Introduction | Lifecycle | Distribution / Map | Treatment | Loiasis
Introduction
Onchocerciasis causes intense itching, disfiguring dermatitis, and eye lesions that can result in blindness.1 Where the prevalence of blindness is high, young, able-bodied people – fearful of contracting onchocerciasis – leave their villages, often abandoning rich farmland. Without able workers, production is greatly diminished, increasing poverty and famine.1,2
The disease is caused by the parasitic filarial worm Onchocerca volvulus and is transmitted to humans through the bites of black flies of the genus Simulium which breed in fast flowing rivers.3 People who frequent these rivers to fish, bathe, or collect water are at increased risk of infection.1
Disease Information and Statistics
The World Health Organization (WHO) estimates that worldwide there are:
• More than 100 million people - At risk for infection
• 18 million people - Infected
• 800,000 people - Visually impaired
• 270,000 people - Blind1
Diagnosis
Definitive diagnosis of onchocerciasis is made by examination for microfilariae in skin biopsies.1,3 For epidemiological surveys, palpation for nodules, a less invasive diagnostic procedure, is used.
Eye Manifestations
Worldwide, onchocerciasis is the second leading cause of blindness of infectious origin.4 Microfilariae that migrate to the eye, and subsequently die there, cause the severe inflammation and scarring that can lead to impaired vision and blindness.1 The life expectancy of a person who becomes blind in endemic areas may be shortened by about 10 years.
Onchocerciasis is commonly referred to as “river blindness” because of the blindness it causes and its association with habitation near rivers where black flies breed.
Skin Manifestations
The chronic dermatitis caused by onchocerciasis can be extremely distressing and debilitating and is heralded by intense itching.
The dying microfilariae provoke a subcutaneous inflammatory response that eventually causes the skin to itch, and to become swollen and chronically thickened, a condition often called “lizard skin”. The skin may also become lax due to the destruction of its elastic fibers. Over time the skin may lose some of its pigment, which on dark skin gives rise to an appearance commonly referred to as “leopard skin”.
Nodules in the subcutaneous tissue vary in size from one to five centimeters in diameter, and an infected individual may have as many as several hundred. Nodules may be detected over the skull, ribs, elbows, hips, thighs, and knees. Non-palpable nodules may be located within muscles, causing deep-seated pain. The region of the body most affected by nodules varies from one geographic locale to another and is dependent upon the biting habits of the area’s black flies and the clothing worn by inhabitants.
References
1. World Health Organization. Onchocerciasis and its control, report of a WHO Expert Committee on Onchocerciasis Control. Geneva: World Health Organization; 1995. Technical Report Series No.: 852.
2. World Health Organization. Onchocerciasis Control Programme 1974-1994. Geneva: World Health Organization; 1995. Document No.: WHO/OCP/CTD/94.1.
3. Despommier DD, Gwadz RW, Hotez PJ, Knirsch CA. Parasitic Diseases. 4th ed. New York: Springer-Verlag; 2000. p. 138-142.
4. Thylefors B, Negrel A-D, Pararajasegaram R, Dadzie KY. Global data on blindness. Bulletine of the World Health Organization 1995;73(1): 115-121. | 
