Hi, I'm new here.
I've a mouse just like you. My little Mikey is just one years old.
Is May having trouble moving? is she still pretty active when you handle her or she doesn't like moving too much?
Sometimes, when Mikey gets injured, he would twitch profusely for a second, then stop.
I don't think it's cruel though. Let her fight her way off the disease first. It's not fair to end her life just because we think but not feel what they're really going through.