Sounds like yet another nabkiller. As it stands I never leave the base without a horse, and will trample your little noodly annoyances. This just widens the gap between the weak and the strong further. Also if you don't notice and run away immediately, (imagine you're walking through the woods early world to preserve stamina), the adder can posion you in two bites.
While there needs to be more adder availability to deal with swamp fever, I suggest just expanding their biome. Less interesting but also less frustrating.
To further this, what does the player learn from this danger? Step on a leaf, your life will be brief? My first world back in legacy, I died because I ran out of food, started starving, and attacked an ant hill in desperation. Lesson learned, stockpile food.