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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...

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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.

Read more at Wikipedia.org


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Ivermectin approved for two parasitic diseases - strongyloidiasis; onchocerciasis - Update - Brief Article
From FDA Consumer, 3/1/97

The antiparasitic animal drug ivermectin was approved by FDA to treat two human parasitic diseases, strongyloidiasis and onchocerciasis.

Strongyloidiasis is common in many tropical countries and is occasionally acquired in some areas of the United States. Infection is usually confined to the small intestine. It can persist for many years, causing abdominal pain, diarrhea, and elevated levels of white blood cells. In people with weakened immune systems, the infection can spread through the body and be fatal.

Onchocerciasis, commonly known as river blindness, is most often found in Africa and South and Central America. The parasite is transmitted by the bite of a black fly, which deposits immature forms of the parasite under the skin. There, the worms mature and the adult females produce more larvae. The immature worms migrate under the skin, causing intense itching. They can also migrate to the eyes, causing inflammation and blindness.

Ivermectin is better tolerated than other drugs approved to treat these conditions, and it is more effective than the previously approved treatment for onchocerciasis. In clinical studies, a single dose of ivermectin cured between 64 and 100 percent of patients with strongyloidiasis who had normal immune systems. In studies of onchocerciasis, a single dose of the drug reduced the number of larvae under the skin an average of 83 percent at three days and 99.5 percent at three months. Adverse reactions to the drug differ depending on which disease is being treated but can include skin rashes, itching, dizziness, diarrhea, and nausea.

Merck & Co. Inc., of West Point, Pa., sells ivermectin in the United States under the trade name Stromectol. (Elsewhere it is marketed as Mectizan.) Before its approval on Nov. 25, 1996, ivermectin had been available here for limited human use as an investigational drug.

Ivermectin has been used worldwide since 1987 to treat river blindness. More than 5.2 million people have received the drug through the World Health Organization's Onchocerciasis Control Program and Merck's Mectizan Donation Program.

(See also "Treating Tropical Diseases" in the January-February 1997 FDA Consumer.)

COPYRIGHT 1997 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group

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