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Author: Marie Hartley, Staff Writer, 2010.
Onchocerciasis is a chronic and slowly progressive skin and eye disease caused by a worm called Onchocerca volvulus. It is one of the main causes of filariasis. O. volvulus is transmitted to humans through the bite of a blackfly (of the genus Simulium). Blackflies are found near fast-flowing rivers in the inter-tropical zones. About 90% of the disease occurs in Africa, the remainder in 6 countries in Latin America, and Yemen in the Arabian Peninsula. Of the 85 million people who live in endemic areas, an estimated 18 million are currently infested; 4 million have a skin disease, and 2 million are blind or visually impaired.
Short-term travellers to endemic areas are at low risk of onchocerciasis, as multiple bites are required for infection. Travellers who visit endemic areas for extended periods and live or work near blackfly habitats are at greater risk of infection.
Within the human body, the adult worms live in subcutaneous nodules (lumps) scattered around the body. The female worm produces thousands of larval worms (called microfilariae) which migrate to the skin and eye. Microfilariae that have migrated to the skin can be ingested by blackflies. Within the fly, the larvae then develop further over two weeks. When the blackfly bites another human, the larvae enter through the wound, penetrate the tissues, and develop into adult worms.
The female worms can live for up to 15 years and produce hundreds of microfilariae each day. The microfilariae can survive 2 to 3 years, and their death is very toxic to the skin and the eye, producing severe itching and eye lesions. After repeated years of exposure, these lesions may lead to skin disfiguration and irreversible blindness. Initial infestation may occur in childhood and produce no symptoms for long periods.
Six different patterns of skin disease have been described. Two or more patterns may be present in a single patient, and the patterns may evolve and change over time.
|Acute papular onchodermatitis||Widespread itchy eczema-like rash with multiple small itchy papules (lumps) which progress to become vesicles (blisters) and pustules (pus-filled blisters). The face, trunk, and extremities are often affected.|
|Chronic papular onchodermatitis||Severely itchy rash with scattered flat-topped papules and areas of darker pigmentation. The shoulders, buttocks, and extremities are typically affected. The most common pattern of skin disease.|
|Lichenified onchodermatitis||Thickened scaly and hyperpigmented itchy plaques. The lower extremities are commonly affected, and lymph nodes are often enlarged.|
|Onchocercal atrophy||Large areas of wrinkled thin, dry inelastic skin. Commonly affects buttocks and lower back.|
|Onchocercal depigmentation||Also called “leopard skin”. Areas of pigment loss, with islands of normally pigmented skin surrounding hair follicles. Often affects the shins in a symmetrical pattern and is not usually itchy.|
|Palpable onchocercal nodules||Subcutaneous lumps found over bony prominences, which contain the adult worms. The subcutaneous nodules range in size from a few millimetres to several centimetres, and each contains 2 to 4 adult worms that can reach a length of 80 cm.|
Other classic clinical presentations of onchocerciasis include:
The dead microfilariae produce inflammation and bleeding in most internal tissues in the eye. Symptoms range from itching, redness, pain, and photophobia (light sensitivity) to blurring of vision, night blindness, glaucoma, visual field loss (restricted area of vision), and eventually blindness.
Onchocerciasis can be diagnosed by a variety of methods.
Onchocerciasis is treated with an oral medicine called ivermectin. Ivermectin kills microfilariae, but not the adult worms. Treatment stops the progression of the disease. One dose of ivermectin every 3-12 months is required.
It has recently been discovered that a bacteria called Wolbachia have a symbiotic relationship with O. volvulus. Wolbachia appear to have evolved to become essential to the fertility of their worm hosts. Promising results have been seen so far with antibiotic treatments (predominantly with doxycycline) targeting Wolbachia.
No vaccines or medications are available for preventing infection. Blackflies bite during the day, so preventive strategies include:
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