If you're looking for leeches (to get leech blood) just listen, cuz they make a sound in the water when you're close to them (it sounds similar to someone making but bubbles inside the water). Everytime I go to the swamp i always find them quick and easy due to this and I hope this help other people too.
Info: (made by me) / They dont carry swamp fever but drains your stamina along your health. Make sure to get rid of em by using a cooking pot or campire (recommend campfire its easy to craft)
Deseased Leech (A red leech)
Info: They carry swamp fever so be carefull seing this variant. To cure the swamp fever use lesser antidote if you dont have.. you have no choice but to you nameless venom nameless venom can be dropped by nameless (his name is just nameless)
Drops (both variants): Raw meat, Hide (i think) and leech blood. (Deaseased leeches drop more leech blood.)
Uses of leech blood: Fishing (can attract pirana’s saber tooths’s and normal fish so highly reccomended for fishing. Amber is used too)