Define onchocerciasis?

Onchocerciasis (River Blindness)

Onchocerciasis is a vector-borne parasitic neglected tropical disease caused by the filarial nematode Onchocerca volvulus, transmitted through the bite of black flies of the genus Simulium that breed in fast-flowing water bodies. 1

Epidemiology and Global Burden

• Approximately 37 million people worldwide are infected with onchocerciasis, with 200 million people at risk of contracting the infection. 1
• 99% of the total global risk and burden occurs in Africa, specifically south of the Sahara. 1
• The disease also exists in scattered foci in Yemen and the Yanomami area of Brazil, Venezuela, and the Bolivian Republic. 1
• Onchocerciasis is the world's second leading infectious cause of blindness after trachoma, affecting approximately 600,000 people with visual impairment, half of whom are partially or totally blind. 1, 2

Clinical Manifestations

Dermatologic Features

• Diffuse dermatitis with severe pruritus and excoriation results in skin hypo- or hyperpigmentation, lichenification, and ultimately depigmentation ("leopard skin"). 1
• Subcutaneous nodules (onchocercomas) appear on bony prominences, head, and trunk, containing adult worms. 1
• Travelers typically present with mild to intense pruritus and limb swelling only, rather than the full spectrum of disease seen in endemic populations. 1

Ocular Disease

• Ocular manifestations include keratitis, uveitis, and choroidoretinitis, with eventual blindness—hence the name "river blindness." 1
• The incubation period ranges from 8 to 20 months before clinical symptoms appear. 1

Systemic Impact

• The disease is associated with reduced life expectancy and causes high mortality among onchocerciasis-blind individuals. 1
• Onchocerciasis has been linked to epilepsy in endemic populations. 1
• Psychosocial and economic impacts include social stigma of infected persons and their families, disturbed sleep, reduced earnings among infected adults, poor school performance and higher dropout rates among infected school-aged children, and high health costs. 1
• Communities migrate away from fertile arable land in fear of infection, causing drops in agricultural yields and perpetuating poverty. 1

Transmission and Vector Biology

• The disease is transmitted through the bite of black flies of the genus Simulium, which breed exclusively in fast-flowing water bodies such as rivers and streams. 1
• The black fly vector introduces infective third-stage larvae with its saliva into the skin during blood feeding. 3
• Within the human body, adult female worms (macrofilariae) produce thousands of larvae (microfilariae) that migrate through skin and eye tissues. 3

Diagnostic Approach

• Microscopic visualization of microfilariae following incubation of skin snips in normal saline is the primary diagnostic method, though sensitivity is low, especially in recent infection. 1
• Slit lamp examination can identify microfilariae in the eye, but this is rarely positive in travelers. 1, 4
• Serology is available but does not differentiate between filarial species. 1, 4
• A unique biomarker, N-acetyltyramine-O,β-glucuronide (NATOG), can be detected in urine using liquid chromatography-MS-based metabolome analysis and tracks O. volvulus metabolism in both worms and humans. 5

Public Health Significance

• Onchocerciasis is classified as a disease of major public health importance and a serious vector-borne neglected tropical disease of public health and socioeconomic concern. 1
• The WHO established targets for onchocerciasis elimination in selected endemic countries by 2020 and in remaining African countries by 2025. 1
• Rigorous regional interventions over the past four decades have been effective in bringing the disease burden under control; it is currently not a public health problem in most endemic areas in Africa. 1