association and the host is harmed in some way. If human infection is important to complete the life cycle of the organism, humans are known as the obligate host. On the other hand, human infection can be accidental with humans not required to complete the typical life cycle of the parasite worm. The best-known example of accidental infection in humans is toxocariasis. This section will review important aspects of ocular parasitic disease, with special emphasis on pediatric disease, where information is available.

16.4.1  Toxocariasis

Toxocara, of which two species, toxocara canis and toxocara cati, are involved in human infections, is the most commonly recognized parasitic cause of visual loss [268, 269]. The natural hosts are dogs in T. canis and cats in T. cati [270, 271]. Human infection is accidental, as completion of the life cycle can occur in the primary host alone. Human infection is mostly confined to young children, and infection is common [271]. Almost all reported cases of ocular involvement in humans have implicated toxocara canis as the causative organism. The extent to which T. cati is responsible for the disease is not known. T. canis is a ubiquitous canine parasite affecting not only dogs but also affecting wolves, foxes, and other canines.

Both organisms are nematodes of the ascarid group. The worm may reach 10 cm in length and secrete an average of 20,000 eggs each day. The eggs initially are embryonated, and further development is climate dependent. Under suitable temperatures, the ova become infective in a few weeks and remains so for months [272]. Adult dogs become infected through ingestion of larval infected tissue. Ingestion of larvae or immature adult worms in the vomitus or feces of infected puppies can also produce infection. In utero, transmission in puppies can occur by transplacental migration of the larvae from the pregnant pitch and mature egg – laying worms may be present in puppies as early as 3 weeks after birth.

Children are usually infected as a result of ingestion of ova present in contaminated play areas, on the coats of infected pets, or on toys. Occasionally, infection can occur by oral contact with the face of an infected puppy. Following ingestion, ova develop into first stage larva. The larva penetrates the gut mucosa to enter the portal circulation and intestinal lymphatic channels.

Larvae lodge mainly in the liver, but ultimately may reach the lung, brain, or the eye. The acute phase of larval dissemination produces visceral larval migrans (VLM). The affected child may be acutely ill, but the course is mostly benign. Ocular involvement occurs through hematogenous dissemination. Recovery of the organism from tissues like the cornea or the vitreous suggests that direct involvement of the ocular tissue is also possible [273]. Ocular toxocariasis and VLM do not frequently coexist, and ocular involvement is typically an isolated event. An interval of time ranging from few months to 10 years elapses before ocular involvement occurs [274, 275].

16.4.1.1  Ocular Involvement

Although ocular toxocariasis may rarely present with anterior segment involvement, optic nerve involvement, or diffuse chorioretinitis, the three most common clinical presentations are chronic endophthalmitis, central granuloma, and peripheral granuloma, all of which are almost always unilateral.

Chronic endophthalmitis is the most common presentation occurring in about two thirds of reported cases [276]. The factors that lead to a severe inflammatory response as opposed to a small, localized granuloma are not known but could be related to initial site of the larva, host defense mechanism, and activity of the worm [277]. The reaction may remain localized and slowly subside into a dense gliotic mass or may progress unabated to produce significant and irreversible damage, including phthisis bulbi. Patients with chronic endophthalmitis present invariably with leukocoria, visual loss, and occasionally strabismus. The retina is usually detached as a result of serous exudation. The infection is usually unilateral, and the age of presentation ranges from 2 to 9 years [277].

Anterior chamber reaction may range from mild with minimal cells and flare to severe with hypopyon and fibrin membrane formation. Leukocoria is the result of severe vitreous inflammation, cyclitic membrane formation, and/or retinal detachment. Occasionally, an inflammatory mass can be seen in the vitreous cavity [277, 278]. In some cases, inflammation may subside spontaneously, permitting the recognition of a localized granuloma.

Central granulomas typically present at a later age compared with chronic endophthalmitis, with age at diagnosis ranging from 6 to 14 years. The principal

complaint is usually diminution of vision or strabismus. Involved eyes are usually uninflamed with findings limited to the posterior pole. Rarely, the lesion may progress from a state of minimal inflammation to severe endophthalmitis with total retinal detachment and dense leukocoria [276, 277]. Posterior pole lesions can involve any area including the fovea or the juxtapapillary region. The lesion is grayish white or whitish in color and is of variable size ranging from one disc diameter to several disc diameters in size (Fig. 16.13). Vitreous involvement may be mild or severe with epiretinal membrane formation [317]. Rarely, focal posterior lesions can cause sufficient traction on the surrounding retina to produce retinal breaks and detachment [277].

Peripheral granuloma formation can occur in the peripheral retina or the pars plana [280] (Fig. 16.14). The larvae most likely enter the eye via the ciliary circulation in such cases. Some peripheral lesions are asymptomatic; but in most cases, vision is significantly reduced in the affected eye, and strabismus is common. The cause of vision loss may be vitreous inflammation in the acute phase or macular destruction due to

Fig. 16.13  Posterior pole lesion in toxocariasis

Fig. 16.14  Peripheral granuloma in a toxocara retinitis with a fibrous band attaching to the optic nerve

traction, epiretinal macular membrane formation, or rhegmatogenous retinal detachment.

Minimal aqueous reaction may occasionally be present, and rarely, posterior synechiae or complicated cataract may occur. Typically, the peripheral lesion is dense, white, and elevated. The mass is located at various depths within the retina or choroid and projects towards the vitreous. Dense vitreous cells or strands are invariably noted. Many are directed posteriorly and are attached to either the disc or the surrounding retina. Elevated folds of the retina are frequently seen and often extend from the mass to the disc. Vitreous membranes may radiate circumferentially around the vitreous. When this configuration is present inferiorly, the picture may be difficult to distinguish from pars planitis. Multiple sites of involvement may be seen in some patients [277].

Involvement of the optic nerve causing optic neuritis with adjacent retinal detachment and overlying vitreous haze has also been reported [281–283]. Rare reports of toxocara canis involvement of the anterior segment have been published [284]. It is probable that the organism becomes lodged in the end arteries of the corneoscleral limbus, with larval migration into clear cornea [285] or that direct involvement of the eye has occurred.

Although toxocara canis has been mentioned as a possible cause of diffuse unilateral subacute neuroretinitis (DUSN) [286], no convincing evidence has linked the organism to this disease entity. Factors that make T. canis unlikely include absence of clinical similarity between DUSN and ocular toxocariasis, absence of positive serology and lack of epidemiological similarities between toxocariasis and DUSN [287].

Pathologically there is a focus of granulomatous inflammation within the retina with a central zone of necrosis in which toxocara larvae may be present. Eosinophils are a major component of the cellular infiltrate and may exert a cytotoxic effect on the larvae [288, 289]. In chronic endophthalmitis, the retina is usually detached as a result of serous exudation, the later sometimes containing cholesterol crystals [288].

16.4.1.2  Diagnosis

No ova or larvae can be recovered from human feces because the larva does not mature in the human gastrointestinal tract. While leukocytosis and eosinophilia