Leech
The leech was an animal that attached itself to another creature, usually absorbing some of its blood. It was sometimes used in medicine, though the practice had ceased by 2374.

The leech native to Earth consumed hemoglobin present in the blood of many other Earth organisms. ()

Doctor Phlox was fond of leeches as a snack. In 2154, he snacked on some leeches while dictating a letter to Dr. Jeremy Lucas. He also fed one to a begging Porthos after warning him that it would clear out the dog's intestinal tract. ()

While experiencing Q's take on Robin Hood in 2367, Maid Marian (Vash)'s servant suggested using leeches to relieve her fever caused by the brain sickness she feared Marian had. ()

The Emergency Medical Hologram of the  told The Doctor, after arriving in the Alpha Quadrant, that advances in medical equipment since the  left meant that "scalpels and leeches" were not used anymore. ()

In 2377, when The Doctor was forced to work aboard Hospital Ship 4-2, he complained "With technology this primitive you may as well be using leeches." ()

See also 
 Aldebaran mud leech

External link 
 

de:Blutegel
Medical equipment
Earth animals