Stingrat

When a rat ingests bloodsap - either directly by nibble on bark it's spilled on or indirectly by eating a bug - they mutate into stingrats.
Visually, the main difference between rats and stingrats is the lack of hair. Over the course of the mutation, a rat's fur falls out in chunks, leaving them pink and wrinkled. They grow up to 1.5x larger and leaner, resulting in a less round profile. Their teeth become sharper as well, allowing them to bite through material that would have previously given them trouble.
Their name comes from the hundreds of tiny stinging hairs that grow in place of fur. These hairs are too fine and translucent to be seen from a distance, but touching one results in a shock of pain, like grasping a nettle. The hairs cover the rats' backs and are most densely gathered on their tails. Their tails become weapons, capable of whipping at a predator and inflicting a shock of pain. Even when dead, picking up a stingrat bare-handed results in pain and a rash that lasts a day or two.
The biggest danger from stingrats, though, is their ability to spread infection. Like their non-mutated brethren, stingrats infiltrate human dwellings in search of food. If a human eats food that has been contaminated by the urine or feces, infection could follow.
In Kaleina, all ships setting sail from a mainland port are required to be inspected by the Border Rangers and receive a document declaring it vermin-free. This document is required in order to dock at a protected island.
Ships coming from outside Kaleina are required to dock in the harbour, and a constable from the Royal Kaleinen Constabulary will inspect the ship and issue it a vermin-free document at that time.
Despite these measures, cases always slip through. Containing them to only certain islands, and attempting to eradicate them, is a major issue for governments.