I have never encountered a single Leech in my entire playtime on Ark. I’ve taken dino’s at both night and day through both swamps on Ragnarok, swam through them and stood next to the shore hoping to get attacked. (I’m on official by the way) and I have Swamp Fever somehow.
These things are the worst. They’ll attack you very sneakily, and I hate having to take time off of my task just to build a campfire to get the stupid thing off. Even though it just takes a little bit of time to get the thing off, it’s still extremely annoying. Luckily I haven’t encountered a diseased leech yet, so there’s that.