Former U.S. President Jimmy Carter is appealing for other donors to join Britain in a multi-million dollar campaign to wipe out guinea worm, a crippling and painful parasitic disease that now exists only in four African countries.

At a press briefing in London on Wednesday, British officials are expected to pledge 20 million pounds (US$31 million) over four years to the cause _ but only if other donors also open their wallets.

The global campaign to eradicate guinea worm started in 1980, when there were about 3.5 million cases of the disease, also known as dracunculiasis, every year across Africa and Asia.

Since then, cases have dropped by more than 99 percent, but the disease remains a problem in South Sudan, Ethiopia, Mali and Chad. Last year, there were 1,797 cases.

The Carter Center and partners, including the World Health Organization and the U.S. Centers for Disease Control and Prevention, aim to get rid of guinea worm disease by 2015.

There is no treatment or cure; the disease is eliminated by stopping people from drinking dirty water and by preventing infected people from wading into water and spreading the disease. Health campaigns that focus on changing behavior are often more difficult to implement than those that rely on medicines or vaccines.

Smallpox is the only disease in history to have been eradicated, while another effort to get rid of polio is also ongoing.

People get infected with guinea worm when they drink water infected with the larvae of the parasite.

About a year after someone is infected, the spaghetti-like worm, which can grow up to 1 meter in length, bursts out of their foot. That painful process can take months, often leaves the patient bedridden, and involves winding the worm around a stick so it doesn't break.

Guinea worm disease "prevents people from escaping poverty," Carter said in a statement. "I welcome the challenge laid down by the British government. I call on other donors to match their efforts."

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Online:

http://www.cartercenter.org

http://www.who.int/dracunculiasis/en/