I - Rules
(see WOD, p.6)
For every wound token lost due to a Leech attack, the target also loses 1 fatigue (or suffers 1 additional wound, ignoring armor, if the target is out of fatigue) and the attacker is healed of 1 wound.
II - References
Heroes with the Leech ability: none.
Monsters with the Leech ability: Ferrox (master only).