Onchocerca

Onchocerca is a genus of filarial nematodes in the family Onchocercidae. The most important human pathogen is Onchocerca volvulus, which causes onchocerciasis, commonly known as river blindness. Other Onchocerca species infect animals, including cattle and dogs, and are studied as models for parasitic filariae.

Transmission and life cycle: Adult worms reside in subcutaneous nodules and produce microfilariae that migrate through the skin and can reach the eyes. Blackflies of the genus Simulium transmit the parasite during blood feeding; microfilariae ingested by the fly develop into infectious L3 larvae and are transmitted to a new human host. Adults can live many years in nodules, sustaining a chronic infection and a continuous microfilarial load.

Clinical features: The most common symptoms are chronic pruritus and dermatitis, often leading to skin changes such as lichenification. Ocular involvement can progress from corneal scar formation to sclerosing endophthalmitis and progressive vision loss, potentially causing river blindness. Disease severity correlates with microfilarial burden and inflammatory responses to both microfilariae and dead worms.

Diagnosis and treatment: Diagnosis relies on detecting microfilariae in skin snips or nodules, with supporting ophthalmologic findings and, in some settings, serology or PCR. Ivermectin is the main microfilaricidal therapy, reducing skin microfilariae but not reliably killing adult worms. Doxycycline targets the Wolbachia endosymbionts essential for worm survival and fertility, often sterilizing or weakening adult worms. Moxidectin is another agent with longer-lasting activity. Public health control includes mass drug administration, vector control, and monitoring to reduce transmission and work toward elimination in endemic areas.

Epidemiology and control: Onchocerciasis is endemic in tropical regions of sub-Saharan Africa, parts of Latin America, and some pockets in the Middle East. Elimination efforts combine pharmacologic treatment with vector control and community engagement.