ESSENTIALS OF DIAGNOSIS
Conjunctivitis progressing to blindness.
Severe pruritus; skin excoriations, thickening, and depigmentation; and subcutaneous nodules.
Microfilariae in skin snips and on slit-lamp examination; adult worms in subcutaneous nodules.
Onchocerciasis, or river blindness, is caused by Onchocerca volvulus. An estimated 37 million persons are infected, of whom 3–4 million have skin disease, 500,000 have severe visual impairment, and 300,000 are blinded. Over 99% of infections are in sub-Saharan Africa, especially the West African savanna, with about half of cases in Nigeria and Congo. In some hyperendemic African villages, close to 100% of individuals are infected, and 10% or more of the population is blind. The disease is also prevalent in the southwestern Arabian Peninsula and Latin America, including southern Mexico, Guatemala, Venezuela, Colombia, Ecuador, and northwestern Brazil. Onchocerciasis is transmitted by simulium flies (blackflies). These insects breed in fast-flowing streams and bite during the day.
After the bite of an infected blackfly, larvae are deposited in the skin (eFigure 35–48), where adults develop over 6–12 months. Adult worms live in subcutaneous connective tissue or muscle nodules for a decade or more. Microfilariae are released from the nodules and migrate through subcutaneous and ocular tissues. Disease is due to responses to worms and to intracellular Wolbachia bacteria.
Life cycle of Onchocerca volvulus (blinding worm). During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound
. In subcutaneous tissues, the larvae
develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues
. Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33–50 cm in length and 270–400 mcm in diameter, while males measure 19–42 mm by 130–210 mcm. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years. The microfilariae, measuring 220–360 mcm by 5–9 mcm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues
. A blackfly ingests the microfilariae during a blood meal
. After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic muscles
. There, the microfilariae develop into first-stage larvae
and subsequently into third-stage infective larvae
. The third-stage infective larvae migrate to the blackfly's proboscis
and can infect another human when the fly takes a blood meal
. (Global Health, Division of Parasitic Diseases and Malaria, CDC.)
After an incubation period of up to 1–3 years, the disease typically produces an erythematous, ...