Ophthalmologic Manifestations of Onchocerciasis Clinical Presentation

• A detailed history and complete ophthalmologic examination should be performed whenever possible.

• Skin manifestations

  • The earliest skin involvement is mild pruritus and may be followed by papular rash, erosions, and lichenification. In highly endemic communities, the prevalence of itching increased until age 20 years and then plateaued.
  • The classic dermal lesion is referred to as leopard skin, which represents islands of hyperpigmented skin around hair follicles with surrounding depigmentation, most commonly located on the shins. These cutaneous manifestations affect mobility and social acceptability.
• Ocular manifestations

  • By slit lamp biomicroscopy, microfilariae can be seen within the cornea, migrating freely in the anterior chamber and vitreous humor. Although live microfilariae cause minimum reaction, dead microfilariae are associated with a severe inflammatory response.
  • Onchocerciasis-related blindness results from decades of infection and reinfection. Keratitis is one of the most devastating consequences of onchocerciasis. Various epidemiologic studies have found visible corneal or anterior chamber microfilariae (5-45%), punctate keratitis (35%), and sclerosing keratitis (10-15%).
  • Larval death induces an inflammation that results in corneal opacification and neovascularization.
    • Dead worms are surrounded by inflammatory infiltrates in the superficial stroma. Lymphocytes and eosinophils migrate to the peripheral cornea where the infection is more dense. A sclerosing keratitis follows, which may involve the visual axis over time. Corneal neovascularization and opacification with interstitial keratitis lead to corneal blindness.
    • The development of keratitis is dependent upon the previous immunization and the presence of sensitized T lymphocytes. It is associated with a predominance of T-helper type 2 (Th2) response.
    • Eosinophils are the predominant inflammatory cells in the cornea after injection of the parasite antigen. The severity of keratitis seems to be associated with the number of eosinophils in the cornea.
    • Neutrophils are prominent early in the inflammatory response and mediate keratitis in the absence of eosinophils.
  • Anterior uveitis: Early in the disease, nongranulomatous or granulomatous inflammation may result from invasion of the iris and ciliary body by the microfilariae. The frequency of iritis is about 10-20%. Posterior synechiae may distort the pupil inferiorly giving a classical pear-shaped iris. Seclusio pupillae and iris bombe with secondary angle-closure glaucoma may occur. The common sequelae include occlusio and seclusio pupillae, iris atrophy, iris bombe, inflammatory glaucoma, and cataract.
  • Chorioretinitis is seen in 10-25% of patients with ocular involvement.
  • The choroid is characterized by progressive inflammation associated with loss of pigment, pallor of the vessels from orange to yellow or white, and loss of choriocapillaris.
  • The retinal pigment epithelium is affected in the early stage of infection. Areas of hypopigmentation and hyperpigmentation, mottling, and confluent atrophy characterize it. Dark brown clumps of pigment are seen in a diffusely pale retinal pigment epithelium. Photoreceptors initially are lost, followed by loss of the inner retinal layers.
• In the early stage, chorioretinitis may mimic any other diffuse chorioretinal process, such as histoplasmosis, toxoplasmosis, or retinitis pigmentosa.
• Commonly involving the temporal retina, onchocerciasis tends to spare the macula; as a result, central visual acuity is maintained until late in the disease.
• The pathogenesis of onchocercal chorioretinopathy is not well understood. The following mechanisms have been hypothesized to cause the characteristic chorioretinopathy:

  • Inflammatory reaction to dead microfilariae
  • Deposits of immune complexes
  • Autoimmunity
  • Secretory-excretory products of microfilariae
  • Eosinophil-derived toxic effector molecules
  • Autoimmunity is supported as a mechanism for several reasons. First, the burden of microfilariae is not associated directly with the degree of chorioretinopathy. Second, the chorioretinopathy continues to progress even after effective treatment with reduction of microfilariae and vector control. Finally, retinal disease can persist throughout life unless controlled with anti-inflammatory agents.
  • Evidence favoring the autoreactivity theory for the development of onchocercal chorioretinopathy is derived from the detection of retina-specific autoantibodies in the human-infected sera.
    • Onchocercal human uveitis is associated with uveitopathogenic peptides, retinal S-antigen, and interphotoreceptor retinoid binding protein (IRBP). This evidence led Vingtain to demonstrate the presence of higher levels of S-antigen antibodies in patients with onchocerciasis who also had involvement of the posterior segment. However, these findings were not replicated.
    • Autoantibodies may have resulted from activation of T and B cells seen in parasitic infections. About 30% of B cells produce antibodies that are able to bind to autoantigens.
    • Hypergammaglobulinemia is a feature of O volvulus infection. Strong evidence exists that the chorioretinopathy is an inflammatory reaction to dead microfilariae because treatment with microfilaricidal induces characteristic retinal pathology. Similar progressive pathology also is seen after antimicrobial therapy of Toxocara uveitis in children.
    • Microfilariae load is associated with the presence of new chorioretinal lesions. Even though the incidence of new lesions is reduced after antimicrobial chemotherapy, extension and progression of existing disease is common.
    • This observation suggests that although chorioretinopathy is likely to be initiated by the presence of microfilariae, persistence is a self-perpetuating autoimmune process that does not require the presence of microfilariae.
  • Optic atrophy is seen in 10-25% of patients with ocular manifestation.
    • Loss of the ganglion cell axons leads to optic nerve head damage.
    • The optic nerve also may be involved by the infectious process or by treatment-induced inflammatory optic neuritis.
    • A heavy load of microfilariae may cause optic neuropathy.

Causes are discussed in Physical .