LRB has a good one on "delusory parasitosis"...
I was convinced that I was infested. ‘Infested’ was the word, I thought, as well as ‘contaminated’. The pubic lice multiplied to a plethora and became imaginatively licensed to inhabit my entire body. They crawled on my arms, my torso, my legs, my hair, sometimes my face and neck. They had become all-rounder lice. Not even lice, if someone had pointed out the impossible ethology I had invented for them. They were … I didn’t know what they were, but they were. Insects, lice-like, flea-like, tic-like crawling creatures that lived on me, and indeed, in me. I thought they burrowed under my skin and emerged to wander about on the surface in the dark of night or under cover of my clothes. I felt them, tickling me in specific parts, and the redness I saw when I finished scratching my skin convinced me that they were there (so easy now to write that rational sentence). I saw them, always out of the corner of my eye. I became most distressed at night. I
would feel their presence and then turn on the light quickly to catch them, but of course they had burrowed back into my skin by the time I could focus. It was a malevolent game of hide and seek. They had super-lice powers: they sensed me looking for them, and always dodged me. I was sure I saw them, yet I could never quite say what they looked like. I found evidence of them, even occasionally caught one and killed it as you do a flea, squeezing it between my fingers. Then I would put it in my palm and examine it carefully under a light. I saw it was something, a mote, a dot, black, white, grey, but never quite well enough to be sure exactly what. It could have been a flake of skin, a speck of dust, a tiny thread, but I knew it wasn’t. In that special way you know when you really know or are crazy.