**2. Anatomy**

*Loa loa, Toxocara canis and Toxocara cati*) [1, 8, 10–12]. In addition, case studies of ocular infections caused by other nematodes (*Angiostrongylus cantonensis, Dirofilaria repens*, *Trichinella spiralis, Thelazia callipaeda, Baylisascaris procyonis, Wuchereria bancrofti* and *B. malayi), c*estodes (*T.soli‐ um cysticercus*, *Echinococcus granulosus,* and *Multiceps multiceps* larvae) and trematodes (*Fasciola hepatica and Schistosoma* species) have been reported from different geographical areas [1, 8, 13–15]. The ectoparasites infecting the eye include larvae of flies [16] (*Oestrus ovis*, *Rhinoestrus purpureus, Dermatobia hominis, Chrysomia bezziana, Lucilia* spp., *Cuterebra, Hypoderma, Cochlio‐ myia, Wohlfahrtia, Gastrophilus*), *Phthirus pubis*, hard and soft ticks (belonging to class Arach‐ nida) [1, 17]. Ocular pentastomiasis caused by the larval stage of Pentastomida, the crustaceanrelated parasites, is reported to cause permanent loss of vision due to the retinal detachment or lens subluxation [18]. Further, with the advent of HIV/AIDS (human immunodeficiency virus/acquired immune deficiency syndrome), few ocular infections have also been reported

Ocular parasitic infections have been widely reported from different geographical areas (Table 1), mainly depending on the endemicity of the parasite(s). The prevalence depends primarily on the geographical distribution of the parasite, socioeconomic environment and immune status of the patient. The common modes of infection are direct contact (blepharoconjunctivitis caused by *Leishmania, Acanthamoeba* keratitis, microsporidial infections, infestation caused by lice and mites) [21–23], through blood stream (Toxoplasma chorioretinitis, retinal involvement in malaria, uveitis caused by *Toxocara)* [1, 23, 24], congenital transmission (Toxoplasmosis) and zoonotic transmission (primarily infectious diseases of animals that can naturally be trans‐ mitted to humans) [25]. In addition, few of the helminths that may lead to ocular infection are transmitted by vectors (onchocerciasis, dirofilariasis and thelaziasis), consumption of conta‐ minated food (sparganosis, trichinellosis) and indirectly from the environment (fascioliasis,

Adult and/or larval stages of the parasites may reside in human ocular tissues externally or in the ocular globe. The clinical symptoms and signs vary, depending on the etiological agent and the ocular tissue/part involved. However, local defense mechanisms and host immune responses play role in establishing the infection. The pathology in the eye can occur due to direct damage by the infecting pathogen, indirectly by toxic products, immune mediated or ectopic localization by ectoparasites. The clinical diagnosis usually mimics other pathologies due to numerous etiologies both infectious and non-infectious, which can cause conjunctivitis, keratitis, uveitis and endopthalmitis [26]. Thus, a high index of clinician suspicion is required for infective parasite etiology in patients having inflammation in the eye. In addition, eye can be involved in various systemic disorders and thorough ocular examination along with history of travel to the endemic area, risk factors and other associated medical illness that help in establishing the preliminary diagnosis. However, confirmatory diagnosis is usually achieved by direct demonstration of parasite in clinical samples and/or pathological changes observed by either slit lamp or biopsy examination [1, 8, 27, 28]. The antigen and antibody detection in ocular fluids and/or serum usually substantiates the clinical diagnosis in few parasitic infections (Toxoplasmosis, malaria, leishmaniasis, ocular gnathostomiasis, cysticercosis, toxocariasis, echinococcosis) [1, 10, 29, 30]. Molecular techniques including detection of parasite DNA by polymerase chain reaction (PCR) have added new dimensions in the diagnosis and species identification [31–36]. The treatment of choice is mostly surgical excision,

in HIV-infected patients [19, 20].

42 Advances in Common Eye Infections

ascariasis and echinococcosis).

Diagrammatic representation of human eye depicting significant ocular parasitic infections is shown in Figure 1.

**Figure 1.** Human eye anatomy depicting significant ocular parasitic infections.

#### **2.1. Orbits**

The eye balls along with extraocular muscles, nerves, blood vessels and fat are situated in the bony cavities known as orbits. The periosteal covering of the bony orbit fuses with orbital septum and duramater. Abscess due to infectious agent can localize in the space beneath the periostium. The paranasal sinuses are separated from it by the floor, medial wall and roof of the orbit and may act as the source of orbital infection. Lamina papyracea are the thinnest bony walls, which separate orbit from ethmoidal sinuses. Thus, any breach in it causes the ingress of sinus microbiota to orbital tissue leading to infection. Orbital cellulitis can also be caused by direct extension of the infection from the ethmoidal sinuses to the orbital cavity. The lateral wall of the sphenoidal sinus constitutes the medial wall of the optic canal and infection of the former can percolate to the latter causing optic nerve damage and visual loss. There are various apertures present in the orbital cavity, which provides the route of communication with the adjacent structures. The superior and inferior orbital fissures, the lacrimal fossa, nasolacrimal duct and the optic canal constitute such important apertures [1, 42–46].

#### **2.2. Blood supply**

The ophthalmic artery and its branches constitute main arterial supply of orbit. The majority of the venous drainage occurs through superior ophthalmic vein, which drains into cavernous sinus that is located just posterior to the orbital apex. Veins from the facial region and many anterior ophthalmic veins anastomose and drain into cavernous sinus through superior orbital vein. Thus, cavernous sinus is prone to infection from facial region and also from the orbital region through the superior ophthalmic vein leading to a serious complication.

