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River Blindness

This is a disease caused by a filarial worm, Onchocerca volvulus. The filarial worm causes chronic skin disease and eye lesions that may lead to blindness. This disease is also known as River Blindness.


Transmission

This disease occurs when a female blackfly of the Simulium species bites a human. The worms are inoculated into humans in larvae form and take about a year to develop into adult worms. These mature adult worms produce millions of tiny worms called microfilairae that migrate throughout the body, mainly through the skin and invade the eyes.

Prevalence

World wide about 18 million people are infected, of whom about 270,000 are blind and a further 500,000 are visually impaired. Infection and disease are most common in tropical regions of Africa. Smaller foci exist in Yemen, Guatemala, Colombia, Venezuela, southern, Mexico and the Brazilian Amazon. Onchocerciasis is the second leading cause of blindness worldwide.

Symptoms And Signs

Initially, in the microfilarial stage, a Popular , intensely itchy rash develops and this may be the only symptom in lightly infected persons .With repeated infections characteristic subcutaneous nodules of various sizes appear. These nodules contain adult worms that are visible or palpable and are otherwise asymptomatic. The nodules can be unsightly and disfiguring.

Lichennification, premature wrinkling, skin atrophy, patchy hyopigmentation, massive enlargement of the inguinal or femoral nodes and transitory localized areas of edema and erythema can occur. Ocular (Eye) lesions represent the most serious manifestation of the disease, which may range from mild visual impairment to complete blindness. The patient complains of lacrimation, photophobia and a foreign body sensation in the eye. Sclerosing keratitis, the fibrotic reaction on the cornea and retinal damage are the main causes of blindness in these patients.

Diagnosis

This is established by demonstrating the microfilarie in snips of bloodless tissue obtained from the nodules. A simple test is done by taking a small skin snip and placing it in a drop of saline, microfilariae can be seen leaving the skin .

The microfilarie can also be identified in the anterior chamber of the eye by slit -lamp examination. Mazzotis test is suggestive of the diagnosis. This test is positive if pruritis and a rash develop within a few hours of taking 50-100mg of dietthylcarbamazine DEC.

Polymerase chain reaction (PCR)- based methods to detect parasite DNA in skin snips is more sensitive than standard methods.

Prevention

Personal protections against bites by the blackfly by wearing protective wear and liberally using insect repellants is recommended when one is in an endemic area.

Killing of the vector, the blackfly by spraying the breeding groups and clearing vegetation. No effective prophylaxis is available.

Links:
CDC - Onchocerciasis
The Carter Center River Blindness (Onchocerciasis) Program
Onchocerciasis By WHO

Last Updated on Tuesday 18th January 2011