Leeches could hold the key to a new treatment for rheumatoid arthritis. New Zealand scientists claim to have discovered a chemical in a European species of leech which can slow the growth of cells in joints ravaged by the disease. The European leech has long been valued for its medical applications. Recently, anticoagulants found in the animal's saliva have been used to reduce the chances of blood clots in patients with heart problems. The worms have also been used to help restore blood flow to severed damaged limbs.