onchocerciasis (river blindness)

Etiology: - Onchocerca, especially Onchocerca volvulus Epidemiology: 1) equatorial Africa 2) Saudi Arabia 3) Central & South America 4) infection is transmitted by the bite of the blackfly (Simulium) 5) the blackfly develops along freely flowing rivers & streams, particularly rapids Pathology: 1) infects primary the skin, eyes & lymph nodes 2) infective larvae from the blackfly develop into adults in subcutaneous nodules 3) 7 months to 3 years after infection, the gravid female releases microfilariae that migrate out of the nodule & concentrate in the dermis 4) infection is transmitted when a female blackfly ingests microfilaria from the host's skin 5) males are 3-6 cm in lenth, females 40-60 cm in length 6) adults live up to 18 years, average 9 years 7) damage is induced by microfilariae, not adults 8) inflammatory changes in the skin, with fibrosis & atrophy 9) neovascularization & corneal scarring lead to corneal opacities 10) inflammation in anterior chamber of the eye & posterior chamber of the eye results in anterior uveitis, chorioretinitis & optic atrophy Clinical manifestations: 1) skin manifestations a) subcutaneous nodule (onchocermata) at site of blackfly bite 1] firm & non tender 2] contain the adult worms 3] for every palpable nodule, there are 4 deeper ones b) pruritus & rash 2) eye manifestations a) visual impairment with moderate to heavy infestation b) conjunctivitis & photophobia (early) 3) mild to moderate lymphadenopathy 4) cachexia 5) no periodicity of infection Laboratory: - diagnosis is made by recovery of an adult worm in an excised nodule or of microfilaria on skin snips a) material is incubated in tissue culture medium or saline on a glass slide for 2-4 hours or overnight b) microfilariae emerge from the skin & may be seen under light microscopy c) no sheaths Management: 1) ivermectin 150 ug/kg PO single dose annually or semiannually 2) suramin if total irradication indicated 2) surgical excision of nodules on head because of proximity to eye

References

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