The limit of the zapper in its current state is that...
The limit of the zapper in its current state is that it relies on electricity and so conductiveness of the material.
The best illustration of how the current travels in the body is to look at a lightning photo : many small paths and one big path.
Even if you put electrodes on the head, you won't be able to use enough intensity without risking to damage the brain, the nervous system, the eyes, the ears.
With any combinations, the head area remains a place where
parasites can hide because you won't be able to use enough intensity without possible side effects. (i have had my sight becomes white with the eyes wide open when putting an electrode on the skin of the forehead.)
Maybe an electromagnetic device is the solution for this limit, but this will require a lot of power !
Keep researching parazapper, i am for this tech even if it is not ready yet.