What is Onchocerciasis?

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  • Written By: Dulce Corazon
  • Edited By: W. Everett
  • Last Modified Date: 09 October 2019
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Nematodes, also known as round worms, are parasites known to cause infection around the world. One example of a round worm is Onchocerca volvulus (O. volvulus), which is the cause of onchocerciasis in humans. Onchocerciasis, also referred to as river blindness, is an infection transmitted to people by female Simulian blackflies.

These female blackflies usually bite during daytime and are usually found breeding along flowing streams and rivers. Persons of any age and gender living near these areas have increased risks of getting the disease. Infection with onchocerciasis is commonly seen in Africa, Yemen, Venezuela, and Brazil.

The disease cannot be transmitted from person to person. Female blackflies are mostly the vehicle for the spread of onchocerciasis. When infected blackflies bite into the skin, they transfer the infective larvae or immature parasites into the blood. These parasites then migrate into the tissues under the skin forming hard lumps or nodules as they slowly mature. It takes about six months to a year before O. volvulus becomes an adult worm.

Adult male and female worms, which can live inside the host for about 15 years, then mate and release thousands of young parasites called microfilariae, into the nearby tissues. These microfilaria are capable of moving around the body for one to two years. Most of them migrate into the skin and some into the eyes. As they die, they cause certain reactions inside the body, and these usually manifest as onchocerciasis symptoms.


Symptoms of onchocerciasis usually occur several months after being bitten. These include rashes, swelling, and itchiness in scattered areas of the skin, hyperpigmentation or darkening of skin areas, presence of hard lumps or nodules, and loss of elasticity in the skin. Infected individuals may also present with dark skin patches on the lower part of the legs resembling the skin of the leopard. Dying microfilariae in the eye often cause reactions leading to the development of eye inflammations. When severe, this can sometimes result in blindness, hence the term river blindness.

When one or several female blackflies bite into the skin of infected individuals, they become infected with microfilariae. Inside the female blackflies, these microfilariae then undergo stages of growth until they become infective larvae in about three weeks. As infected blackflies bite into a person during this time, they then transfer the parasite into the skin, thus completing the life cycle of the O. volvulus.

Diagnosis of onchocerciasis can be done by testing the blood of infected individuals or by taking tissue samples for examination. Treatment of onchocerciasis is frequently the responsibility of infectious disease specialists, ophthalmologists, and dermatologists. Ophthalmologists are doctors who treat eye problems, and dermatologists deal with skin disorders. Medications that can kill the microfilariae are often administered in order to halt the progress of symptoms.


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