USMLE – Onchocerciasis
Onchocerca volvulus, a filarial nematode transmitted by black flies, has been a major cause of blindness in equatorial Africa, where the parasite once infected 20 million persons. Recently, an aggressive campaign of ivermectin treatment has dramatically reduced the incidence of Onchocerca infection and may lead to eradication of this disease in some areas.
Adult O. volvulus parasites mate in the dermis, where they are surrounded by a mixed infiltrate of host cells that produces a characteristic subcutaneous nodule (onchocercoma). The major pathologic process, however, which includes blindness and chronic pruritic dermatitis, is caused by large numbers of microfilariae, released by females, that accumulate in the skin and in the eye chambers. Punctate keratitis is caused by inflammation around a degenerating microfilaria. It is sometimes accentuated by treatment with antifilarial drugs (Mazzotti reaction), resulting in blindness. Damage to the retina, which is disproportionate to the number of parasites in the posterior eye chamber, is probably caused by release of host cytokines including IL-12.
Severe infection with O. volvulus causes chronic, itchy dermatitis with focal darkening or loss of pigment and scaling referred to as leopard, lizard, or elephant skin. Foci of epidermal atrophy and elastic fiber breakdown may alternate with areas of hyperkeratosis, hyperpigmentation with pigment incontinence, dermal atrophy, and fibrosis. The subcutaneous onchocercoma is composed of a fibrous capsule surrounding adult worms and a mixed chronic inflammatory infiltrate that includes fibrin, neutrophils, eosinophils, lymphocytes, and giant cells. The progressive eye lesions begin with punctate keratitis along with small, fluffy opacities of the cornea caused by degenerating microfilariae, which evoke an eosinophilic infiltrate. This is followed by a sclerosing keratitis that opacifies the cornea, beginning at the scleral limbus. Microfilariae in the anterior chamber cause iridocyclitis and glaucoma, whereas involvement of the choroid and retina results in atrophy and loss of vision.