Clin Microbiol Rev. 2000 October; 13(4): 662–685.
Copyright © 2000, American Society for Microbiology
Fungal and Parasitic Infections of the Eye
Section of Infectious Diseases, Veterans Affairs Medical Center, Kansas City, Missouri1; University of Kansas School of Medicine, Kansas City,2 and Lawrence Memorial Hospital, Lawrence,3 Kansas; and Department of Ophthalmology, Washington Hospital Center, Washington, D.C.4
*Corresponding author. Mailing address: Research, Veterans Affairs Medical Center, 4801 Linwood Blvd., Kansas City, MO 64128. Phone: (816) 861-4700, ext. 6713. Fax: (816) 922-4687. E-mail: (816) 861-4700, ext. 6713. Fax: (816) 922-4687. E-mail: email@example.com
The unique structure of the human eye as well as exposure of the eye directly to the environment renders it vulnerable to a number of uncommon infectious diseases caused by fungi and parasites. Host defenses directed against these microorganisms, once anatomical barriers are breached, are often insufficient to prevent loss of vision. Therefore, the timely identification and treatment of the involved microorganisms are paramount. The anatomy of the eye and its surrounding structures is presented with an emphasis upon the association of the anatomy with specific infection of fungi and parasites. For example, filamentous fungal infections of the eye are usually due to penetrating trauma by objects contaminated by vegetable matter of the cornea or globe or, by extension, of infection from adjacent paranasal sinuses. Fungal endophthalmitis and chorioretinitis, on the other hand, are usually the result of antecedent fungemia seeding the ocular tissue. Candida spp. are the most common cause of endogenous endophthalmitis, although initial infection with the dimorphic fungi may lead to infection and scarring of the chorioretina. Contact lens wear is associated with keratitis caused by yeasts, filamentous fungi, and Acanthamoebae spp. Most parasitic infections of the eye, however, arise following bloodborne carriage of the microorganism to the eye or adjacent structures.
This is a comprehensive review of the fungal and parasitic diseases of the eye. Numerous fungi and parasites infect the eye either by direct introduction through trauma or surgery, by extension from infected adjacent tissues, or by hematogenous dissemination to the eye. The majority of the clinically important species of fungi and parasites involved in eye infections are reviewed in this article. The fungi are discussed in relation to the anatomical part of the eye involved in disease, whereas parasites are discussed by the diseases they cause. Emphasis has been placed on literature published within this decade, but prior noteworthy reviews and case reports are included. A glossary of the ophthalmologic terms used is provided at the end of the paper (Appendix A). We suggest that the works of Beard and Quickert (26a) and Snell and Lemp (252a) be consulted as references concerning the anatomy of the eye.
ANATOMY OF THE EYE AND ITS RELATIONSHIP TO INFECTIOUS PROCESSES
Orbits, Their Soft Tissue Contents, and Adjacent Structures
The orbits are pear-shaped bony cavities that contain the globes, extraocular muscles, nerves, fat and blood vessels (Fig. (Fig.1,1, left). The walls of the orbit are comprised of seven bones. The periosteal covering of the orbital bony cavity fuses anteriorly with the orbital septum and posteriorly with the dura mater. Abscesses can localize in the subperiosteal space. The roof, medial wall, and floor of the orbit separate it from adjacent paranasal sinuses, including the maxillary, frontal, ethmoid, and sphenoid sinuses. The paranasal sinuses arise from and drain into the nasal cavity. Thus, an intimate anatomical relationship exists between the orbit and the adjacent paranasal sinuses, and the latter may be the source of an orbital infection (Fig. (Fig.1,1, right).
The thinnest bony walls of the orbit are the lamina papyracea, which cover the ethmoid sinuses. They are commonly involved in any fracture of the orbit from force to the periorbital area. As a result of fracture, sinus microbiota has direct ingress to the orbital tissues. Infections of the ethmoid sinus in children commonly extend through the lamina papyracea (without fracture), causing orbital cellulitis. The lateral wall of the sphenoid is also the medial wall of the optic canal. Therefore, infections of the sphenoid sinus may impinge on the optic nerve, resulting in visual loss or visual field abnormalities.
There are several important communications through apertures in the bony orbit to adjacent structures, including the superior and inferior orbital fissures, the lacrimal fossa and nasolacrimal duct, and the optic canal. These apertures may serve as a direct passage for an infectious process between the orbit and surrounding structures.
