Onchocerciasis

Onchocerciasis or river blindness is the world's second leading infectious cause of blindness. It is caused by Onchocerca volvulus, a parasitic worm that can live for up to fourteen years in the human body. more...

Life cycle

The life cycle of O. volvulus begins when a parasitised female Black fly of the genus Simulium takes a blood meal. Saliva containing stage three O. volvulus larvae passes into the blood of the host. From here the larvae migrate to the subcutaneous tissue where they form nodules and then mature into adult worms over a period of one to three months. After the worms have matured they mate, the female worm producing between 1000 and 1900 eggs per day. The eggs mature internally to form stage one microfilariae, which are released from the female's body one at a time.

The microfilariae migrate from the location of the nodule to the skin where they wait to be taken up by a black fly. Once in the black fly they moult twice within seven days and then move to its mouthparts to be retransmitted.

Causes of morbidity

When the microfilariae migrate to the skin they are a target for the immune system. White blood cells release various cytokines that have the effect of damaging the surrounding tissue and causing inflammation. This kills the microfilariae but is the cause of the morbidity associated with this disease.

In the skin this can cause intense itching that leads to the skin becoming swollen and chronically thickened, a condition often called lizard skin. The skin may also become lax as a result of the loss of elastic fibres. Over time the skin may lose some of its pigment; on dark skin this gives rise to a condition known as leopard skin.

The symptom that gives the disease its common name river blindness is also caused by the immune system's reaction to the microfilariae. The surface of the cornea is another area to which the microfilariae migrate, where they are also attacked by the immune system. In the area that is damaged, punctate keratitis occurs, which clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time the entire cornea may become opaque, thus leading to blindness.

Treatment and control

The treatment for onchocerciasis is ivermectin (mectizan); infected people can be treated once every twelve months. The drug paralyses the microfillariae and prevents them from causing itching. In addition, while the drug does not kill the adult worm, it does prevent them from producing additional offspring. The drug therefore prevents both morbidity and transmission.

Since 1988, ivermectin has been provided free of charge by Merck & Co. through the Mectizan Donation Program (MDP). The MDP works together with ministries of health and non-governmental development organsations such as the World Health Organisation to provide free mectizan to those who need it in endemic areas.

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