some people will develop an inflammatory response to the mite along their eyelid margin, which leads to crusting, itching, and redness of the eyelids and eyes. Such inflammation has been shown to respond to vigorous eyelid hygiene with dilute tea tree oil or over-the-counter preparations of dilute hypochlorous acid.
Just as on the skin, infestation of the eyelids and eyelashes is possible with the louse arthropod Pthirus pubis, known as phthiriasis palpebrarum. This pubic louse is usually acquired through direct sexual contact or indirectly through shared towels or clothing. Though rare, infestation of the eyelashes can occur from direct extension of the head or body louse Pediculus humanus as well. Lice can be manually removed but are usually treated with a pediculicide. Any ointment can be applied to smother the lice, but it must be applied twice daily for 10 days to be effective since the incubation period of lice is 7–10 days.
Acanthamoeba is a protozoan found in fresh water and soil. They exist either in a motile or dormant cyst form. Acanthamoeba causes amebic keratitis, or corneal infection, and the majority of cases are associated with contact lens use. Contact lenses should always be removed at the end of the day and placed into designated contact lens cleaning solutions. However, if contact lenses are improperly sanitized, for example, by placing them in or rinsing them with tap water or by placing them in homemade saline contact lens solution, then the wearer is at high risk for contracting Acanthamoeba. The protozoan adheres to the epithelium of the cornea and invades the corneal stroma, or the layer underneath the epithelium. People with this infection present to their eye doctor with severe corneal pain, often out of proportion to the findings on examination. They are extremely photophobic, or light sensitive. On exam, a ring-shaped whitish infiltrate is seen, usually in the central cornea. The protozoan classically causes inflammation of the corneal nerves, called radial perineuritis. The key to treatment is prompt diagnosis, but this is often delayed because the clinical features of the disease can overlap with other corneal infections, such as those caused by the herpes viruses. Treatment involves debridement, or removal, of infected corneal tissue and three to four months of antiamoebic therapy. Medication regimens include a combination of hexamidine, polyhexamethylene biguanide, neomycin, and voriconazole.
Toxoplasmosis is the most common cause of infectious inflammation of the retina and the choroidal layer under the retina of the eye. The parasite Toxoplasma gondii, which is a single-celled intracellular protozoan distributed worldwide, is the causative agent, and infection in immunocompromised individuals such as AIDS patients can lead to encephalitis. The common household cat is the definitive host of T. gondii, while humans and other animals are intermediate hosts. T. gondii exist in three forms: a cystic form that lives in the soil, an infectious form, and a tissue cyst, which is a latent form. The soil form of cysts reproduce sexually and are found in the intestinal mucosa of cats, where they are shed in feces and contaminate the environment. These cysts can then be ingested by intermediate hosts (humans) or reingested by cats. Human infection can also be acquired by ingesting undercooked, infected meat or contaminated water, fruit, or vegetables. The infectious form travels within the circulatory system and can invade all host tissues. However, in immunocompetent human hosts their replication ceases, and most organisms are eliminated. A small number remain in the dormant state as tissue cysts. In some human hosts T. gondii causes robust inflammation of the retina, choroid, and vitreous. Vision is blurred and hazy. Complications of this infection include cataracts, retinal detachment, and neovascularization, or abnormal blood vessel growth, plus retinal scarring. In immunocompetent hosts the infection resolves on its own in six to eight weeks. However, treatment is given to limit progression and scarring. It involves a medication regimen of three drugs: pyrimethamine, sulfadiazine, and prednisone.Eye diseases can also be caused by helminths, a general term referring to parasitic worms. Loiasis, or eye worm infection, is a nematode (roundworm)-caused disease in Africa affecting skin and eyes. The Loa loa worm penetrates the skin when a person is bitten by an African deer fly or mango fly carrying the worm larvae. Once inside, the larvae migrate through the subcutaneous tissue until they mature and produce offspring, which can be picked up when another fly bites to continue the life cycle. Occasionally, maturing worms will migrate across the surface of the eye. The worms are visible and painful in the eye but do not cause blindness.