Onchocerciasis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]
Overview
Onchocerciasis is a Neglected Tropical Disease (NTD) caused by the parasitic worm Onchocerca volvulus. It is transmitted through repeated bites by black flies of the genus Simulium. The disease is called River Blindness because the black fly that transmits the infection lives and breeds near fast-flowing streams and rivers and the infection can result in blindness. In addition to visual impairment or blindness, onchocerciasis causes skin disease, including nodules under the skin or debilitating itching. Worldwide onchocerciasis is second only to trachoma as an infectious cause of blindness.
Historical Perspective
Dr. Rodolfo Robles Valverde's study on patients with river blindness in Guatemala led to the discovery that the disease is caused by filaria of O. volvulus, and sheds light on the life cycle and transmission of the parasite.
Classification
Onchocerciasis may be classified into different types depending on the cutaneous lesions seen.
Pathophysiology
The disease spreads from person to person by the bite of a black fly. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person's skin enter and infect the black fly. The larvae develop over 2 weeks in the fly to a stage that is infectious to humans. An infectious black fly will typically drop larvae when biting a person. The larvae then penetrate the skin to infect the person. Because the worms reproduce only in humans but need to complete some of their development inside the black fly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual.
Causes
Onchocerca volvulus is a nematode that causes onchocerciasis mostly in Africa. Onchocerca volvulus, along with most filarial nematodes, share an endosymbiotic relationship with the bacterium Wolbachia. In the absence of Wolbachia, larval development of the O. volvulus is disrupted or ceased.
Differentiating Onchocerciasis from other Diseases
Patients with onchocerciasis may manifests with skin rash, eye disease and skin swelling. It needs to be differentiated from other diseases with similar manifestations.
Epidemiology and Demographics
Onchocercal infections are found in tropical climates. About 99% of onchocerciasis cases occur in Africa. Also, the parasite is found in limited areas in the America and in Yemen in the Middle East.
Risk Factors
Those most at risk are people who live in areas where the parasite is spread followed by adventure travelers, missionaries, and peace corps volunteers who are exposed for long periods of time (generally more than 3 months) to black fly bites in areas where the parasite exists. The disease is most intensely transmitted in remote rural African agricultural villages which are located near rapidly flowing streams.
Natural History, Complications and Prognosis
There can be inflammation of the optic nerve resulting in vision loss, particularly peripheral vision, and eventually blindness.
Diagnosis
History and Symptoms
Infected persons may be without symptoms. Those with symptoms will usually have one or more of the three manifestations: skin rash (usually itchy), eye disease, and nodules under the skin. The most serious manifestation consists of lesions in the eye that can lead to visual impairment andblindness.
Other Diagnostic Studies
The gold standard test for the diagnosis of onchocerciasis remains the skin snip biopsy. The biopsy is performed using a sclerocorneal biopsy punch or by elevating a small cone of skin (3 mm in diameter) with a needle and shaving it off with a scalpel. There are antibody tests that can assist in the diagnosis of onchocerciasis, though many are not available outside the research setting
Treatment
Medical Therapy
The treatment of choice for onchocerciasis is ivermectin, which has been shown to reduce the occurrence of blindness and to reduce the occurrence and severity of skin symptoms. An evolving treatment is doxycycline, which has been shown in studies to kill Wolbachia, an endosymbiotic rickettsia-like bacteria that appears to be required for the survival of the O.volvulus macrofilariae and for embryogenesis. Doxycycline does not kill the microfilariae, so treatment with ivermectin would be needed to result in a more rapid decrease of symptoms.
Primary Prevention
There are no vaccines or medications available to prevent becoming infected with O. volvulus. The best prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when blackflies bite, and wearing permethrin- treated clothing.