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Life Cycle of Onchocerciasis

Onchocerciasis, a form of filariasis, is caused by the parasitic filarial nematode, Onchocerca volvulus. Also known as river blindness, the illness is found mainly in tropical areas of Africa and can also be found in Central America and the Middle East. Immature larvae of the worms can cause blindness, skin rashes, lesions and intense itching.

  1. Initial Infection

    • To begin the life cycle, an infected blackfly will bite and feed on the blood of a host. Larvae will be introduced onto the skin from the fly's proboscis and will enter into the open wound. Within the subcutaneous tissues of the host, the larvae will develop into adult worms.

    The Adult Stage within Humans

    • The adult worms will then form and live in nodules within the subcutaneous tissues of the host for up to 15 years. Nodules can contain many male and female worms. According to the Centers for Disease Control, males are approximately 20 to 40 mm in length, while females average 30 to 50 cm. Within the nodules, the worms mate and females produce microfilariae.

    The Microfilarial Stage within Humans

    • Microfilariae reside mainly within the skin and lymphatic system but can also be found in the blood, saliva and urine. They can live for up to two years. The Centers for Disease Control states that they are 220 to 360 μm in length. Eventually, a blackfly bites the host, ingesting infected blood.

    The Blackfly Stages

    • Blackflies of the Simulium genus serve as the vector for Onchocerciasis. Once infected, the microfilariae penetrate the fly's midgut and migrate to its thoracic muscles. There, the larvae develop through three stages to become infective larvae. These larvae then migrate to the fly's proboscis, where they wait for the fly to bite another host, thereby beginning the life cycle again.

    Diagnosis and Treatment

    • According to the World Health Organization (WHO), a combination of insecticide usage to control blackfly populations and drugs to treat the disease is proving to be an effective way to control Onchocerciasis.

      Diagnosis can be accomplished by searching for microfilariae in a recently collected blood sample or skin biopsy. Blood tests that look for specific antigens can also assist in diagnosing the illness. Ivermectin is the drug of choice for treating the disease in humans.

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