Onchocerciasis is a parasitic infection of the skin and eyes caused by Onchocerca volvulus, which is endemic in parts of West and Central Africa, with foci in East Africa and Latin America. In hyperendemic areas 60% of the population are infected: in such areas blindness rates may vary widely, but may be as high as 15%. Microfilariae invade all parts of the eye: the cornea (keratitis), the anterior chamber (iritis), the retina (chorioretinitis), and the optic nerve (optic neuritis). Blindness and irreversible eye lesions are most often found in men of 30 and older. Try to identify the patients at high risk from the blinding complications, and to arrest their disease.

ONCHOCERCIASIS A patient from an endemic area complains of itching, with or without a rash. He may have skin nodules, night blindness (enquire about this), gradual loss of vision in both his eyes, and sometimes tearing and photophobia. Look for microfilariae in snips from his skin.

CORNEA. He has a sclerosing keratitis with opacification of the lower third of his cornea. Tongues of opacification invade his cornea from the 3 and 9 o'clock positions, or from anywhere in the lower half, where they may form an apron across his cornea. If he is not treated, opacification slowly advances upwards over his pupil, until all that may be left is a clear area at 12 o'clock.

Slit lamp microscopy shows 0.5 mm linear and fluffy opacities at all levels in the stroma, and minute wriggling microfilariae in his anterior chamber.

IRIS. The final stage is a small, non-reactive, down drawn, pear-shaped pupil. Earlier stages are a loss of pigment in the margin of his pupil, exudation in front of, across, or behind it, posterior adhesions which turn it inwards, and small keratic precipitates (KP). A gelatinous exudate sometimes drags the lower margin of his pupil down and everts it.

Posterior synechiae and peripheral anterior synechiae lead to secondary glaucoma.

RETINA AND OPTIC DISCS. Look for: (1) Diffuse white areas and black pigmented ones most marked temporal to his macula (the Ridley fundus). (2) Optic atrophy, with sheathing of the vessels close to the nerve.

TREATMENT. Ivermectin (''Mectizan') is the drug of choice. Ongoing (1988) community trials are using an annual dose of 150 [gm]g/kg with encouraging results. The traditional drugs diethylcarbamazine (DEC) and suramin cause so many complications, including visual deterioration, that they are not recommended. Nodulectomy is usually only recommended for children with head nodules.

If he has an onchocercal iritis, dilate his pupil with atropine and give him topical steroids (24.5). If he develops secondary glaucoma (24.6), treat his onchocerciasis and then refer him for surgery.