History
Clinicians should have high suspicion for river blindness among people living in endemic areas with the following symptoms:
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Itchy skin rashes
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Subcutaneous nodules
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Vision changes
Physical
Skin manifestations
Skin manifestations of onchocerciasis include the following:
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Subcutaneous nodules
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Diffuse onchodermatitis: Raised intensely pruritic papules, vesicles, and pustules
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Skin atrophy
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“Hanging groin”: Drooping of the inguinal skin
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Sowdah: Severe pruritus and darkened skin, usually confined to one limb
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Leopard skin: Bilateral, symmetric, patchy depigmentation of the shins
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Lymphadenopathy
Ocular manifestations
The two subtypes of ocular disease include savanna and rainforest types. The African Savanna type causes blindness 2.5-5 times more frequently and is usually caused by punctate of sclerosing keratitis. In the African rainforest subtype, ocular involvement occurs earlier but blindness is not as common. When it does happen, blindness is usually secondary to posterior segment disease such as chorioretinitis, vascular sheathing, or optic atrophy.
Via 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, believed to be secondary to the release of Wolbachia antigens when microfilariae die in the tissues of the eye. The ocular manifestations of onchocerciasis, as reported by Kayembe et al [7] , include chorioretinitis (seen in approximately 20% of infected hosts, making it the most prevalent isolated condition), punctate keratitis (13.8%), white intraretinal deposits (10.4%), and iridocyclitis (8%). Anterior segment lesions are more prevalent than posterior lesions as a whole and appear earlier in life. A correlation between the intraretinal deposits and the severity of the chorioretinitis has been found. [7]
As mentioned above, larval death induces the 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. Sclerosing keratitis follows, which may involve the visual axis over time. Corneal neovascularization and opacification with interstitial keratitis lead to corneal blindness.
Keratitis
The development of keratitis depends on 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.
Iridocyclitis: Early in the disease, nongranulomatous or granulomatous inflammation may result from invasion of the iris and ciliary body by the microfilariae. 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
Chorioretinitis 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 are initially 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:
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Inflammatory reaction to dead microfilariae
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Deposits of immune complexes
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Autoimmunity
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Secretory-excretory products of microfilariae
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Eosinophil-derived toxic effector molecules
Autoimmunity is supported as a mechanism for several reasons. First, the burden of microfilariae is not directly associated 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.
Uveitis
Onchocercal human uveitis is associated with uveitopathogenic peptides, retinal S-antigen, and interphotoreceptor retinoid binding protein (IRBP). This evidence led Vingtain and Thillaye [8] 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
Loss of the ganglion cell axons leads to optic nerve head damage.
The optic nerve may also be involved by the infectious process or by treatment-induced inflammatory optic neuritis.
A heavy load of microfilariae may cause optic neuropathy.
