Onchocerca volvulus

Taxonomy, Common Name, Disease

Hosts

humans

Distribution

Africa, Arabia, Guatemala, Mexico, Venezuela, Colombia

Life Cycle

Adult worms are located under the skin, where they become encapsulated by host reactions. The unsheathed microfilariae remain in the skin, where they can be ingested by the black fly intermediate hosts Simulium sp. when they take a blood meal. The first stage juvenile migrates from the intestinal tract of the fly to its thoracic muscles. There it molts to the second (sausage) stage and then molts again to the infective, filariform stage. 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.

Symptoms-Pathogenicity

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.

Management

Surgery to remove nodules. Drug therapy (Suramin) kills adults and causes a slow disappearance of the microfilariae. Control of black flies. Diagnosis is by "bloodless" snips of the skin.

Importance

More than 30 million people are infected in Africa. The disease is not fatal but causes disfigurement and blindness in many cases. In some small communities in Africa and Central America, most of the people of middle age and over are blind.

Characteristics

The morphology is similar to that of W. bancrofti. The worms characteristically are knotted together in pairs or groups in the subcutaneous tissues. They are slender and blunt at both ends. Lips and a buccal capsule are absent, and two circles of four papillae each surround the mouth. The esophagus is not conspicuously divided. Males are 19 to 42 cm long by 130 to 210 um wide; females are 33.5 to 50 cm long by 270 to 400 um wide, with the vulva just behind the posterior end of the esophagus. The tail of the male is curled ventrad and lacks alae; it bears four pairs of adanal and six or eight pairs of postanal papillae. The microfilariae are unsheathed.