a bloodsucking leech,
of Europe, introduced into the northeastern U.S., usually green with brown stripes, up to 4 inches (10 cm) long: once used by physicians to bleed patients.
a large European freshwater leech,
formerly used in medical bloodletting
Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
The same result has followed from keeping together different varieties of the medicinal leech.