Blood Supply of the Orbits
The blood supply to the orbit is primarily through the ophthalmic artery and its branches. The majority of orbital venous drainage is via the superior ophthalmic vein, which courses through the superior fissure to the cavernous sinus. The cavernous sinus is a venous plexus located posterior to the apex of the orbit. As the primary venous system receiving orbital drainage, the cavernous sinus is susceptible to venous thrombosis secondary to direct intravascular extension of infection. Veins from the face and many anterior orbital veins anastamose and become tributaries of the superior orbital vein. Thus, facial infections may lead through these communications to infection of the cavernous sinus, which may be a lethal complication.
Eyelids and Lacrimal System
The eyelids possess two protective anatomical barriers preventing the penetration of pathogens beyond the anterior surface of the globe. The first is the orbital septum, a thin multilayer fibrous tissue that divides the orbit from the eyelid into preseptal and postseptal spaces and serves as a physical barrier to prevent infections from spreading posteriorly into orbital fat. The second is the conjunctiva that is reflected back on itself. This prevents material on the anterior surface of the globe from freely moving posteriorly along its surface.
The lacrimal system is comprised of the lacrimal gland, accessory glands, and the excretory system. The lacrimal gland secretes tears that pass down over the cornea and enter the lacrimal excretory system at the puncta. The puncta drains tears into the canalicular system that leads to the lacrimal sac. Tears in the lacrimal sac drain to the nose. The lacrimal system thus forms a direct passage from the anterior ocular adnexa to the nasal cavity. With total nasolacrimal duct obstruction, infected material in the sac may reflux onto the ocular surface.
The Globe, Including the Sclera and Choroid
The adult human eye is approximately 24 mm in anterior-posterior length and is 6 mm3 in volume. The basic structure of the globe consists of three concentric layers or tunics. The outermost tunic is comprised of the cornea and sclera. The middle tunic is the uveal tract. It consists of the choroid, ciliary body, and iris. The innermost tunic is the retina (Fig. (Fig.1,1, left).
The posterior outer layer of the globe is the sclera, which is comprised of collagen and ground substance. The scleral width ranges from 0.3 to 1.0 mm. The sclera is essentially avascular except for superficial episcleral vessels and the intrascleral vascular plexus. The choroid is a vascular tunic that comprises the posterior portion of the uveal tract. The purpose of this highly vascularized tissue is to provide nutritive support to the outer layer of the retina. The blood flow and oxygenation of the choroid are very high compared to the other tissues in the body. Because of these qualities, the choroid may serve as a fertile site for the proliferation of hematogenously spread pathogens.
Anterior Chamber, Aqueous Humor, Cornea, and Iris
The anterior chamber is a space bordered anteriorly by the cornea and posteriorly by the iris diaphragm and pupil and is filled with aqueous humor (Fig. (Fig.1,1, left). The aqueous humor, produced by nonpigmented ciliary epithelium in the posterior chamber, passes through the pupillary aperture into the anterior chamber, where it exits. The cornea is avascular, and its stroma is composed of highly organized collagen fibrils. A tear film comprised of three layers covers the anterior surface of the cornea.
The iris is the anterior extension of the ciliary body that forms a contractile diaphragm in front of the anterior surface of the lens. It separates the anterior and posterior chambers. The central aperture in the iris is the pupil, which constantly changes size in response to light intensity.
Posterior Chamber, Lens, and Vitreous Humor
The posterior chamber is bordered anteriorly by the iris diaphragm and pupil and posteriorly by the lens and zonules (Fig. (Fig.1,1, left). The lens is an avascular biconcave crystalline structure centrally located in the posterior chamber. It continues to grow throughout life, receiving nutrition from the aqueous and vitreous humors.
The vitreous is a gel-like substance occupying the posterior segment of the eye. It consists of a collagen framework interspersed with hyaluronic acid. In its normal state, it is optically clear, whereas during intraocular inflammation it may become hazy.
Retina and Optic Nerve
The retina is the innermost coat of the ocular tunics. It is a thin, transparent, net-like membrane that captures light energy. The retina is comprised of 10 layers, with the layer nearest the interior of the globe containing the photoreceptors called rods and cones. The inner half of the retina receives its blood supply from the central retinal artery, and the outer half receives its blood from the choroid.
