Onchocerciasis, commonly known as “river blindness”, is caused by a parasitic worm acquired after the bite of an infected blackfly. More than 99% of infected people live in 31 African countries. The disease also exists in some areas in Latin America and Yemen.
About 21 million people are infected per year. 14.6 million of the infected people developped skin disease and 1.15 million had vision loss.
What causes Onchocerciasis
Onchocerciasis, also known as river blindness, is caused by a parasitic worm Onchocerca volvulus. Humans get infected from the repeated bites of a blackfly of Simulium genus. Usually, many bites are required before infection occurs. These flies live near rivers, hence the common name of the disease.
Onchocerca volvulus has a complex life cycle where humans are the definite hosts. The parasite enters the human body in the form of a larvae, which later can develop into adult parasites.
The adult worms produce microfilariae after around 10 months of infection. The adult worms get encapsulated and form subcutaneous nodules in humans and they can live up to 15 years shedding microfilariae, which travel to the skin and eyes causing the major symptoms of the disease.
Where are Onchocerca found
Around 21 million people worldwide are infected with onchocerciasis and most of the cases are localized in Sub-Saharan Africa. It is also found in Central and South America along with Yemen. About 99% of onchocerciasis cases occur in Africa.
Blackflies, the transmitters of the disease, breed in rivers and streams and usually bite during the daytime.
Onchocerca originated in Africa and was exported to the Americas by the slave trade, as part of the Columbian exchange that introduced other old world diseases such as yellow fever into the New World
What are the symptoms of Onchocerciasis
The course of the disease varies from person to person and depends on the immune reaction to the parasite and its symbiont Wolbachia bacteria. Some infected people may experience no clinical symptoms, while others might have a strong immune response, which causes the associated pathologies.
Intense itching, swelling and inflammation on skin, along with blindness are some of the symptoms of onchocerciasis. Subcutaneous and muscle nodules where the adult worms reside do not typically cause significant symptoms. The itchy skin and severe rashes may permanently damage the skin leaving papules, scars and hyperpigmentation.
Microfilariae are able to induce strong immune responses when they migrate out of the nodule. Ocular onchocerciasis develops due to the migration of microfilariae to cornea, which leads to the inflammation of the cornea. Chronic inflammation makes the cornea opaque, which leads to blindness.
How can Onchocerciasis be prevented
There is no available vaccine for onchocerciasis. The best prevention involves personal protection measures, avoiding bites from flies, using insecticides like N,N-Diethyl-meta-toluamide (DEET) on skin and wearing proper clothing. The clothing can be treated with permethrin.
How is Onchocerciasis diagnosed
Skin snip biopsy is the classical diagnostic test performed for the detection of microfilariae. The test is based on taking a biopsy punch or shaving off a small piece of skin with a scalpel; usually several snips are necessary for diagnostic sensitivity. The piece of tissue is then incubated in a solution to allow microfilariae to emerge, which is followed by microscopic examination for microfilariae.
Nodulectomy or nodule excision allow the identification of adult worms. There are also antibody tests, which however are not available outside the research setting and they can only identify if the person had any past filarial infection but it cannot detect current infection.
Ultrasonography can be used for the detection of deeper nodules which are not palpable.
How is Onchocerciasis treated
Ivermectin paralyses and kills microfilariae, which are shed by the adult worms. 2 doses of Ivermectin (Mectizan) over 3 years every 6 months is used in mass drug administration programs in endemic areas.
An antibiotic called Doxycycline has also been used and is more effective against adults worms compared to Ivermectin. Doxycycline sterilizes adult female worms by depleting Wolbachia, the symbiont bacteria of Onchocerca volvulus. Doxycycline is more laborious in administration as it requires daily dose for at least 4-6 weeks.
Edited by: Malika Khassenova
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