The majority of lesions of onchocerciasis are caused by the host's inflammatory response to dead and dying microfilariae. These dying microfilariae cause local inflammation, leading to damage to surrounding tissues, which accumulates and produces the pathological features of the disease. Early skin changes include perivascular inflammatory infiltrates of eosinophils, plasma cells, histiocytes and lymphocytes; hyperkeratosis; acanthosis, with increased melanin in the upper dermis and dilated tortuous lymph and blood capillaries. This progresses to loss of elastic fibres, fibrosis and eventual atrophy of the epidermis associated with binding of eosinophil-derived protein (MBP, major basic protein) to elastic fibres. Skin lesions may, consequently, show a wide range of histopathological appearances, including acute exudative, granulomatous and fibrotic changes.
Ocular lesions are also thought to be directly or indirectly related to invasion and local death of microfilariae. The snowflake opacities of punctate keratitis are focal collections of lymphocytes and eosinophils, with some transient interstitial oedema around dead or dying micro-filariae. Sclerosing keratitis, by contrast consists of an inflammatory exudate, mainly of lymphocytes and eosinophils, scarring and a fibrovascular pannus formation. Uveitis and chorioretinitis are usually low-grade, chronic non-granulomatous inflammatory processes involving lymphocyte and macrophage infiltration.
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