Characteristics of the organism and its antigens

O. volvulus is a parasitic filarial nematode worm which undergoes a complex set of developmental stages involving two different hosts. Infection is transmitted when the insect vector, female black flies of the genus Simulium, bites the human host and introduces infective larvae which subsequently molt twice and mature to male and female adult worms over a period of 3-18 months. The adult worms arc encapsulated in subcutaneous nodules of host collagenous tissue called onchocercomata. The adult worms are unable to replicate within the human host, where they can survive up to 10-15 years. Following reproduction within the onchocercomata, the females release microfilariae which migrate out of the nodules to invade the host tissue and concentrate in the dermis. Approximately 7-34 months (prepatent period) pass before microfilariae are detected in the skin (patent infection). Completion of the life cycle is initiated when infected individuals are bitten by a female black fly which ingests microfilariae along with host tissues from the skin. A small portion of the ingested microfilariae migrate from the gut to the thoracic muscles of the fly, develop into infective larvae over a period of 6-8 days and then migrate to the head of the fly. When this fly bites another individual the infection is transmitted. Figure 1 illustrates the life cycle of O. volvulus.

Major impediments for characterization of O. volvulus antigens have been its complex life cycle and limited availability in the absence of a suitable in vitro culture system. Since only adult worms

Infective larvae

Figure 1 Life cycle of Onchocerca volvulus. The sizes of different forms of the parasite are: infective larvae 440-700 fim long, 19-28 ju,m wide; male adult worms 3-6 cm long, 130-150 /¿m wide; female adult worms 23-70 cm long, 275-325 )im wide; microfilariae 300-320 nm long, 6-9 ^m wide.

Infective larvae

Figure 1 Life cycle of Onchocerca volvulus. The sizes of different forms of the parasite are: infective larvae 440-700 fim long, 19-28 ju,m wide; male adult worms 3-6 cm long, 130-150 /¿m wide; female adult worms 23-70 cm long, 275-325 )im wide; microfilariae 300-320 nm long, 6-9 ^m wide.

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