The inner cell layer axons in the retina exit the globe to make up the optic nerve (Fig. (Fig.1,1, left). This nerve is surrounded by pia mater, arachnoid, and dura mater meningeal coverings, which are direct extensions from the cranial vault. The optic nerves are vulnerable to infectious processes originating both within the cranial vault and within the orbits.
OCULAR DEFENSE MECHANISMS
The surface of the eye is armed with mechanical and immunologic functions to defend itself against a hostile environment. The defense mechanisms are native and acquired, both generalized and specific (8). It is manifestly obvious that exposed portions of the eye possess a remarkable defense against microorganisms. To breach this defense, trauma in some form is usually required.
The eyelids provide mechanical protection of the ocular surface. The lashes initiate the blink reflex to protect against airborne particles or trauma. The cornea is exquisitely sensitive, and tactile stimulation of its surface will also initiate the blink reflex. The lids sweep over the anterior surface of the globe directing tears, debris, microbes, and allergens to the lacrimal excretory system. Lipids secreted by the meibomian glands maintain the stability of the tear film.
The nonkeratinized squamous epithelium of the conjunctiva and cornea serves as a protective anatomic barrier against pathogens. The basement membrane and cellular junctional complexes of the cornea contribute to its impermeability (184). Indigenous ocular flora of the lids and mucosal ocular surface serve a protective function by limiting the opportunity for pathogenic organisms to colonize the surface (89).
The vascular supply to the surface of the eye is a major conduit of the immune defenses. The ocular inflammatory response involves vascular dilation and exudation of immunologically active substances and cells, including macrophages, polymorphonuclear leukocytes, and lymphocytes (167).
Defenses of the Tear Film
The tear film is comprised of three layers: oil, aqueous, and mucous. These layers are produced by the meibomian glands, the lacrimal glands, and the goblet cells of the conjunctiva, respectively. The aqueous layer comprises the majority of the 7-μm-thick tear film. It is produced at a rate of ∼1 μl per min. The tear pH, ∼7.14 to ∼7.82, likely contributes to the neutralization of toxic substances (167). Tear flow mechanically bathes the anterior surface of the eye, preventing the adherence of microorganisms, and flushes allergens and foreign particles into the lacrimal excretory system. The mucous layer of the tear film entraps foreign material, which facilitates its removal (1). For example, the mucin contained in tears preventsCandida spp. from adhering to contact lenses, likely by entrapping the microorganisms (34). The tear film contains several immunologically active substances that participate in both general and specific ocular defense (Table (Table1).1).
Beneath the protective epithelium of the conjunctiva lie a vascular network and lymphoid structures. The conjunctiva-associated lymphoid tissue is subepithelial tissue packed with B and T lymphocytes. B-cell precursors mature when exposed to local antigen, proceed to regional lymph nodes where they transform into plasma cells, and then return via the bloodstream to the conjunctiva, where they produce their specific immunoglobulin A (IgA). Similarly, T-cell precursors are locally sensitized, travel to regional nodes, and then hematogenously return to the conjunctiva to provide cellular defense (46).
The cornea is avascular and possesses limited immune defenses. The two main components are the Langerhans cells (dendritic cells), which modulate B and T lymphocyte activity in the cornea, and immunoglobulins, which are concentrated in the corneal stroma (167). The corneal surface is covered by a glycocalyx associated with a layer of mucous glycoprotein. A subtype of the IgA cross-links with the mucous glycoprotein to cover and protect the anterior surface of the cornea (184). Upon injury, the corneal epithelium may release a thymocyte-activating factor that incites a local immune response to include polymorphonuclear cells, lymphocytes, and fibroblasts (184).
Cellular Immune Response
Langerhans cells are concentrated in the epithelium of the peripheral cornea and conjunctiva but sparse in the central cornea (184). Like macrophages, they possess receptors for immunoglobulins, complement, and antigen. The Langerhans cell recognizes, phagocytizes, and processes certain antigens for presentation via the epithelial surface and stroma (167). Langerhans cells stimulate helper T and B cells that collaborate with other lymphocytes (killer, suppressor T cells) to enlist a strong cellular immune response. During inflammation Langerhans cells migrate toward the center of the cornea and may participate in the secretion or release of inflammatory mediator substances (105). T cells are mainly present in the conjunctival substantia propria, whereas B cells are more concentrated in the lacrimal gland (167).
