onchocerciasis (river blindness)

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Etiology

equatorial Africa
Saudi Arabia
Central & South America
infection is transmitted by the bite of the blackfly (Simulium)
the blackfly develops along freely flowing rivers & streams, particularly rapids

infects primary the skin, eyes & lymph nodes
infective larvae from the blackfly develop into adults in subcutaneous nodules
7 months to 3 years after infection, the gravid female releases microfilariae that migrate out of the nodule & concentrate in the dermis
infection is transmitted when a female blackfly ingests microfilaria from the host's skin
males are 3-6 cm in lenth, females 40-60 cm in length
adults live up to 18 years, average 9 years
damage is induced by microfilariae, not adults
inflammatory changes in the skin, with fibrosis & atrophy
neovascularization & corneal scarring lead to corneal opacities
inflammation in anterior chamber of the eye & posterior chamber of the eye results in anterior uveitis, chorioretinitis & optic atrophy

skin manifestations
- subcutaneous nodule (onchocermata) at site of blackfly bite
  - firm & non tender
  - contain the adult worms
  - for every palpable nodule, there are 4 deeper ones
- pruritus & rash
eye manifestations
- visual impairment with moderate to heavy infestation
- conjunctivitis & photophobia (early)
mild to moderate lymphadenopathy
cachexia
no periodicity of infection

diagnosis is made by recovery of an adult worm in an excised nodule or of microfilaria on skin snips
- material is incubated in tissue culture medium or saline on a glass slide for 2-4 hours or overnight
- microfilariae emerge from the skin & may be seen under light microscopy
- no sheaths

↑ Harrison's Principles of Internal Medicine, 14th ed. Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 1174