#### **2.3. Eyelids**

The eyelids impart two protective anatomical barriers, i.e., orbital septum and conjunctiva. Former divides the orbit from the eyelid into preseptal and postseptal spaces and provides a physical barrier to infectious agents and latter one is reflected back on itself, which provides protection by hindering the free movement of the material posteriorly from the anterior surface of the globe.

#### **2.4. Lacrimal system**

Lacrimal system consists of lacrimal gland, accessory gland and excretory system. Tears are secreted by lacrimal gland, which flows over the cornea and finally drain into nasal cavity by nasolacrimal duct through lacrimal sac. Any obstruction to the nasolacrimal duct can lead to regurgitation of the accumulated fluid onto the ocular surface leading to increased chances of infection.

#### **2.5. Layers of eye ball**

The basic structure of eye ball or globe consists of three concentric layers. The outermost covering is composed of sclera and cornea. The middle covering is composed of uveal tract, consisting of choroid, ciliary body and iris. The inner most covering is retina. The sclera is almost avascular except for the presence of superficial small blood vessels. The choroid is a highly vascular structure and provides nutrition and oxygenation to the retina beneath it. Due to these qualities, choroid serves as a fertile area for the proliferation of various pathogens, which spread by hematogenous route.

#### **2.6. Anterior and posterior chambers**

Anterior segment of the eye in front of the vitreous humor comprises anterior one-third of the eye and is further divided into anterior chamber and posterior chamber. Anterior chamber is the space between posterior surface of cornea and the iris, whereas posterior chamber is the space between iris and the front of vitreous. The aqueous humor is produced by non-pig‐ mented ciliary epithelium in the posterior chamber and drains through the pupillary aperture into the anterior chamber. Cornea is composed of well-organized collagen fibrils, which is avascular in nature. Lens is also an avascular crystalline structure, which continues to grow throughout life. Thus, aqueous humor fills these spaces and provides nutrition to the sur‐ rounding structures.

#### **2.7. Vitreous humor**

septum and duramater. Abscess due to infectious agent can localize in the space beneath the periostium. The paranasal sinuses are separated from it by the floor, medial wall and roof of the orbit and may act as the source of orbital infection. Lamina papyracea are the thinnest bony walls, which separate orbit from ethmoidal sinuses. Thus, any breach in it causes the ingress of sinus microbiota to orbital tissue leading to infection. Orbital cellulitis can also be caused by direct extension of the infection from the ethmoidal sinuses to the orbital cavity. The lateral wall of the sphenoidal sinus constitutes the medial wall of the optic canal and infection of the former can percolate to the latter causing optic nerve damage and visual loss. There are various apertures present in the orbital cavity, which provides the route of communication with the adjacent structures. The superior and inferior orbital fissures, the lacrimal fossa, nasolacrimal

The ophthalmic artery and its branches constitute main arterial supply of orbit. The majority of the venous drainage occurs through superior ophthalmic vein, which drains into cavernous sinus that is located just posterior to the orbital apex. Veins from the facial region and many anterior ophthalmic veins anastomose and drain into cavernous sinus through superior orbital vein. Thus, cavernous sinus is prone to infection from facial region and also from the orbital

The eyelids impart two protective anatomical barriers, i.e., orbital septum and conjunctiva. Former divides the orbit from the eyelid into preseptal and postseptal spaces and provides a physical barrier to infectious agents and latter one is reflected back on itself, which provides protection by hindering the free movement of the material posteriorly from the anterior surface

Lacrimal system consists of lacrimal gland, accessory gland and excretory system. Tears are secreted by lacrimal gland, which flows over the cornea and finally drain into nasal cavity by nasolacrimal duct through lacrimal sac. Any obstruction to the nasolacrimal duct can lead to regurgitation of the accumulated fluid onto the ocular surface leading to increased chances of

The basic structure of eye ball or globe consists of three concentric layers. The outermost covering is composed of sclera and cornea. The middle covering is composed of uveal tract, consisting of choroid, ciliary body and iris. The inner most covering is retina. The sclera is almost avascular except for the presence of superficial small blood vessels. The choroid is a highly vascular structure and provides nutrition and oxygenation to the retina beneath it. Due

duct and the optic canal constitute such important apertures [1, 42–46].

region through the superior ophthalmic vein leading to a serious complication.

**2.2. Blood supply**

44 Advances in Common Eye Infections

**2.3. Eyelids**

of the globe.

infection.

**2.4. Lacrimal system**

**2.5. Layers of eye ball**

It is a gel-like substance present in front of retina and posterior to the lens in the posterior segment of the eye. It is optically clear and is composed of collagen framework interspersed with hyaluronic acid. During intraocular inflammation, it becomes hazy and may cause impairment of vision.

#### **2.8. Retina and optic nerve**

Retina constitutes the innermost covering of the eye ball and captures the light energy with the help of rods and cones. The outer half of the retina is supplied by central retinal artery, whereas inner half receives its blood supply from the choroid.

The optic nerve is formed by axons of the inner cell layer that exits the globe. It is covered by all the three meningeal coverings, which are direct extensions of the brain coverings. Thus, it is vulnerable to infections originating from both within cranial vault and within orbits.
