WordNet sense 1 (a follower who hangs around a host (without benefit to the host) in hope of gain or advantage):

parasite, sponge, sponger

WordNet sense 2 (carnivorous or bloodsucking aquatic or terrestrial worms typically having a sucker at each end):

bloodsucker, hirudinean

WordNet sense 3 (draw blood):

bleed, phlebotomize, phlebotomise