Polymorphonuclear leukocytes possess the ability to ingest and kill microorganisms by two main pathways. The absence of polymorphonuclear leukocytes is associated with fungemia with Candida, Aspergillus, and Fusariumspp. The oxygen-dependent pathway is based on postphagocytic intracellular production of oxygen radicals (oxidants). The oxygen-independent pathway is based mainly on the function of antimicrobial proteins called defensins. Defensins are peptides that possess broad-spectrum antimicrobial activity in vitro, killing a variety of gram-positive and gram-negative bacteria and some fungi (167), including a wide range of ocular pathogens (60).
FUNGAL INFECTIONS OF THE EYE
Epidemiology of Fungal Eye Infections
Ophthalmologists and optometrists, in particular, and clinicians, in general, must be knowledgeable of the pathogenesis of fungal eye infections. Mycotic eye infections are commonplace. For example, the yeast Candida albicans is the most common cause of endogenous endophthalmitis. Filamentous fungi, such as Fusarium solaniand Aspergillus flavus, may constitute up to one-third of all cases of traumatic infectious keratitis (157). Furthermore, patients with AIDS may contract many different fungal infections of the eye and adjacent structures (Table (Table2).2).
In fungal eye disease, the pathogenesis of the infections is inextricably linked to the epidemiology. Therefore, in discussing the epidemiology of fungal eye infections, it is worthwhile at the outset to state several proposed pathogenetic principles of fungal eye disease. (i) It is likely that sustained fungemia with even saprophytic fungi will lead to endophthalmitis. (ii) At the time of initial infection with some of the dimorphic, pathogenic fungi, such as Histoplasma capsulatum and Coccidioides immitis, an unrecognized fungemia occurs and often leads to endophthalmitis. (iii) The paranasal sinuses, because of their direct communication with the ambient air, harbor saprophytic fungi, which may erode the bony walls of the sinus and invade the eye in certain circumstances, e.g., in a patient with neutropenia. (iv) Trauma, either from vegetable matter or surgery, may introduce saprophytic fungi into the cornea and/or adjacent tissue, giving rise to invasive disease.
The epidemiology of endogenous endophthalmitis reflects both the natural habitats of the involved fungi and the habits and health status of the patients (Table (Table3).3).Candida endogenous endophthalmitis occurs as a direct result of the success of modern medical practice that sustains patients' lives with broad-spectrum antibiotics, indwelling central venous lines, parenteral nutrition, abdominal surgery, and cytotoxic chemotherapy. The recent origin of this disease is established by the fact that Candida endophthalmitis was first recognized clinically in 1958 (275).Candida and Aspergillus spp. also cause endophthalmitis in intravenous drug users. Virtually any intravascular prosthesis or device may become contaminated by bloodborne opportunistic fungi, and fungemia arising from such infection may lead to endogenous endophthalmitis.
Endogenous endophthalmitis occurring as part of disseminated disease with the dimorphic fungi H. capsulatum, Blastomyces dermatitidis, and C. immitis is uncommon. Patients with disease from these fungi have resided in or traveled through the respective areas of endemicity. These are the Ohio and Mississippi river valleys for H. capsulatum; the Lower Sonoran Life Zone for C. immitis, including southern parts of California, Arizona, New Mexico, West Texas, and parts of Mexico and Argentina; and the Southeast and Midwest of the United States for B. dermatitidis. Residence along a waterway may be another important association for exposure to B. dermatitidis (67). H. capsulatum may flourish in bird and bat droppings; therefore, exposure to the fungus may occur through one's occupation, for example, demolition of old bird-infested buildings, or one's hobby, such as camping or spelunking.
Exogenous endophthalmitis, on the other hand, results from trauma to the globe or preceding keratitis. It may also occur as a postoperative complication of lens removal, prosthetic lens implantation, or corneal transplantation. The vast majority of postoperative eye infections are due to coagulase-negative Staphylococcus; however, outbreaks of fungal exogenous endophthalmitis continue to occur episodically. These have been due to perioperative contamination of lens prostheses (204) or contamination of fluids used for irrigation (260) of the perioperative and postoperative eye. Candida species are particularly likely to occur in this setting, and infection may be enhanced by the pre- and postoperative use of topical corticosteroids and antibacterial agents.
Mycotic keratitis is usually caused by filamentous fungi and occurs in conjunction with trauma to the cornea with vegetable matter. In the tropics it is common in male agricultural workers. The fungal genera causing keratitis in the tropics are more diverse and include some, such as Lasiodiplodia theobromae, that do not grow in temperate regions. Eye trauma is the cause of fungal keratitis in temperate areas as well, but the common fungal genera involved are Fusarium, Alternaria, andAspergillus (71, 293). Keratitis caused by yeasts such as the Candida spp. almost always occur in previously abnormal eyes, e.g., in patients with dry eye, chronic corneal ulceration, or corneal scarring.
