Onchocerciasis (river blindness)

Nigerian man blinded by onchocerciasis

Introduction

Onchocerciasis causes intense itching, disfiguring dermatitis, and eye lesions that can result in blindness. 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. 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. People who frequent these rivers to fish, bathe, or collect water are at increased risk of infection.

Disease Information and Statistics

The World Health Organization (WHO) estimates that worldwide there are:

• More than 100 million people - At risk for infection

• 37 million people - Infected

• 800,000 people - Visually impaired

• 270,000 people - Blind

Diagnosis

Definitive diagnosis of onchocerciasis is made by examination for microfilariae in skin biopsies. 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. Microfilariae that migrate to the eye, and subsequently die there, cause the severe inflammation and scarring that can lead to impaired vision and blindness. 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 due to the intense and relentless 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”.

Over time, nodules that contain adult worms form in the subcutaneous tissue. The nodules 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. Bulletin of the World Health Organization 1995;73(1): 115-121.

Mectizan for Onchocerciasis

A single, annual dose of Mectizan of 150 to 200 mcg/kg has been found to effectively reduce the microfilarial density in the skin to near zero after one month and to successfully maintain a low microfilarial level for up to 12 months. Reduction of microfilarial density in the skin can significantly reduce parasite transmission.

At present, the specific mechanism of action of Mectizan against O. volvulus has not been clearly delineated. It has been determined that Mectizan has a direct toxic effect on microfilariae and interferes with their development and release from the uterus of adult female worms.

Mectizan also has a very beneficial effect on symptoms and clinical manifestations of onchocerciasis. It relieves the intense itching from the disease, and it clears microfilariae from the eye, which halts the progression towards blindness except in very advanced cases.