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Onchocerca volvulus is a tissue dwelling nematode, the microfilariae of which are found predominantly in the skin and eye. Onchocerciasis has a focal distribution in Africa and South America. It is endemic in West Africa, equatorial and East Africa and in the Sudan. A small focus is known also in Yemen. It occurs in Central America and in parts of Venezuela and Colombia.
Distribution of onchocerciasis
Photo 1. Aquatie stages of the vectors. The filarial worm is transmitted by Simulium or 'Buffalo flies'. Species of the S. damnosum complex are the veetors in West Africa, S. naevei in East Africa, and S. ochraceum and S. rnetallicum in Central and South America. The larvae (1) and the pupae (2) are attached to submerged objects in fast running water from which they extract oxygen through head filaments(x 6)
Photo 2. Aquatie stages of the vectors (pupae).
Photo 3. Adult simulium damnosum.
Photo 4. Typical locality of Simulium in West Africa.
Typical locality of Simulium in West Africa Fast moving, highly oxygenated water in streams, rivers, waterfalls, etc provides the essential ecological environment. The figure shows a branch of the Upper Volta river in Ghana during the rainy season. In some hyperendemic villages in West Africa a third of all adults may be blinded by onchocerciasis.
Photo 5. Simulium pupae attached to rocks. Similar river in the dry season showing stone boulders in the bed of the stream which, by causing eddies in the water, provide enough oxygenation to permit the Simulium to survive while the river level is low.
Photo 6. Simulium pupae on a crab. In East Africa larvae and pupae of the S. neavei complex are attached to fresh-water crabs.
Photo 7. Onchocerca nodules on iliac crests. The adult filariae become encapsulated-in fibrous material which forms nodules in the subcutaneous tissues. They are found predominantly in the lower part of the body in Africa, while in South and Central America they are more commonly found on the head and upper trunk.
Photo 8. Macroscopic section of nodule. In this gross section of a nodule the adult worms are seen entwined. (x 2)
Photo 9. Transverse section of nodule. A microscope section through an onchocercal nodule, showing adult worms and microfilariae. (x20)
Photo 10. Microfilaria in skin biopsy. The microfilariae migrate to the skin and the eve.
Photo 11. Onchocercal changes in skin. Onchocerciasis is characterised by lesions in two main sites, the skin and the eye. A pruriginous condition commonly called 'craw craw' in Africa involves irregular, broad areas of the skin where small papules form around the microfilariae.
Photo 12. "Elephant" skin. Thickening and wrinkling of the skin give rise to the 'lizard' or 'elephant' skin appearance.
Photo 13. Chronic dermatitis of knee. In long standing lesions, a chronic oncho- dermatitis with gross convoluted appearance of the skin and lichénification may
Photo 14. Depigmentation. Prétibial atrophy and depigmentation in a patient with late (burnt-out) onchocerciasis. This condition is sometimes called 'leopard skin'.
Photo 15. Tissue paper skin. In chronic infections atrophy of the skin may occur resulting in a 'tissue paper' appearance.
Photo 16. Hanging groin. Involvement of the inguinocrural glands can result in an appearance described as 'hanging groin'.
Photo 17. Hanging groin and scrotal elephantiasis.
Photo 18. Sowda. A peculiar feature ot onchocerciasis in the Yemen is a condition known as Sowda which is characterised by hyperpigmentation, usually of one of the lower limbs, often accompanied by inguinocrural lymphadenopathy.
In Central America 'Erisipela de la costa' is characterised by an erythematous appearance of the face or upper trunk. It occurs in heavily infected young people, usually under 20 years of age. Purplish tinged plaques or papules may be observed in Central America in patients usually of an older age group. This condition is known as 'mal morado'.
Photo 19. Early corneal involvement. The tissue reaction associated with dead microfilariae in the cornea gives rise to a number of 'snowflake'-like opacities as seen in the figure. This punctate keratitis may clear with time.
Photo 20. Selerosing keratitis. Heavy microfilarial infection of the cornea leads to the development of progressive, sclerosing keratitis which commonly produces blindness.
Photo 21. Optic atrophy. A variety of choroidoretinal lesions may follow damage by microfilariae to the anterior segments of the eye, and finally optic atrophy may develop as seen in this eye.
Photo 22. Skin snip technique. With a razor blade or scalpel, a tiny piece of skin, often from the back of the shoulders, iliac regions or calf, is snipped off and placed in a drop of saline on a microscope slide under a cover slip.
Photo 23. Skin snip technique (2).
Photo 24. Living microfilariae of O.volvulus. After some minutes actively moving microfilariae emerge from the skin into the surrounding saline. (x600)
Photo 25. Slit lamp examination. Slit lamp examination often reveals numerous microfilariae in the anterior chamber of the eye. They are best looked for in the inferior medial quadrant.
Photo 26. Nodulectomy.
Nodulectomy Serious eye changes can be prevented in some early cases by excising the nodules containing the adult worms, thus preventing the continuing production of the microfilariae which are the actual pathogenic agents in this disease. Nodulectomy has been widely employed in Central and South America.
See also: Nematodes — the filariases.
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