Bloodborne Infections: Endogenous Endophthalmitis
Endogenous endophthalmitis is uncommon; however, fungi cause this disease more often than gram-positive or gram-negative bacteria. The term endogenous endophthalmitis implies that bloodborne spread of microorganisms to the eye has occurred. Therefore, infection in the eye is the result of metastatic spread of infection from a distant site, for example, infected heart valves or the urinary tract. In this manner the eye becomes the site of numerous microabscesses. This mechanism of infection is to be contrasted to exogenous endophthalmitis (see below), which arises from the direct introduction of a microorganism(s) into the eye during trauma or surgery. Endogenous endophthalmitis is further distinguished from exogenous endophthalmitis by occurring in a greater number of immunocompromised patients, e.g., patients receiving chemotherapy or total parenteral nutrition, or intravenous drug abusers (Table (Table4).4).
Endophthalmitis is recognized clinically by the presence of one or more creamy-white, well-circumscribed lesions of the choroid and retina, often accompanied by inflammatory infiltrates in the vitreous. These lesions can be detected using an ophthalmoscope after dilating the pupils. Often, there is inflammation in the anterior chamber manifested by the presence of a hypopyon. Typical lesions of chorioretinitis are shown in Fig. Fig.2,2, left. Patients complain of eye pain and may have blurred vision or spots in their visual fields. Patients with endogenous endophthalmitis may have positive blood cultures antedating eye symptoms or signs. In the absence of a positive blood culture or characteristic clinical syndrome, aspiration of the vitreous (or biopsy) may be necessary to establish the causative microorganism.
Why the eye is a common end organ target of fungemia is unknown. However, in a rabbit model of C. albicans endophthalmitis, more fungal elements are found in the eye per gram of tissue than are found in the kidneys of the same animals. Since C. albicans is believed to have a marked tropism for the kidneys and endothelium, the great number of organisms in the eye bespeak a tropism for the eye as well (142,143). The candidal lesions in the rabbit are identical to those found in humans demonstrating a focal chorioretinitis (Fig. (Fig.2,2, middle left), with a mixture of granulomatous and suppurative host reactions (76). The infection likely begins in the choroid and progresses anteriorly to the retinal layers (226). This may be related mechanistically to the fact that the outer retinal layers, i.e., those considered to be infected first, receive blood from a high-flow system (150 mm/s), whereas the inner layers receive blood from a low-flow system (25 mm/s). It should be noted that drainage from the retinal layers is entirely through the venous system as there is no lymphatic system serving the inside of the globe.
The most common cause of endogenous fungal endophthalmitis is C. albicans(287). Endogenous fungal endophthalmitis by definition follows fungemia; therefore, it is important to note that Candida species are the fourth most common cause of positive nosocomial blood cultures in the United States, exceeding the number of positive cultures of any single gram-negative bacterial genus (21). It is estimated that some 120,000 patients contract disseminated candidiasis (i.e., candidemia) per year in North America (58), and the usual estimates of the incidence of candidal endophthalmitis in patients with candidemia are around 30% (32, 196; J. R. Griffin, R. Y. Foos, and T. H. Pettit, presented at the 22nd Concilium Ophthalmologicum, 1974); thus, the disease is fairly common. If the definition of chorioretinitis is more stringent, i.e., if nonspecific lesions such as cotton wool spots and retinal hemorrhages are eliminated, the incidence is much less, on the order of 9.3% (70, 226).
The pathogenesis of candidemia remains unknown but is likely multifactorial. There are characteristic clinical features of patients with candidemia, with one or another feature being found in each patient. These include the use of broad-spectrum antibiotics that eliminate competing normal microbiota of the host, the presence of central venous catheters, the administration of total parenteral nutrition, prior abdominal surgery, and/or neutropenia (164). One or all of these factors are sufficient to place a patient at risk for candidemia and, hence, for endophthalmitis. Neutropenia, although a risk factor for candidemia, reduces the incidence of candidal endophthalmitis in the rabbit model (122) and perhaps in patients as well (78). This suggests that the chorioretinal lesions are probably a reflection of a vigorous host response rather than just the sheer number of infecting microorganisms.
