Bloodletting by the freshwater leech Hirudo medicinalis
is used to control swelling subsequent to the reattachment of
severed fingers or transplanted tissue. Saliva of bloodsucking
leeches contains the anticoagulant hirudin as well as an anesthetic.
This leech harbors a symbiotic bacterium Aeromonas
hydrophila; not only does the bacterium digest blood, but also
produces an antibiotic that kills other bacteria.