Onchocerciasis
Onchocerciasis affects up to 37million people worldwide and is most abundant in Africa. It is responsible for skin disease and blindness. It is caused by a large worm known as Onchocerca volvulus. which is transmitted to humans by black flies, which breed in rivers and streams, hence the name - river blindness.
When an infected black fly bites, infective larvae (L3) crawl into the bite wound. These then migrate through the subcutaneous tissue and mature into adult worms located under the skin in self-contained nodules (onchocercomas). New worms can form new nodules or willcluster together inexisting nodules. If the site of the worm nest is over bone – such as the skull, hips or knee – then a prominent nodule can appear. The smaller male worm (about 4cmlong) may migrate between nodules when mating with the larger females (about 40cm long). A female worm is capable of producing up to 1000 microfilariae (juvenile worms, stage L1) per day. These microfilariae then migrate through skin tissues all around the body.
Disease occurs when the microfilariae die with the resulting inflammation causing dermatits, intense itching and depigmentation. These symptoms create conditions like lizard skin and leopard skin. If the microfilariae migrate to the eye their death results in inflammation induced by Wolbachia leading ultimately to blindness.
Links
Liverpool School of Tropical Medicine
The Bill & Melinda Gates Foundation