Onchocerciasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Overview

The disease spreads from person to person by the bite of a black fly. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person's skin enter and infect the black fly. The larvae develop over 2 weeks in the fly to a stage that is infectious to humans. An infectious black fly will typically drop larvae when biting a person. The larvae then penetrate the skin to infect the person. Because the worms reproduce only in humans but need to complete some of their development inside the black fly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual.

Pathophysiology

O volvulus LifeCycle.gif

During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound (1). In subcutaneous tissues the larvae (2) develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues (3). Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130 to 210 μm. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues (4). A blackfly ingests the microfilariae during a blood meal (5). After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic muscles (6). There the microfilariae develop into first-stage larvae (7) and subsequently into third-stage infective larvae (8) . The third-stage infective larvae migrate to the blackfly's proboscis (9) and can infect another human when the fly takes a blood meal (1).

Vector

The disease spreads from person to person by the bite of a black fly. Black flies bite during the day. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person's skin enter and infect the blackfly.


Shown below is an image of blackfly.

Black fly 1.jpg

References


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