pedunculated papillomatous or polypoid lesions in the conjunctiva, which may be associated with similar lesions in the nose and nasopharynx.

Septate Filamentous Fungi

A significant majority of external ocular mold infections are caused by septate fungi. Fusarium species (eg, Fusarium solani and Fusarium oxysporum) are common pathogens encountered in warm, humid environments as a cause of fulminant keratitis. Among the genera that have been isolated from the external eye are Aspergillus, Alternaria, Curvularia, Paecilomyces, Scedosporium, and

Phialophora. Some filamentous fungal corneal infections are more indolent. Most cases of oculomycosis occur following trauma with vegetative matter and, less frequently, with contact lens usage.

Nonseptate Filamentous Fungi

Nonseptate filamentous fungi include the Mucor, Rhizopus, and Absidia species in class Zygomycetes, order Mucorales, family Mucoraceae. These ubiquitous fungi are an uncommon cause of external ocular infections, but they can cause life-threatening infections of the paranasal sinuses, brain, and orbit in immunocompromised patients, with particular predilection for those with failure of normal phagocytic responses due to acidosis from diabetes mellitus or renal failure. Fungal invasion of blood vessels results in ischemic necrosis (blackened char) of affected tissues.

Pneumocystis jiroveci (previously Pneumocystis carinii) was formerly classified as Protozoa, but gene sequencing has placed the organism firmly in the Fungi kingdom. P carinii is an important cause of choroiditis in HIV-infected individuals, the incidence of which is declining in the era of antiretroviral therapy.

Thomas PA, Geraldine P. Oculomycosis. In: Collier L, Balows A, Sussman M, eds. Topley & Wilson’s Microbiology and Microbial Infections. 10th ed. Medical Mycology, ed Merz WG, Hay RJ. London: Hodder Arnold; 2005:chap 16.

Parasitology

Protozoa

Acanthamoeba species are protozoa (unicellular eukaryotes) that can cause an isolated infection of the human cornea as their primary disease in humans. Other conditions have been described, such as disseminated dermatitis, visceral infestation, and encephalitis unrelated to ocular disease. The Acanthamoeba life cycle includes a motile trophozoite form (15–45 μm in diameter) and a dormant cyst form (10–25 μm in diameter) (Fig 5-7). The cysts are double-walled and very hardy, resistant to most environmental extremes and toxins, including chlorine. Classification of Acanthamoeba species has been based on morphology, but molecular methods are more accurate and increasingly utilized.

Figure 5-7 Acanthamoeba cyst. (Diff-Quick stain ×100.) (Courtesy of Elmer Y. Tu, MD.)

Microsporidia are obligate intracellular parasites and have recently been linked to fungi. Their spores enter eukaryotic cells through a polar tube that opens a hole in the eukaryotic cell membrane. Growth and differentiation of the sporoplasm result in the formation of intracellular spores that may be liberated by lysis of the host cell. Of the phylum Microspora, the following genera have been implicated in human infection: Nosema, Encephalitozoon, Pleistophora, Vittaforma (formerly Nosema corneum), Trachipleistophora, Enterocytozoon, and unclassified microsporida.

Toxoplasma gondii causes one of the most common parasitic infections of humans and is a common cause of chorioretinitis (see BCSC Section 9, Intraocular Inflammation and Uveitis). Cats shed oocysts in their feces after ingestion of T gondii. Oocysts may be ingested by human-food animals such as swine; the cyst-containing meat of these animals is then eaten by humans. Alternatively, cysts may be ingested directly by human contact with cat feces or feces-contaminated water. Transplacental transmission to the fetus of T gondii tachyzoites can result in a devastating fetal infection. See BCSC Section 6, Pediatric Ophthalmology and Strabismus, for discussion of the consequences of maternal transmission of toxoplasmosis.

Leishmania species

Cutaneous leishmaniasis is transmitted by the bite of its vector, the female sandfly, in endemic areas of tropical Asia, Africa, and Latin America. Leishmania organisms hide within the phagolysosomal system of macrophages. An infected eyelid ulcer may become granulomatous. Scrapings or biopsy material can show intracellular parasites by Giemsa or immunofluorescent stains. The parasites can sometimes be isolated on blood agar or insect tissue culture medium.

Helminths

Onchocerciasis is caused by onchocercal filariae transmitted by the bite of the blackfly (Simulium), which lays its eggs on vegetation in fast-flowing rivers (hence the common name river blindness) and is endemic in parts of sub-Saharan Africa, the Middle East, and Latin America. Microfilariae penetrate the skin and mature in nodules at the site of the bite for approximately 1 year, after which mating produces microfilariae offspring (≈300 μm in length)—up to 1500 a day per female (100 cm in length). These worms can live as long as 15 years in the human host; thus, diagnosis can be made