Diseased leeches drop like 10x more of these than regular leeches, but they spawn with triple the health (120 with every level in health +24) and move faster than a regular leech so you might have to use firearms in case your melee can't one shot them and if you don't want to risk getting swamp fever. Also if you don't play old ark mobile ignore this next part:

They can also spawn in swamp dungeons, as after enough time has passed one will spawn in the fountain looking pool areas with decorative lilypads and mangrove trees. If you go into the water and walk to the center of the pool then step out, then you might see a leech or 2 chasing after you. They will also spawn at a much higher level than normal, similar to creatures in the snow and swamp cave. If a leech latches onto you while you are attempting to kill it then walk in a poison vent then walk out as this will deal 5 damage to you but remove any leeches latched onto you.

If you are on singleplayer you can also farm leech blood in this area by leaving and rejoining your game, then a leech will automatically spawn in the center of the pool and you can kill it, then leave and rejoin your game again. You can do this infinitely until the dungeon timer in the top right of your screen runs out and you die instantly.