During the introduction of total parenteral nutrition in the 1970s there was a marked increase in the number of patients with Candida endophthalmitis (77), which is likely related to the prolonged use of central venous catheters. Candida endophthalmitis has also been reported to occur after induced abortion (49), in the postpartum state (43), following treatment of toxic megacolon (123), and as a consequence of intravenous drug abuse. An addict's use of intravenous brown heroin often leads to a characteristic syndrome, at one time common in Europe, that includes pustular cutaneous lesions, endophthalmitis, and osteomyelitis. C. albicans can be isolated from all of these lesions (74). The microorganisms in this syndrome may be acquired from the drug abuser's own skin surface (79). Candida endophthalmitis may also occur after intravenous placement of a foreign device, such as a pacemaker (243), and following repeated intramuscular injections of medications, such as anabolic steroids (285). Species of Candida other than C. albicans are capable of causing endogenous endophthalmitis and may do so in proportion to their ability to cause candidemia (20, 53, 133, 243).
Although Candida species are clearly the most-common causes of endogenous endophthalmitis, other fungi are occasionally encountered. Aspergillus species are the second most-common cause of fungal endophthalmitis (291). Aspergillus spp. may be less capable of causing endophthalmitis than Candida spp.; an example of this is the rabbit endogenous endophthalmitis model, in which larger inocula ofAspergillus spp. are required to cause the disease than with C. albicans (93). Many species of Aspergillus have been reported to cause endophthalmitis, butAspergillus flavus is probably the most common (219), followed by Aspergillus fumigatus, Aspergillus niger, Aspergillus terreus, Aspergillus glaucus (281), andAspergillus nidulans (271). Endogenous Aspergillus endophthalmitis may be encountered in neutropenic patients or in patients taking pharmacologic doses of corticosteroids, often for chronic lung disease. Aspergillus endophthalmitis has even been reported to occur following severe periodontitis, although entry ofAspergillus spp. into the bloodstream through the mouth certainly is not common (172). Intravenous drug addicts are at particular risk for disseminated aspergillosis (69). Aspergillus endophthalmitis has been reported in addicts abusing a mixture of intravenous cocaine, pentazocine, and tripelennamine. Three such individuals from Louisville, Ky., were infected with A. flavus in this manner (23). Patients receiving large doses of corticosteroids for lung disease may have negative blood cultures but evidence of severe Aspergillus endogenous endophthalmitis. Endophthalmitis, therefore, is the sole manifestation of disseminated disease and must be established by aspiration of the vitreous (281). Aspergillus endophthalmitis has also arisen in recipients of solid-organ transplants, in which the donated organ was the likely source of the fungus (16, 139). Pathologic specimens of invasive aspergillosis usually demonstrate angioinvasion by the hyphae, and thusAspergillus species may possess a tropism for vascular tissue (279).
The emerging pathogens of the genus Fusarium have been reported to cause endophthalmitis in neutropenic hosts (160), in an intravenous drug abuser (94), and in a patient with AIDS (106). Penicillium spp. also have caused endogenous endophthalmitis in an intravenous drug abuser (265). As mentioned in connection with C. albicans, endogenous endophthalmitis may occur from fungi seeding the bloodstream from a catheter or endocarditis. Pseudallescheria boydii has caused endophthalmitis from an infected porcine allograft of the aortic valve (259) and even in a patient without risk factors for the disease (193).
The four dimorphic fungi H. capsulatum (165), B. dermatitidis (158), Sporothrix schenckii (2), and C. immitis (96) as well as Cryptococcus neoformans (59) may cause endogenous endophthalmitis as part of disseminated disease. Within the region of H. capsulatum endemicity in North America, roughly the Ohio and Mississippi river valleys, there is a well-described syndrome attributed to infection with H. capsulatum. This entity is known as presumed ocular histoplasmosis (POH), which occurs in immunocompetent individuals and is recognized by the presence of multiple diskiform atrophic chorioretinal scars without vitreous or aqueous humor inflammation. POH is said to affect 2,000 new individuals a year in areas of endemicity and in some cases may lead to visual loss and blindness (165). The lesions are usually burned out, but not all of them are static and some may reactivate (41). The lesions are thought to arise from the hematogenous spread of the fungus following initial infection. The initial infection, acquired by inhalation of microconidia into the lung, spreads throughout the body, including the eye, and is soon controlled by a competent host immune response (