Keywords

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Synonyms

Parasitic diseases; Parasitic infections

Definition

Parasitic disease of the eye includes any infection, infestation, or inflammation (e.g., neuro- and chorioretinitis, uveitis, keratitis) caused by any number of organisms (ranging from protozoans to helminths).

Structure

The manifestations of parasitic infections can either stem from systemic infection or be ocularly localized. Infections are wide ranging in their presentation. Detailed below are some of the most common etiologic agents and their ocular manifestations.

Clinical Relevance

  1. (A)

    Parasitic retinitis and its causative organisms.

    Toxoplasmosis (causative agent: Toxoplasma gondii) produces a necrotizing retinitis and is the most common cause of infectious retinitis in immunocompetent patients. Common signs include decreased vision, photophobia, floaters, vascular sheathing, full-thickness retinal necrosis, and a fluffy, yellow-white retinal lesion with overlying vitreous reaction. Toxoplasmosis infection is categorized as either acquired (from eating undercooked meat) or congenital (from transplacental transmission). T. gondii is an obligate intracellular protozoan, and its most definitive host is domesticated felines, though it can also infest other mammals (principally livestock) as well as humans. Infection is initiated by ingestion of the oocysts from feces of the definitive host in contaminated food or water or from the ingestion of tissue cysts (which include bradyzoites), that are the product of infection in the animal itself. Tissue disruption is caused by the active form of the protozoan, called tachyzoites.

    Complications of toxoplasma infection include direct involvement of the macula or secondary optic nerve head involvement due to a juxtapapillary lesion. Less common complications include primary optic nerve head involvement, which can present as anterior ischemic optic neuropathy or if the inflammatory focus of the primary infection can induce occlusion of a larger blood vessel, triggering ischemia. Cases of choroidal neovascularization, serous retinal detachment, tractional retinal detachment, and macular edema have all been reported in conjunction with toxoplasma.

    Treatment is geared toward reducing the acute inflammation and the risk of permanent visual loss by reducing scarring. Small lesions do not usually require treatment if they are peripherally located, though severe or posterior infections are treated for up to 6 weeks with pyrimethamine and trisulfapyrimidine, and patients are usually given leucovorin calcium to prevent bone marrow suppression. Clindamycin is an alternative therapy. Complications (including increased intraocular pressure) are managed as they would be otherwise.

    Taenia solium is a common tapeworm that infects swine and humans. Like many cestodes, its life cycle begins in humans with the ingestion of eggs or gravid proglottids. Larvae migrate into the host tissue to cause cysticercosis. It can occasionally cause conjunctivitis but usually invades the retina, choroid, or vitreous. Diagnosis is made from a positive complement fixation or precipitin test or by demonstrating the presence of the organism in the gastrointestinal tract. Eosinophilia is usually present as well.

    Treatment for T. solium requires excision of the ocular lesion and niclosamide (or praziquantel) for the tapeworm infection.

    Parasitic conjunctivitis and its causative organisms (Garcia et al. 2013).

    Pthirus pubis infection is caused by pubic lice and may infest the cilia and the margins of the eyelids. A toxic follicular conjunctivitis in children and a papillary conjunctivitis in adults is caused by the release of an chemical irritant. Intense itching is usually present. Diagnosis is confirmed by finding either the adult organism or oval-shaped nits.

    Treatment for P. pubis includes lindane or pyrethrins, applied to both the pubic area and the lash margins after removal of the nits. All clothes and accessories that may have come in contact should be washed carefully.

    Ophthalmomyiasis is an infection caused by the larvae of flies (common species include Musca domestica, Fannia, and Oestrus ovis), which can invade either necrotic or healthy tissue. External wounds and ingestion are the most common routes of transmission. The infection usually occurs on the surface of the eye, though cases have been reported with infection in intraocular tissues.

    Treatment is the removal of the larvae after the application of a topical anesthetic (Kean et al. 1991).

    Thelazia californiensis is a roundworm that usually inhabits the eyes of domesticated dogs, but also infects a range of mammals (feral and domestic). Accidental infection through contact has been reported. Treatment is the removal of the larvae after the application of a topical anesthetic.

    Loa loa is a common eye worm in sub-Saharan Africa and resides in the connective tissue of primates. A vector fly (such as the horse or mango fly) ingests microfilariae from an infected human host, which then move to the fat body of the insect and develop into larvae. The infective form of the larvae travels to the proboscis and is then transmitted to an uninfected host upon a bite. The infective form matures into microfilariae which disseminate in various host tissues. Diagnosis is made by identifying the organism in a blood smear, usually corroborated by mild eosinophilia (Cook et al. 2009). Treatment is diethylcarbamazine.

    Ascaris lumbricoides is a roundworm that can cause a very painful and toxic conjunctivitis from the exposure to infected tissue of other animals and must be treated by rapid and thorough irrigation of the conjunctiva (Gunn and Pitt 2012).

    Trichinella spiralis is a nematode parasite and more commonly the cause of trichinosis, which can be asymptomatic or cause general enteric symptoms. It completes its life cycle in a single host, beginning with the ingestion of cysts in infected meat. The larvae in these cysts are released in the stomach and migrate to the intestines, where they reproduce as anaerobic or facultative anaerobes. The larvae then gain access to the circulation and migrate throughout the host tissues and can cause myocarditis or encephalitis as well as infection of the muscle and cutaneous tissue. Most patients have a chemosis (a pale swelling over the lateral and medial rectus muscles) which may last a week or more (Gunn and Pitt 2012).

    Schistosoma haematobium is more commonly the cause of schistosomiasis (bilharziasis), which is endemic in the regions bordering the Nile River. It is a trematode (flatworm) that uses snails as its intermediate host. Infective free-swimming larval cercariae burrow into the human skin and enter the bloodstream and migrate to the liver. They then migrate to the urinary bladder to reproduce and be further disseminated. S. haematobium infection results in a characteristic granuloma that has lymphocytes, plasma cells, giant cells, and eosinophils surrounding the ova’s various different stages. Conjunctival lesions are similar to Trichinella (Cook et al. 2009).

    Treatment of choice includes praziquantel or antimonials and excision of the granuloma.

Cross-References