the electric current eventually finds most of itself in the main blood arteries (in the groin area and the arteries between the shoulders) which is where it is concentrated enough to have maximum effects against microbes. Electric current, like the current of a stream or river, seeks the path of least resistance from point A to B. So if the blood in veins and arteries is more conductive (ie: offering less resistance to electric current flow) then current will tend to take that route instead of rougher going in lymph muscles tendons bones etc.
I have the advantage (if we can call this an "advantage"...) to be infected in many parts of the body.
And when i electrise my body with a high enough intensity i can feel the worms contract their bodies and move to avoid the current (if i don't try to kill them with herbs or electricity, they don't move much...)
So even if i thought the same thing as you before, i have to modify my position now. With a higher intensity (between 15mA to 25mA) the current seems to reach parts which are not in the direct path of the main blood veins and arteries, like in the head for example.
PS : your zapper produces less die off and do not disturb the worms as much as the ultimate zapper (with the kind of roundworms i am infected with), but it seems it has a higher allowed intensity. That's strange, maybe it is because of AC ?