These are not as rare as you think. Every other I get yoinked off a mount by a kapro I end up running into at least one. That being said I don't get yoinked off mounts often but when I do in the swamp I find a leech. After puttering around on my pelagornis I managed to find enough to get 30ish leech blood.
To the post that said they weren’t sure if swamp fever exists in mobile, it does. I’ve gotten it on mobile. Also I was killing animals in the swamp with my dire wolf and it said killed infected leech in the kill feed.