Hi,
I have had experience with what we call "blow flies". They usually affect cows. If I remember correctly, they lay an egg on the surface that borrows into the skin as it matures, making a rather deep hole. They grow and eat their way out. Very hard to do but I learned to leave the entire process alone and everything would be fine. If tampered with, the larvae (or whatever the horrid thing is inside, eating away) dies and then the problems occur because it will not exit the host when dead and infection results. I think the adults feed on dead carcasses. Lovely, huh? I wonder if that is the same kind of situation your kitty had?