are constant laboratory findings in VLM, eosinophilia is generally absent in ocular toxocariasis. While immunodiagnosis is the prime serological method for detecting VLM, these tests are mainly confirmatory in ocular toxocariasis where clinical examination is usually distinctive. A negative or weakly positive ELIZA performed on the serum can evolve in ocular toxocariasis, making these tests less likely to rule out toxocariasis. It is also possible to have positive aqueous and vitreous titers even when the serum ELIZA titers are negative. Intraocular fluid titers higher than serum titers help to confirm the diagnosis and emphasize the role of local antibody production [290].

16.4.1.3  Differential Diagnosis

Toxocariasis can sometimes be confused with a variety of other ocular conditions, including retinoblastoma, advanced retinopathy of prematurity, Coats disease, and persistent fetal vasculature. Peripheral granuloma can sometimes be confused with familial exudative vitreoretinopathy, and when located inferiorly with circumferential vitreous bands involving the inferior retina, may mimic pars planitis. Chronic endophthalmitis due to toxocariasis should be differentiated from other causes of anterior uveitis in children, particularly juvenile rheumatoid arthritis, and should also be differentiated from exogenous and endogenous endophthalmitis.

16.4.1.4  Treatment

Treatment strategy depends on vision, the extent of intraocular inflammation, and the degree of macular involvement. When intraocular inflammation is severe, corticosteroids are the mainstay of treatment. Systemic corticosteroids are typically administered and may help to reduce the severity of vitritis and development of vitreous bands and membranes by suppressing the destructive inflammatory response to the parasite. Since the disease typically occurs in young children, close monitoring for local and systemic corticosteroid side effects is important.

The antihelminthic drugs thiabendazole and diethylcarbamazine have both been used. Although clinical improvement can occur with the use of these drugs, many clinicians question the real effect of these agents,

citing the fact that clinical improvement can occur as the disease runs its natural course [291]. The rationale of antihelminthic treatment is to destroy viable nematodes and eliminate further migration of the larvae. The parasite, however, may persist despite antihelminthic treatment.

Pars plana vitrectomy has been used to treat epiretinal membranes, tractional retinal detachment, impending tractional macular detachment, chronic vitreous inflammation, and dense vitreous membranes. Removal of all components of the toxocara granuloma can be successful in the treatment of ocular toxocariasis and is possible with pars plana vitrectomy, combined with subretinal surgical techniques [292].

Ocular toxocariasis can be prevented by avoiding contact with puppy feces for the first 4 months of the puppie’slife.Piperazine,Thiabendazole,andIvermectin are effective in eradicating adult worms from puppies [293]. Antihelminthic drug appears to decrease transmission of the organism to puppies if administered to the pregnant female during late gestation and early lactation.

16.4.2  Onchocerciasis

Onchocerciasis or river blindness is a major and preventable cause of blindness. About 1 million people are blind or are rendered visually handicapped as a result of this parasitic infection. More than 80 million people live in endemic areas and are therefore at risk for this disease [294]. Onchocerciasis is found across equatorial Africa and central and south America. In Africa, the disease is endemic in Mali, Niger, Chad, Sudan, and Mallawi. The disease spreads along rivers where the water is well oxygenated, which is important for the larval stage to develop. In central and south America, the disease is endemic in Mexico, Guatemala, Venezuela, Brazil, Columbia, and the Ecuador. In Asia, one small focus is present in Yemen [294].

The disease is caused by the filarial nematode onchocerca volvulus (Fig. 16.15) and is transmitted to man by a vector fly, simulium, more commonly known as the black fly. Involvement of the eye is due to microfilarial invasion of either the anterior or posterior segment. Much of the blindness from onchocerciasis is due to advanced lesions of the posterior segment, particularly chorioretinitis. Onchocercal chorioretinitis

appear to be a chronic, indolent, low-grade, and ­progressive inflammation rather than active fulminant disease [295].

Humans are the natural sources of adult worms (onchocerca volvulus), which are incarcerated within a nodule of dense fibrous tissue known as onchocercoma (Fig. 16.16). The fertilized female releases a large number of microfilaria, which migrates through the superficial dermis. Further development is dependent on the

Fig. 16.15  Filarial nematode onchocerca volvulus (with courtesy of Babalola Oe, MD)

Fig. 16.16  Onchocercoma with coiled warms (with courtesy of Babalola Oe, MD)

larvae being ingested by blackflies feeding on the skin. After mottling twice, third stage larvae can infect other individuals the next time the fly bites. The larva migrates within the body for about a year developing into adult worms, which then settle in a nodule where male and female worms mate. The female then produces millions of microfilaria that migrate into the subcutaneous tissue but also may be found in the eyes and visceral organs. Live microfilaria produces few problems; dead microfilaria can provoke an intense inflammatory reaction. Microfilaria within the skin is often associated with a pruritic rash and with multiple papules, proceeding through intradermal edema, to atrophy and loss of skin elasticity.

16.4.2.1  Ocular Manifestations

Anterior segment manifestations are related to the presence of microfilaria that reaches the cornea and anterior chamber from the periorbital skin and conjunctiva. Manifestations include punctate keratitis occurring as a response to dying larvae, [296] sclerosing keratitis [297], and nongranulomatous or, less likely, granulomatous iridocyclitis [298].

Advanced lesions of the posterior segment consist of retinal pigment epithelial disturbance, chorioretinitis, chorioretinal atrophy, subretinal fibrosis, and optic atrophy. The macula tends to be spared (Fig. 16.17). Intraretinal pigmentation is common in patients with ocular onchocerciasis. Two types of intraretinal pigmentation are described. Brown intraretinal pigmentation and black pigmentation (bone spicules). In the mildest form, intraretinal pigment has a fine brown granular appearance and is found diffusely in different areas of the retina except the macula. Pathological