September 25, 2016
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Onchocerciasis (cont.)

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What causes onchocerciasis?

The cause of onchocerciasis is the transfer of larvae of the parasite Onchocerca volvulus by the female blackfly when the fly gets a blood meal (bites) a human. The larvae enter the subcutaneous tissues and develop into adult male and female worms (filarial nematodes). These reproduce in the human tissue and form microfilariae that migrate to other areas of connective tissue and occasionally to the blood, urine, and sputum. In addition, both the adult worms and the microfilaria are colonized with bacteria termed Wolbachia that help these parasites survive. When the worms die, a host immune response ensues that can destroy optical tissue in the eye. The life cycle of the parasites is continued when a blackfly bites a human and, during its blood meal, ingests microfilariae. The complex life cycle is shown below.

Picture of the life cycle of O. volvulus
Picture of the life cycle of O. volvulus; SOURCE: CDC

What are risk factors for onchocerciasis?

Living in and visiting sub-Sahara Africa and the few other areas where the blackfly is endemic is a major risk factor for developing onchocerciasis. The disease usually occurs after repeated exposures to blackfly bites so short-term travelers through these areas pose little risk; however, missionaries, volunteer health workers, and others who may spend a few months in the areas have an increased risk of infection.

Is onchocerciasis contagious?

Onchocerciasis is not contagious person to person. Transmission of the disease is through the bite of female blackflies (usually occurring during the daytime near rapidly flowing rivers and streams). Multiple bites are usually needed before being infected.

Medically Reviewed by a Doctor on 9/16/2016

Source: MedicineNet.com
http://www.medicinenet.com/onchocerciasis/article.htm

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