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Onchocerciasis

Overview

Onchocerciasis is a parasitic infection of a nematode worm, causing disease to the eye and skin. The causative organism is the filarial worm, Onchocerca volvulus. Onchocerciasis is transmitted to humans through the bite of an infected blackfly of the Simulium species, which release larvae into the bloodstream. Blackflies breed in fast-flowing streams and rivers which gives Onchocherciasis the alternative name of ‘river blindness.’ They are commonly found in remote villages near fertile land. In vivo, the female worm produces thousands of larval worms (microfilariae) which migrate to the skin and the eye to present with symptoms of blindness and skin disease.

 

Around 90% of reported cases occur in Africa but Onchocerciasis is also present in Latin America and Yemen. The World Health Organization (WHO) has estimated that 25 million people are infected with O. volvulus worldwide, of which 300,000 are blind and 800,000 have visual impairment. Around 123 million people are at risk of infection. 1974 saw the introduction of the Onchocerciasis Control Programme (OCP) enabling onchocerciasis to be brought under control in West Africa. The work of OCP was mainly the use of insecticides against blackfly larvae as vector control. Large-scale distribution of ivermectin was also provided. In 1995, the African Programme for Onchocerciasis Control (APOC) was launched where the aim was shifted from vector control to treatment in remaining endemic areas.

 

Signs and symptoms

The death of the microfilariae is very toxic to the skin and eye and produces:

  • Skin inflammation

  • Pruritus

  • Subcutaneous nodules

  • Loss of elastic tissue (drooping of the skin)

  • Patchy skin depigmentation (referred to as lizard/leopard skin)

  • Lymph node inflammation

  • Ocular lesions

  • Eosinophilia

  • Visual impairment / blindness due to clouding of the cornea

 

Causes

An infected Simulium blackfly introduces filarial larvae of Onchocerca volvulus

through the bite wound when feeding on human blood. Once in vivo, larvae

develop into adult filariae in the subcutaneous tissues. Adult filariae can live in

the subcutaneous nodules for approximately 15 years. In the subcutaneous

nodules, the female worms produce microfilariae and can continue to do so for

approximately 9 years. The microfilariae have a life span that can reach up to 2

years and are occasionally found in peripheral blood, urine, and sputum but

usually inhabit the skin and lymphatic connective tissues. The microfilariae and

adult worms are colonised with Wolbachia bacteria which aid parasitic survival. When microfilariae die in vivo, the immune response of the host results in destruction of optical tissues and hence visual impairment or blindness. Microfilariae can be ingested by another blackfly when an infected human is bitten. Larvae then develop inside the stomach of the blackfly and the infective cycle continues.

 

Risk factors / at risk groups

  • Living in/visiting sub-Saharan African countries

  • Living near breeding sites e.g. rivers

  • Agricultural occupations

  • Increased risk of infection with increased bites from infected blackflies

 

Diagnosis / microbiology testing

  • Skin snip – 1-2mg biopsy of the skin placed in saline causes larvae to emerge

  • Polymerase chain reaction used where larvae are not visualised in skin snip test

  • Surgical removal and examination of skin nodules

  • Slit-lamp examination of anterior of eye

  • Antibody tests (not widely available or specific to onchocerciasis)

 

Treatment

People who are found to be infected with O. volvulus should be treated in order to prevent the long-term skin damage and blindness. The recommended treatment is the antiparasitic, ivermectin, which reduces the risk of blindness and reduces occurrence and severity of skin symptoms. Ivermectin should be administered every 6 months for 10-15 years (the adult worm lifespan) or until diagnostic tests remain clear. Ivermectin kills microfilariae and larvae but has no effect on the adult worms.

It has been suggested that individuals may benefit from a 6 week course of doxycycline after each ivermectin dose. Doxycycline has been shown to kill the Wolbachia bacteria which the adult worms rely on for survival.

A recent drug, moxidectin, has been found to kill the Onchocerca volvulus worms but has not yet been approved for human use.

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