The unsheathed microfilariae remain in the skin, where they can be ingested by the black fly intermediate hosts Simulium sp. The first stage juvenile migrates from the intestinal tract of the fly to its thoracic muscles. The filariform juvenile moves to the labium of the fly and can infect a new host when the insect next feeds. Mature worms appear in the skin in less than a year. If adults are located over bony prominences such as at a joint or the skull, a prominent nodule appears.
Nodules consist mainly of collagen fibers surrounding one to several adult worms.
The presence of microfilariae in the skin often results in a severe dermatitis caused either by allergic responses or toxic effects after the death of the juveniles. The worst complication involves the eyes. Lesions of the eye take many years to develop, most affected persons are over 40 years old. Invasion of the cornea by microfilariae causes inflammation of sclera, or white of the eye, followed by an invasion of fibrous tissue, leading to extensive vascularization of the cornea, which, in turn, severely impairs vision. Subsequent fibrosis may lead to complete blindness.