Onchocerciasis (River Blindness)

Onchocerciasis is a chronic parasitic disease caused by the filarial nematode Onchocerca volvulus, transmitted to humans via repeated bites of black flies (Simulium species) that breed in fast-flowing rivers, hence the term river blindness. It is the second leading infectious cause of blindness worldwide after trachoma, affecting approximately 21 million people globally with 99% in sub-Saharan Africa, particularly West and Central Africa, also endemic in Yemen and previously in Latin America (now eliminated through MDA programs).

Pathogenesis involves adult female worms (length 30-80 cm) and male worms (length 3-5 cm) residing in subcutaneous nodules (onchocercomas) where they live up to 15 years and produce millions of microfilariae that migrate through skin, eyes, and lymphatic system. Microfilariae cause inflammatory reactions when they die (often from drug treatment), releasing Wolbachia endosymbiotic bacteria that trigger pathology. Clinical features include onchodermatitis (severe pruritus, papular dermatitis, lichenification, leopard skin depigmentation, lizard skin atrophic changes), onchocercomas (subcutaneous nodules), lymphadenopathy with hanging groin, and ocular involvement (punctate keratitis, sclerosing keratitis, iridocyclitis, chorioretinitis, optic atrophy) progressing to blindness.

Diagnosis requires skin snip biopsy with microscopic examination for emerging microfilariae, slit-lamp examination of anterior chamber for ocular microfilariae, antibody serology (Ov-16), nodule excision histopathology, and PCR-based assays. Treatment is ivermectin 150 μg/kg orally as single dose every 6-12 months for 10-15 years (life span of adult worms), targeting microfilariae but not adult worms. Doxycycline 100 mg daily for 4-6 weeks targets Wolbachia endosymbionts and sterilizes adult worms. Mass drug administration (MDA) programs with annual ivermectin have reduced disease burden significantly. Surgical excision of accessible head and neck nodules complements pharmacotherapy.