Onchocerciasis is a filarial nematode infection (see Overview of Filarial Nematode Infections) with Onchocerca volvulus. Symptoms are subcutaneous nodules, pruritus, dermatitis, adenopathy, lymphatic obstruction, and eye lesions that may lead to blindness. Diagnosis is by finding microfilariae in skin snips, the cornea, or the anterior chamber of the eye; identifying adult worms in subcutaneous nodules; or using PCR or DNA probes. Treatment is with ivermectin.
Onchocerciasis is spread by blackflies (Simulium sp) that breed in swiftly flowing streams (hence, the term river blindness). Infective larvae inoculated into the skin during the bite of a blackfly develop into adult worms in 12 to 18 mo. Adult female worms may live up to 15 yr in subcutaneous nodules. Females are 33 to 50 cm long; males are 19 to 42 mm long. Mature female worms produce microfilariae that migrate mainly through the skin and invade the eyes.
About 25 million people are infected; about 300,000 are blind, and an additional 800,000 are visually impaired. Onchocerciasis is the 2nd leading cause of blindness worldwide (after trachoma). Onchocerciasis is most common in tropical and sub-Saharan regions of Africa. Small foci exist in Yemen, southern Mexico, Guatemala, Ecuador, Colombia, Venezuela, and the Brazilian Amazon. Blindness due to onchocerciasis is fairly rare in the Americas.
Symptoms and Signs
Onchocerciasis typically affects
The subcutaneous (or deeper) nodules (onchocercoma) that contain adult worms may be visible or palpable but are otherwise asymptomatic. They are composed of inflammatory cells and fibrotic tissue in various proportions. Old nodules may caseate or calcify.
Onchocercal dermatitis is caused by the microfilarial stage of the parasite. Intense pruritus may be the only symptom in lightly infected people. Skin lesions usually consist of a nondescript maculopapular rash with secondary excoriations, scaling ulcerations and lichenification, and mild to moderate lymphadenopathy. Premature wrinkling, skin atrophy, enlargement of inguinal or femoral nodes, lymphatic obstruction, patchy hypopigmentation, and transitory localized areas of edema and erythema can occur.
Onchocercal dermatitis is generalized in most patients, but a localized and sharply delineated form of eczematous dermatitis with hyperkeratosis, scaling, and pigment changes (Sowdah) is common in Yemen and Sudan.
Ocular involvement ranges from mild visual impairment to complete blindness. Lesions of the anterior portion of the eye include
Chorioretinitis, optic neuritis, and optic atrophy may also occur.
Demonstration of microfilariae in skin snips is the traditional diagnostic method; multiple samples are usually taken (see Table 1: Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections). Microfilariae may also be visible in the cornea and anterior chamber of the eye during slit-lamp examination. PCR-based methods to detect parasite DNA in skin snips are more sensitive than standard techniques but are available only in research settings. Antibody detection is of limited value; there is substantial antigenic cross-reactivity among filaria and other helminths, and a positive serologic test does not distinguish between past and current infection. Palpable nodules (or deep nodules detected by ultrasonography or MRI) can be excised and examined for adult worms, but this procedure is rarely necessary.
Ivermectin is given as a single oral dose of 150 mcg/kg, repeated q 6 to 12 mo. Ivermectin reduces microfilariae in the skin and eyes and decreases production of microfilariae for many months. It does not kill adult female worms, but cumulative doses decrease their fertility. The optimal duration of therapy is uncertain. Although annual treatment could theoretically be continued for the life span of female worms (10 to 14 yr), it is often stopped after several years if pruritis has resolved and no evidence of microfilariae is detected by skin biopsy or eye examination. Adverse effects are qualitatively similar to those of diethylcarbamazine (DEC) but are much less common and less severe. DEC is not used for onchocerciasis because it can cause a severe hypersensitivity (Mazzotti) reaction, which can further damage skin and eyes and lead to cardiovascular collapse.
Doxycycline can kill the endosymbiont bacteria Wolbachia, which O. volvulus requires for survival and embryogenesis. A newer regimen includes one dose of ivermectin 150 mcg/kg, followed in 1 wk by doxycycline 100 mg po once/day or bid for 6 wk; ivermectin is then continued at yearly intervals as above.
No drug has been shown to protect against infection with O. volvulus. However, annual or semiannual administration of ivermectin effectively controls disease and may decrease transmission. Surgical removal of accessible onchocercomas can reduce skin microfilaria counts, but it has been replaced by ivermectin therapy.
Simulium bites can be minimized by avoiding fly-infested areas, by wearing protective clothing, and possibly by liberally applying insect repellents.
Last full review/revision September 2013 by Richard D. Pearson, MD
Content last modified October 2013