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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Tissue Nematodes

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Onchocerciasis : Clinical Aspects

The subcutaneous nodules that harbor the adult worms can be located anywhere on the body, generally over bony prominences.

ONCHOCERCIASIS : CLINICAL ASPECTS

MANIFESTATIONS

The subcutaneous nodules that harbor the adult worms can be located anywhere on the body, generally over bony prominences. In Mexico and Guatemala, where the fly vector typically bites the upper part of the body, they are concentrated on the head; in South America and Africa, they are found primarily on the trunk and legs. Although nodules may number in the hundreds, most infected individuals have less than 10. They are firm, freely movable, and measure 1 to 3 cm in diameter. Unless the nodule is located over a joint, pain and tenderness are unusual. Of greater consequence to the patient are the side effects of the presence of microfilariae in the tissues. An immediate hypersensitivity reaction to antigens released by dead or dying parasites results in acute and chronic inflammatory reaction. In the skin, this reaction is manifested as a papular or erysipelas-like rash with severe itching. In time, the skin thickens and lichenifies. As subepidermal elastic tissue is lost, wrinkles and large skin folds or hanging groins are formed. In parts of Africa, fibrosing, obstructive lymphadenitis may result in elephantiasis. Invasion of the eye, however, causes the most devastating lesions. Punctate keratitis, iritis, and chorioretinitis can lead to a decrease in vi-sual acuity and, in time, total blindness. In Central America, eye lesions may be seen in up to 30% of infected patients. In certain communities in West Africa, 85% of the population has ocular lesions and 50% of the adult male population is blind.

DIAGNOSIS

The diagnosis is made by demonstrating the microfilariae in a thin skin snips taken from an involved area. When the eye is involved, the organism may sometimes be seen in theanterior chamber with the help of a slit lamp.

TREATMENT AND PREVENTION

Traditionally, DEC has been used to kill the microfilariae. Treatment was begun with very small doses to prevent rapid parasite destruction and the attendant allergic consequences.

This consideration was particularly important when the eye was involved; a treatmentinduced inflammatory reaction can damage it further. The newer microfilaremic agent, ivermectin, has been demonstrated to be more effective than DEC and does not appear to induce the severe allergic manifestations seen with the latter agent. Because it does not kill the adult worm, retreatment over a period of several years is necessary. No satisfactory methods of control have yet been developed. Application of insecticides to the vector’s breeding waters must be sustained for decades to disrupt transmission permanently, because the parasite is so long-lived within humans. With the introduction of ivermectin, mass treatment or chemoprophylaxis is now possible. 

There are no effective vaccines or chemoprophylactic agents. Annual mass distribution of ivermectin over a period of 10 to 15 years may interrupt the transmission cycle. A World Health Organization–funded Simulium larva control program utilizing aerial insecticides has succeeded in interrupting transmission of onchocerciasis in the savanna regions of West Africa.

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