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Onchocerca volvulus

Common name. convoluted filaria

Disease. onchocerciasis, river blindness

Geographic distribution. Mostly in Subsaharan Africa, Yemen, Guatemala, Mexico, Venezuela, Ecuador, Colombia, and Brazil

Infection rate. About 20 to 40 million people are infected.

Life cycle. Infection comes from the bites of female blackflies of the genus Simulium, which transmit the infective larvae of the parasite. Over 10 to 20 months these larvae grow to adult worms, which live for up to 15 years tangled together in fibrous nodules under the skin. The adult females produce a continuous supply of live embryos or microfilariae. Skin microfilariae ingested by biting Simulium develop to infective larvae in seven to ten days and can be transmitted to a new host.

Morphology. Threadlike adult worms measure 5 cm in males, and 50 cm in females.

Pathology and clinical symptoms. The adult worms induce fibroblastic reaction in the host which causes the worms to become enveloped in fibrous scars: onchocercal nodule or onchocercoma of firm, round, nontender, and varying in diameter of 0.5 to 10 cm or more. These nodules tend to occur on anatomic sites where the bone is superficial, such as the scalp, scapulae, elbows, iliac crests, and knees. Heavy and long term infection, in endemic area, can make a irreversible eye changes. Symptomatology of ocular onchocerciasis begins with photophobia and progresses through gradual blurring of vision to blindness.

Diagnosis. Definitive diagnosis depends on finding adult worms in excised nodules, or microfilariae in affected skin.

Prevention. Avoiding the haunts of Simulium by wearing clothing that reduces the area of skin exposed to bites, or by using insecticides and personal insect repellent spray. There is no chemoprophylaxis.

Hae-Seon Nam

Section of onchocercoma due to Onchocerca volvulus. Several female adult worms were surrounded with host fibrotic tissue. Many uterine embryos or microfilariae are seen. In generaly, adult Onchocerca volvulus provoke a fibroblastic reaction in the host which causes the worms to become enveloped in a fibrous scar forming the onchocercal nodule or onchocercoma. x100

Hae-Seon Nam

Cutaneous nodule with Onchocerca volvulus in human. Microfilariae are observed in sectined adult worm. H&E, X100.

Duk-Young Min

Tansverse section of Onchocerca volvulus. There are many microfilaria in the uteri.

Tai Soon Yong

A section of onchocercoma showing internal organs of Onchocerca volvulus adult worms. Note the uteri filled with larvae.

Tai Soon Yong