Pathology and the World Health Organization Collaborating Centre for the Histopathology of Filarial Diseases in Man,

Onchocerciasis- infection by Onchocerca volvulus- has four cardinal manifestations: dermatitis, subcutaneous nodules, sclerosing lymphadenitis, and eyedisease. The first three are discussed here. The dermatitis begins when micro filariae degenerate in the dermis. This process is accompanied by inflammation, with degranulation of eosinophils and depo-sition of the major basic protein of the eosinophil granules on the cuticleof the microfilariae. So far as is known, the chronic effects of onchocerciasis are all a consequence of the degeneration of microfilariae. Subcutaneous nodules contain coiled adult worms and have an outer layer of fibrous scar and a central inflammatory cell exudate, which may cavi-tate. Perfusion of India ink reveals arborization of capillaries around adult worms, which derive nutrition from these networks. Onchocercal lymphadenitis is characterized initially by histiocytic hyperplasia and follicular atrophy and later by fibrosis and obstruction of lymph flow, a condition causing adenolymphocele ("hanging groin") and elephantiasis of the genitalia. Some patients appear to have immune tolerance to degenerating microfilar-iae, perhaps as a result of exposure in utero to microfilarial antigens in the maternal circu-lation. In contrast, other patients (Yemenites, for example) have a localized but intense response to a few micro filariae; these patients are hypersensitive- perhaps because they were not exposed to microfilarial antigens in utero. Autopsy data on infection of deep organs are limited. Onchocerciasis is the infection of humans by the filarial nematode Onchocerca volvulus. Three of the four cardinal manifestations of human onchocerci-asis are discussed here: dermatitis, subcutaneous nodules, and sclerosing lymphadenitis. These and the fourth manifestation- ocular changes- have been reviewed previously [1-13]