Onchocerciasis is a disease caused by infection with the parasitic filarial nematode (worm) Onchocerca volvulus. It is transmitted through repeated bites from blackflies of the genus Simulium causing chronic and progressive skin, eye, and neurological symptoms. The eye disease is often called river blindness. There are about 20 million people infected worldwide with onchocerciasis with 99% in sub-Saharan Africa with some in Latin America and the Middle East. The name ‘river blindness’ is because the blackflies mostly live and breed near faster flowing streams and rivers.
Once developed the adult worms cluster in subcutaneous nodules or ‘nests’ (onchocercomata) and the symptoms are caused by the microfilariae that move around in the subcutaneous tissue and induce intense inflammatory responses when they die. They live from 10 to 15 years.
The symptoms typically appear between 9 months and 2 years after the initial infecting bite (the prepatent phase). Nodules under the skin form around the adult worms. There is usually generalized pruritus that may be severe. A papular rash (onchodermatitis) may occur which eventually may develop into lichenification, loss of skin elasticity, atrophy and maybe a depigmentation. An itchy rash with lumps that resembles eczema can typically affect the face, trunk, hands and the feet.
The eyes may become itchy with redness or photophobia (ocular onchocerciasis). After a number of years there is scarring that progresses to cause visual loss and ultimately blindness (river blindness).
Sometimes there is a weight loss and generalized myalgias.
There is also a high prevalence of epileptic seizures (”river epilepsy”).
The diagnosis is typically made via the history and clinical features with confirmation by demonstration of microfilariae in a skin nodule biopsy.
- There is no vaccine for onchocerciasis.
- Spraying programs have been used to control or eliminate the blackflies.
- Mass dosing of areas prone to outbreaks with ivermectin are often done every six months to interrupt the worm life cycle.
- Preventative measures to prevent blackfly bites with the use of insect repellents (eg diethyltoluamide) on exposed skin and using pants and long-sleeved shirts to cover the arms and legs.
- Wearing clothing that is treated with the insecticide permethrin.
- Short-term travel to a problematic area is not necessarily a problem as repeated bites are usually required to get a sufficient quantity of the parasite for an infection to develop.
- The World Health Organisation runs a number of Onchoceriasis eradication programs.
Ivermectin is usually given every 6 months for the life span of the adult worms (for up to 15 years) or for as long as there is evidence of skin or eye infection. Ivermectin is effective against the larvae but it does not kill the adults, so hence the need to be continued with for the lifespan of the adult.
Doxycycline can be used off-label to kill the Wolbachia bacteria on which the adult worms needs in order to survive.
Treatment with moxidectin (related to ivermectin) is under investigation.
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