What is onchocerciasis?
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 in 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 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 of time and live or work near blackfly habitats are at greater risk of infection.
How is onchocerciasis transmitted?
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 2 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.
What are the clinical features of onchocerciasis?
Skin disease in onchocerciasis
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. 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 contain 2 to 4 adult worms that can reach a length of 80 cm.|
Other classic clinical presentations of onchocerciasis include:
- “Lizard skin” – dry scaly skin resembling ichthyosis
- “Hanging groin” – folds of inelastic atrophic (shrunken) skin in the groin, associated with enlarged lymph nodes.
Eye disease in onchocerciasis
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.
How is onchocerciasis diagnosed?
Onchocerciasis can be diagnosed by a variety of methods:
- Identifying microfilariae in skin shavings from affected areas – however in early or mild disease, when the microfilarial load is small, the diagnosis may be missed.
- Visualising adult worms from excised nodules under a microscope.
- Microfilariae may be directly observed during slit lamp examination of the eye.
- Detection of antibodies against O volvulus in blood samples – however this test cannot reliably distinguish between past and present infection so is best for diagnosing patients with a brief exposure history, such as expatriates.
How is onchocerciasis treated?
Onchocerciasis is treated with an oral medicine called ivermectin. Ivermectin kills microfilariae, but not the adult worms. Treatment stops 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.
How can onchocerciasis be prevented?
No vaccines or medications are available for preventing infection. Blackflies bite during the day so preventive strategies include:
- Use of insect repellents such as DEET
- Wearing long-sleeved shirts and pants