Onchocerca volvulus

a common parasite whose insect vector breeds in fast-flowing rivers and streams in tropical regions of the Americas and Africa, particularly West Africa. It is the etiologic agent of human onchocerciasis and is transmitted by the bites of buffalo gnats of the genus Simulium, in which the parasite passes part of its life cycle.

Cross-section of adult Onchocerca volvulus in skin nodule (H & E stain).