Onchocerciasis

Synonym: River blindness

Onchocerciasis is caused by the filarial nematode Onchocerca volvulus. It is transmitted by the blackfly Simulium spp., which occur in Africa and South America (mainly Venezuela). Humans are the only definitive hosts.¹

• The blackfly transmits the microfilaria larval form of the worm from one host to another.
• Within 1-3 months, larvae develop into adult worm, which are capable of releasing up to 2,000 microfilariae each day for up to 10 years.
• Microfilariae are found mainly in the lymphatics of the subepidermis and also enter the eye, mainly in the anterior chamber.
• Microfilariae are well tolerated when alive but cause an intense inflammatory reaction when dying.²
• The disease is characterised by genital elephantiasis, subcutaneous nodules and ocular lesions.

• Onchocerciasis is confined to West, East and Central Africa, Central South America and Southern Saudi Arabia.
• It has produced blindness in 270,000 and severely impaired vision in 500,000 people.³
• Onchocerciasis is the world's second leading infective cause of blindness. 13 million individuals are affected world-wide.

• Mild infection causes a localised maculopapular rash with itching. This may resolve or progress to a chronic and generalised form with severe itching.
• Initial symptoms are usually a red, papular, itchy rash, followed by subcutaneous nodules.
• Larvae develop into adult worms within palpable nodules, which are usually found over bony prominences in the chest, pelvis and knees. The lesions may heal with hyperpigmentation but lichenified, hyperkeratotic lesions may become widespread and are very itchy and distressing.
• A localised form in Arabia causes a chronic papular dermatitis affecting the arms and legs.
• Long-standing infection causes the skin to become very wrinkled. Skin begins to sag and depigmentation of the pretibial areas is typical in older people living in endemic areas.
• Genital elephantiasis, hydrocoeles and a "hanging groin" may also occur.
• Hanging groin is caused by lymphatic obstruction leading to swelling, loss of elasticity and atrophy of the skin, producing large folds of skin. It may be unilateral or bilateral, and may also involve enlarged lymph nodes. Hanging groin increases the risk of developing an inguinal hernia.
• Light-skinned patients infected on visiting a country may present a year later with intensely itchy, red macular or maculopapular lesions that may be either localised or generalised.
• Fever, muscle or joint pains, weight loss and lymphadenitis may occur.
• Rash sometimes lasts for several months after treatment.
• Eye involvement:
  • Initially presents with lacrimation, photophobia and a sensation of a foreign body in the eye.
  • Features of eye involvement then include conjunctivitis, intraocular microfilariae, punctate keratitis (presents with " snowflake" opacities resulting from an acute inflammatory reaction to dying microfilariae. Resolves with no long-term sequelae), sclerosing keratitis, anterior uveitis chorioretinitis, optic neuritis, optic atrophy, glaucoma and blindness.

• Full blood count: eosinophilia.
• Skin snips are immersed in normal saline and microfilariae can be seen swimming free within 24 hours.
• Onchocerca may be seen on slit lamp examination of the anterior chamber of the eye.
• Examination of excised nodules show adult worms.
• More sensitive techniques include enzyme immunoassay and polymerase chain reaction.
• The Mazzotti test is now seldom used. It involves the giving the patient diethylcarbamazine (DEC), which inhibits neuromuscular transmission in nematodes. Within 2 hours, a positive result produces pruritus and intense inflammation in the areas of dying microfilariae. Other possible effects include vomiting and hypotension, therefore limiting the usefulness of the test.⁴

• Ivermectin; single dose clears microfilariae from skin for several months. A further dose every 6-12 months prevents progression.⁵ Ivermectin only treats microfilariae and not adult worms. There is no treatment for the adult worms.
• Ivermectin is contraindicated in:
  • Pregnancy and breast feeding.
  • In areas co-endemic for O. volvulus and L. loa as severe encephalopathy has been seen in children after administration of ivermectin (appears to be related to level of L. loa load).
• Treatment is commonly associated (usually after the initial treatment) with increased itching, swelling of face or extremities, headache and body pains. There is very little experience of using ivermectin in children.
• Surgical excision of nodules: especially if on the head as potentially close to the eyes.

• Control of blackfly by spraying.
• Avoidance of getting bitten, e.g. nets, insect repellants.
• Availability of ivermectin in endemic areas - ivermectin is given every 6 -12 months.